Adding upstream version 1.7.2+ds.upstream/1.7.2+ds

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

24 files changed, 3241 insertions, 0 deletions

diff --git a/include/git2/sys/alloc.h b/include/git2/sys/alloc.h
new file mode 100644
index 0000000..e7f85b8
--- /dev/null
+++ b/include/git2/sys/alloc.h
@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#ifndef INCLUDE_sys_git_alloc_h__
+#define INCLUDE_sys_git_alloc_h__
+
+#include "git2/common.h"
+
+GIT_BEGIN_DECL
+
+/**
+ * An instance for a custom memory allocator
+ *
+ * Setting the pointers of this structure allows the developer to implement
+ * custom memory allocators. The global memory allocator can be set by using
+ * "GIT_OPT_SET_ALLOCATOR" with the `git_libgit2_opts` function. Keep in mind
+ * that all fields need to be set to a proper function.
+ */
+typedef struct {
+	/** Allocate `n` bytes of memory */
+	void * GIT_CALLBACK(gmalloc)(size_t n, const char *file, int line);
+
+	/**
+	 * This function shall deallocate the old object `ptr` and return a
+	 * pointer to a new object that has the size specified by `size`. In
+	 * case `ptr` is `NULL`, a new array shall be allocated.
+	 */
+	void * GIT_CALLBACK(grealloc)(void *ptr, size_t size, const char *file, int line);
+
+	/**
+	 * This function shall free the memory pointed to by `ptr`. In case
+	 * `ptr` is `NULL`, this shall be a no-op.
+	 */
+	void GIT_CALLBACK(gfree)(void *ptr);
+} git_allocator;
+
+/**
+ * Initialize the allocator structure to use the `stdalloc` pointer.
+ *
+ * Set up the structure so that all of its members are using the standard
+ * "stdalloc" allocator functions. The structure can then be used with
+ * `git_allocator_setup`.
+ *
+ * @param allocator The allocator that is to be initialized.
+ * @return An error code or 0.
+ */
+int git_stdalloc_init_allocator(git_allocator *allocator);
+
+/**
+ * Initialize the allocator structure to use the `crtdbg` pointer.
+ *
+ * Set up the structure so that all of its members are using the "crtdbg"
+ * allocator functions. Note that this allocator is only available on Windows
+ * platforms and only if libgit2 is being compiled with "-DMSVC_CRTDBG".
+ *
+ * @param allocator The allocator that is to be initialized.
+ * @return An error code or 0.
+ */
+int git_win32_crtdbg_init_allocator(git_allocator *allocator);
+
+GIT_END_DECL
+
+#endif
diff --git a/include/git2/sys/commit.h b/include/git2/sys/commit.h
new file mode 100644
index 0000000..ba67106
--- /dev/null
+++ b/include/git2/sys/commit.h
@@ -0,0 +1,80 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_commit_h__
+#define INCLUDE_sys_git_commit_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+#include "git2/oid.h"
+
+/**
+ * @file git2/sys/commit.h
+ * @brief Low-level Git commit creation
+ * @defgroup git_backend Git custom backend APIs
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Create new commit in the repository from a list of `git_oid` values.
+ *
+ * See documentation for `git_commit_create()` for information about the
+ * parameters, as the meaning is identical excepting that `tree` and
+ * `parents` now take `git_oid`.  This is a dangerous API in that nor
+ * the `tree`, neither the `parents` list of `git_oid`s are checked for
+ * validity.
+ *
+ * @see git_commit_create
+ */
+GIT_EXTERN(int) git_commit_create_from_ids(
+	git_oid *id,
+	git_repository *repo,
+	const char *update_ref,
+	const git_signature *author,
+	const git_signature *committer,
+	const char *message_encoding,
+	const char *message,
+	const git_oid *tree,
+	size_t parent_count,
+	const git_oid *parents[]);
+
+/**
+ * Callback function to return parents for commit.
+ *
+ * This is invoked with the count of the number of parents processed so far
+ * along with the user supplied payload.  This should return a git_oid of
+ * the next parent or NULL if all parents have been provided.
+ */
+typedef const git_oid * GIT_CALLBACK(git_commit_parent_callback)(size_t idx, void *payload);
+
+/**
+ * Create a new commit in the repository with an callback to supply parents.
+ *
+ * See documentation for `git_commit_create()` for information about the
+ * parameters, as the meaning is identical excepting that `tree` takes a
+ * `git_oid` and doesn't check for validity, and `parent_cb` is invoked
+ * with `parent_payload` and should return `git_oid` values or NULL to
+ * indicate that all parents are accounted for.
+ *
+ * @see git_commit_create
+ */
+GIT_EXTERN(int) git_commit_create_from_callback(
+	git_oid *id,
+	git_repository *repo,
+	const char *update_ref,
+	const git_signature *author,
+	const git_signature *committer,
+	const char *message_encoding,
+	const char *message,
+	const git_oid *tree,
+	git_commit_parent_callback parent_cb,
+	void *parent_payload);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/sys/commit_graph.h b/include/git2/sys/commit_graph.h
new file mode 100644
index 0000000..06e045f
--- /dev/null
+++ b/include/git2/sys/commit_graph.h
@@ -0,0 +1,184 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_commit_graph_h__
+#define INCLUDE_sys_git_commit_graph_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+
+/**
+ * @file git2/sys/commit_graph.h
+ * @brief Git commit-graph
+ * @defgroup git_commit_graph Git commit-graph APIs
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Opens a `git_commit_graph` from a path to an objects directory.
+ *
+ * This finds, opens, and validates the `commit-graph` file.
+ *
+ * @param cgraph_out the `git_commit_graph` struct to initialize.
+ * @param objects_dir the path to a git objects directory.
+ * @return Zero on success; -1 on failure.
+ */
+GIT_EXTERN(int) git_commit_graph_open(
+	git_commit_graph **cgraph_out,
+	const char *objects_dir
+#ifdef GIT_EXPERIMENTAL_SHA256
+	, git_oid_t oid_type
+#endif
+	);
+
+/**
+ * Frees commit-graph data. This should only be called when memory allocated
+ * using `git_commit_graph_open` is not returned to libgit2 because it was not
+ * associated with the ODB through a successful call to
+ * `git_odb_set_commit_graph`.
+ *
+ * @param cgraph the commit-graph object to free. If NULL, no action is taken.
+ */
+GIT_EXTERN(void) git_commit_graph_free(git_commit_graph *cgraph);
+
+/**
+ * Create a new writer for `commit-graph` files.
+ *
+ * @param out Location to store the writer pointer.
+ * @param objects_info_dir The `objects/info` directory.
+ * The `commit-graph` file will be written in this directory.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_new(
+		git_commit_graph_writer **out,
+		const char *objects_info_dir
+#ifdef GIT_EXPERIMENTAL_SHA256
+	, git_oid_t oid_type
+#endif
+		);
+
+/**
+ * Free the commit-graph writer and its resources.
+ *
+ * @param w The writer to free. If NULL no action is taken.
+ */
+GIT_EXTERN(void) git_commit_graph_writer_free(git_commit_graph_writer *w);
+
+/**
+ * Add an `.idx` file (associated to a packfile) to the writer.
+ *
+ * @param w The writer.
+ * @param repo The repository that owns the `.idx` file.
+ * @param idx_path The path of an `.idx` file.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_add_index_file(
+		git_commit_graph_writer *w,
+		git_repository *repo,
+		const char *idx_path);
+
+/**
+ * Add a revwalk to the writer. This will add all the commits from the revwalk
+ * to the commit-graph.
+ *
+ * @param w The writer.
+ * @param walk The git_revwalk.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_add_revwalk(
+		git_commit_graph_writer *w,
+		git_revwalk *walk);
+
+
+/**
+ * The strategy to use when adding a new set of commits to a pre-existing
+ * commit-graph chain.
+ */
+typedef enum {
+	/**
+	 * Do not split commit-graph files. The other split strategy-related option
+	 * fields are ignored.
+	 */
+	GIT_COMMIT_GRAPH_SPLIT_STRATEGY_SINGLE_FILE = 0
+} git_commit_graph_split_strategy_t;
+
+/**
+ * Options structure for
+ * `git_commit_graph_writer_commit`/`git_commit_graph_writer_dump`.
+ *
+ * Initialize with `GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT`. Alternatively, you
+ * can use `git_commit_graph_writer_options_init`.
+ */
+typedef struct {
+	unsigned int version;
+
+	/**
+	 * The strategy to use when adding new commits to a pre-existing commit-graph
+	 * chain.
+	 */
+	git_commit_graph_split_strategy_t split_strategy;
+
+	/**
+	 * The number of commits in level N is less than X times the number of
+	 * commits in level N + 1. Default is 2.
+	 */
+	float size_multiple;
+
+	/**
+	 * The number of commits in level N + 1 is more than C commits.
+	 * Default is 64000.
+	 */
+	size_t max_commits;
+} git_commit_graph_writer_options;
+
+#define GIT_COMMIT_GRAPH_WRITER_OPTIONS_VERSION 1
+#define GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT { \
+		GIT_COMMIT_GRAPH_WRITER_OPTIONS_VERSION \
+	}
+
+/**
+ * Initialize git_commit_graph_writer_options structure
+ *
+ * Initializes a `git_commit_graph_writer_options` with default values. Equivalent to
+ * creating an instance with `GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT`.
+ *
+ * @param opts The `git_commit_graph_writer_options` struct to initialize.
+ * @param version The struct version; pass `GIT_COMMIT_GRAPH_WRITER_OPTIONS_VERSION`.
+ * @return Zero on success; -1 on failure.
+ */
+GIT_EXTERN(int) git_commit_graph_writer_options_init(
+	git_commit_graph_writer_options *opts,
+	unsigned int version);
+
+/**
+ * Write a `commit-graph` file to a file.
+ *
+ * @param w The writer
+ * @param opts Pointer to git_commit_graph_writer_options struct.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_commit(
+		git_commit_graph_writer *w,
+		git_commit_graph_writer_options *opts);
+
+/**
+ * Dump the contents of the `commit-graph` to an in-memory buffer.
+ *
+ * @param buffer Buffer where to store the contents of the `commit-graph`.
+ * @param w The writer.
+ * @param opts Pointer to git_commit_graph_writer_options struct.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_dump(
+		git_buf *buffer,
+		git_commit_graph_writer *w,
+		git_commit_graph_writer_options *opts);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/sys/config.h b/include/git2/sys/config.h
new file mode 100644
index 0000000..0a9005e
--- /dev/null
+++ b/include/git2/sys/config.h
@@ -0,0 +1,130 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_config_backend_h__
+#define INCLUDE_sys_git_config_backend_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+#include "git2/config.h"
+
+/**
+ * @file git2/sys/config.h
+ * @brief Git config backend routines
+ * @defgroup git_backend Git custom backend APIs
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Every iterator must have this struct as its first element, so the
+ * API can talk to it. You'd define your iterator as
+ *
+ *     struct my_iterator {
+ *             git_config_iterator parent;
+ *             ...
+ *     }
+ *
+ * and assign `iter->parent.backend` to your `git_config_backend`.
+ */
+struct git_config_iterator {
+	git_config_backend *backend;
+	unsigned int flags;
+
+	/**
+	 * Return the current entry and advance the iterator. The
+	 * memory belongs to the library.
+	 */
+	int GIT_CALLBACK(next)(git_config_entry **entry, git_config_iterator *iter);
+
+	/**
+	 * Free the iterator
+	 */
+	void GIT_CALLBACK(free)(git_config_iterator *iter);
+};
+
+/**
+ * Generic backend that implements the interface to
+ * access a configuration file
+ */
+struct git_config_backend {
+	unsigned int version;
+	/** True if this backend is for a snapshot */
+	int readonly;
+	struct git_config *cfg;
+
+	/* Open means open the file/database and parse if necessary */
+	int GIT_CALLBACK(open)(struct git_config_backend *, git_config_level_t level, const git_repository *repo);
+	int GIT_CALLBACK(get)(struct git_config_backend *, const char *key, git_config_entry **entry);
+	int GIT_CALLBACK(set)(struct git_config_backend *, const char *key, const char *value);
+	int GIT_CALLBACK(set_multivar)(git_config_backend *cfg, const char *name, const char *regexp, const char *value);
+	int GIT_CALLBACK(del)(struct git_config_backend *, const char *key);
+	int GIT_CALLBACK(del_multivar)(struct git_config_backend *, const char *key, const char *regexp);
+	int GIT_CALLBACK(iterator)(git_config_iterator **, struct git_config_backend *);
+	/** Produce a read-only version of this backend */
+	int GIT_CALLBACK(snapshot)(struct git_config_backend **, struct git_config_backend *);
+	/**
+	 * Lock this backend.
+	 *
+	 * Prevent any writes to the data store backing this
+	 * backend. Any updates must not be visible to any other
+	 * readers.
+	 */
+	int GIT_CALLBACK(lock)(struct git_config_backend *);
+	/**
+	 * Unlock the data store backing this backend. If success is
+	 * true, the changes should be committed, otherwise rolled
+	 * back.
+	 */
+	int GIT_CALLBACK(unlock)(struct git_config_backend *, int success);
+	void GIT_CALLBACK(free)(struct git_config_backend *);
+};
+#define GIT_CONFIG_BACKEND_VERSION 1
+#define GIT_CONFIG_BACKEND_INIT {GIT_CONFIG_BACKEND_VERSION}
+
+/**
+ * Initializes a `git_config_backend` with default values. Equivalent to
+ * creating an instance with GIT_CONFIG_BACKEND_INIT.
+ *
+ * @param backend the `git_config_backend` struct to initialize.
+ * @param version Version of struct; pass `GIT_CONFIG_BACKEND_VERSION`
+ * @return Zero on success; -1 on failure.
+ */
+GIT_EXTERN(int) git_config_init_backend(
+	git_config_backend *backend,
+	unsigned int version);
+
+/**
+ * Add a generic config file instance to an existing config
+ *
+ * Note that the configuration object will free the file
+ * automatically.
+ *
+ * Further queries on this config object will access each
+ * of the config file instances in order (instances with
+ * a higher priority level will be accessed first).
+ *
+ * @param cfg the configuration to add the file to
+ * @param file the configuration file (backend) to add
+ * @param level the priority level of the backend
+ * @param repo optional repository to allow parsing of
+ *  conditional includes
+ * @param force if a config file already exists for the given
+ *  priority level, replace it
+ * @return 0 on success, GIT_EEXISTS when adding more than one file
+ *  for a given priority level (and force_replace set to 0), or error code
+ */
+GIT_EXTERN(int) git_config_add_backend(
+	git_config *cfg,
+	git_config_backend *file,
+	git_config_level_t level,
+	const git_repository *repo,
+	int force);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/sys/cred.h b/include/git2/sys/cred.h
new file mode 100644
index 0000000..4d2a59a
--- /dev/null
+++ b/include/git2/sys/cred.h
@@ -0,0 +1,15 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_cred_h__
+#define INCLUDE_sys_git_cred_h__
+
+/* These declarations have moved. */
+#ifndef GIT_DEPRECATE_HARD
+# include "git2/sys/credential.h"
+#endif
+
+#endif
diff --git a/include/git2/sys/credential.h b/include/git2/sys/credential.h
new file mode 100644
index 0000000..bb4c9f9
--- /dev/null
+++ b/include/git2/sys/credential.h
@@ -0,0 +1,90 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_credential_h__
+#define INCLUDE_sys_git_credential_h__
+
+#include "git2/common.h"
+#include "git2/credential.h"
+
+/**
+ * @file git2/sys/cred.h
+ * @brief Git credentials low-level implementation
+ * @defgroup git_credential Git credentials low-level implementation
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * The base structure for all credential types
+ */
+struct git_credential {
+	git_credential_t credtype; /**< A type of credential */
+
+	/** The deallocator for this type of credentials */
+	void GIT_CALLBACK(free)(git_credential *cred);
+};
+
+/** A plaintext username and password */
+struct git_credential_userpass_plaintext {
+	git_credential parent; /**< The parent credential */
+	char *username;        /**< The username to authenticate as */
+	char *password;        /**< The password to use */
+};
+
+/** Username-only credential information */
+struct git_credential_username {
+	git_credential parent; /**< The parent credential */
+	char username[1];      /**< The username to authenticate as */
+};
+
+/**
+ * A ssh key from disk
+ */
+struct git_credential_ssh_key {
+	git_credential parent; /**< The parent credential */
+	char *username;        /**< The username to authenticate as */
+	char *publickey;       /**< The path to a public key */
+	char *privatekey;      /**< The path to a private key */
+	char *passphrase;      /**< Passphrase to decrypt the private key */
+};
+
+/**
+ * Keyboard-interactive based ssh authentication
+ */
+struct git_credential_ssh_interactive {
+	git_credential parent; /**< The parent credential */
+	char *username;        /**< The username to authenticate as */
+
+	/**
+	 * Callback used for authentication.
+	 */
+	git_credential_ssh_interactive_cb prompt_callback;
+
+	void *payload;         /**< Payload passed to prompt_callback */
+};
+
+/**
+ * A key with a custom signature function
+ */
+struct git_credential_ssh_custom {
+	git_credential parent; /**< The parent credential */
+	char *username;        /**< The username to authenticate as */
+	char *publickey;       /**< The public key data */
+	size_t publickey_len;  /**< Length of the public key */
+
+	/**
+	 * Callback used to sign the data.
+	 */
+	git_credential_sign_cb sign_callback;
+
+	void *payload;         /**< Payload passed to prompt_callback */
+};
+
+GIT_END_DECL
+
+#endif
diff --git a/include/git2/sys/diff.h b/include/git2/sys/diff.h
new file mode 100644
index 0000000..aefd7b9
--- /dev/null
+++ b/include/git2/sys/diff.h
@@ -0,0 +1,94 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_diff_h__
+#define INCLUDE_sys_git_diff_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+#include "git2/oid.h"
+#include "git2/diff.h"
+#include "git2/status.h"
+
+/**
+ * @file git2/sys/diff.h
+ * @brief Low-level Git diff utilities
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Diff print callback that writes to a git_buf.
+ *
+ * This function is provided not for you to call it directly, but instead
+ * so you can use it as a function pointer to the `git_diff_print` or
+ * `git_patch_print` APIs.  When using those APIs, you specify a callback
+ * to actually handle the diff and/or patch data.
+ *
+ * Use this callback to easily write that data to a `git_buf` buffer.  You
+ * must pass a `git_buf *` value as the payload to the `git_diff_print`
+ * and/or `git_patch_print` function.  The data will be appended to the
+ * buffer (after any existing content).
+ */
+GIT_EXTERN(int) git_diff_print_callback__to_buf(
+	const git_diff_delta *delta,
+	const git_diff_hunk *hunk,
+	const git_diff_line *line,
+	void *payload); /**< payload must be a `git_buf *` */
+
+/**
+ * Diff print callback that writes to stdio FILE handle.
+ *
+ * This function is provided not for you to call it directly, but instead
+ * so you can use it as a function pointer to the `git_diff_print` or
+ * `git_patch_print` APIs.  When using those APIs, you specify a callback
+ * to actually handle the diff and/or patch data.
+ *
+ * Use this callback to easily write that data to a stdio FILE handle.  You
+ * must pass a `FILE *` value (such as `stdout` or `stderr` or the return
+ * value from `fopen()`) as the payload to the `git_diff_print`
+ * and/or `git_patch_print` function.  If you pass NULL, this will write
+ * data to `stdout`.
+ */
+GIT_EXTERN(int) git_diff_print_callback__to_file_handle(
+	const git_diff_delta *delta,
+	const git_diff_hunk *hunk,
+	const git_diff_line *line,
+	void *payload); /**< payload must be a `FILE *` */
+
+
+/**
+ * Performance data from diffing
+ */
+typedef struct {
+	unsigned int version;
+	size_t stat_calls; /**< Number of stat() calls performed */
+	size_t oid_calculations; /**< Number of ID calculations */
+} git_diff_perfdata;
+
+#define GIT_DIFF_PERFDATA_VERSION 1
+#define GIT_DIFF_PERFDATA_INIT {GIT_DIFF_PERFDATA_VERSION,0,0}
+
+/**
+ * Get performance data for a diff object.
+ *
+ * @param out Structure to be filled with diff performance data
+ * @param diff Diff to read performance data from
+ * @return 0 for success, <0 for error
+ */
+GIT_EXTERN(int) git_diff_get_perfdata(
+	git_diff_perfdata *out, const git_diff *diff);
+
+/**
+ * Get performance data for diffs from a git_status_list
+ */
+GIT_EXTERN(int) git_status_list_get_perfdata(
+	git_diff_perfdata *out, const git_status_list *status);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/sys/email.h b/include/git2/sys/email.h
new file mode 100644
index 0000000..6f4a286
--- /dev/null
+++ b/include/git2/sys/email.h
@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_email_h__
+#define INCLUDE_sys_git_email_h__
+
+/**
+ * @file git2/sys/email.h
+ * @brief Advanced git email creation routines
+ * @defgroup git_email Advanced git email creation routines
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Create a diff for a commit in mbox format for sending via email.
+ *
+ * @param out buffer to store the e-mail patch in
+ * @param diff the changes to include in the email
+ * @param patch_idx the patch index
+ * @param patch_count the total number of patches that will be included
+ * @param commit_id the commit id for this change
+ * @param summary the commit message for this change
+ * @param body optional text to include above the diffstat
+ * @param author the person who authored this commit
+ * @param opts email creation options
+ */
+GIT_EXTERN(int) git_email_create_from_diff(
+	git_buf *out,
+	git_diff *diff,
+	size_t patch_idx,
+	size_t patch_count,
+	const git_oid *commit_id,
+	const char *summary,
+	const char *body,
+	const git_signature *author,
+	const git_email_create_options *opts);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/sys/filter.h b/include/git2/sys/filter.h
new file mode 100644
index 0000000..b375941
--- /dev/null
+++ b/include/git2/sys/filter.h
@@ -0,0 +1,353 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_filter_h__
+#define INCLUDE_sys_git_filter_h__
+
+#include "git2/filter.h"
+
+/**
+ * @file git2/sys/filter.h
+ * @brief Git filter backend and plugin routines
+ * @defgroup git_backend Git custom backend APIs
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Look up a filter by name
+ *
+ * @param name The name of the filter
+ * @return Pointer to the filter object or NULL if not found
+ */
+GIT_EXTERN(git_filter *) git_filter_lookup(const char *name);
+
+#define GIT_FILTER_CRLF  "crlf"
+#define GIT_FILTER_IDENT "ident"
+
+/**
+ * This is priority that the internal CRLF filter will be registered with
+ */
+#define GIT_FILTER_CRLF_PRIORITY 0
+
+/**
+ * This is priority that the internal ident filter will be registered with
+ */
+#define GIT_FILTER_IDENT_PRIORITY 100
+
+/**
+ * This is priority to use with a custom filter to imitate a core Git
+ * filter driver, so that it will be run last on checkout and first on
+ * checkin.  You do not have to use this, but it helps compatibility.
+ */
+#define GIT_FILTER_DRIVER_PRIORITY 200
+
+/**
+ * Create a new empty filter list
+ *
+ * Normally you won't use this because `git_filter_list_load` will create
+ * the filter list for you, but you can use this in combination with the
+ * `git_filter_lookup` and `git_filter_list_push` functions to assemble
+ * your own chains of filters.
+ */
+GIT_EXTERN(int) git_filter_list_new(
+	git_filter_list **out,
+	git_repository *repo,
+	git_filter_mode_t mode,
+	uint32_t options);
+
+/**
+ * Add a filter to a filter list with the given payload.
+ *
+ * Normally you won't have to do this because the filter list is created
+ * by calling the "check" function on registered filters when the filter
+ * attributes are set, but this does allow more direct manipulation of
+ * filter lists when desired.
+ *
+ * Note that normally the "check" function can set up a payload for the
+ * filter.  Using this function, you can either pass in a payload if you
+ * know the expected payload format, or you can pass NULL.  Some filters
+ * may fail with a NULL payload.  Good luck!
+ */
+GIT_EXTERN(int) git_filter_list_push(
+	git_filter_list *fl, git_filter *filter, void *payload);
+
+/**
+ * Look up how many filters are in the list
+ *
+ * We will attempt to apply all of these filters to any data passed in,
+ * but note that the filter apply action still has the option of skipping
+ * data that is passed in (for example, the CRLF filter will skip data
+ * that appears to be binary).
+ *
+ * @param fl A filter list
+ * @return The number of filters in the list
+ */
+GIT_EXTERN(size_t) git_filter_list_length(const git_filter_list *fl);
+
+/**
+ * A filter source represents a file/blob to be processed
+ */
+typedef struct git_filter_source git_filter_source;
+
+/**
+ * Get the repository that the source data is coming from.
+ */
+GIT_EXTERN(git_repository *) git_filter_source_repo(const git_filter_source *src);
+
+/**
+ * Get the path that the source data is coming from.
+ */
+GIT_EXTERN(const char *) git_filter_source_path(const git_filter_source *src);
+
+/**
+ * Get the file mode of the source file
+ * If the mode is unknown, this will return 0
+ */
+GIT_EXTERN(uint16_t) git_filter_source_filemode(const git_filter_source *src);
+
+/**
+ * Get the OID of the source
+ * If the OID is unknown (often the case with GIT_FILTER_CLEAN) then
+ * this will return NULL.
+ */
+GIT_EXTERN(const git_oid *) git_filter_source_id(const git_filter_source *src);
+
+/**
+ * Get the git_filter_mode_t to be used
+ */
+GIT_EXTERN(git_filter_mode_t) git_filter_source_mode(const git_filter_source *src);
+
+/**
+ * Get the combination git_filter_flag_t options to be applied
+ */
+GIT_EXTERN(uint32_t) git_filter_source_flags(const git_filter_source *src);
+
+/**
+ * Initialize callback on filter
+ *
+ * Specified as `filter.initialize`, this is an optional callback invoked
+ * before a filter is first used.  It will be called once at most.
+ *
+ * If non-NULL, the filter's `initialize` callback will be invoked right
+ * before the first use of the filter, so you can defer expensive
+ * initialization operations (in case libgit2 is being used in a way that
+ * doesn't need the filter).
+ */
+typedef int GIT_CALLBACK(git_filter_init_fn)(git_filter *self);
+
+/**
+ * Shutdown callback on filter
+ *
+ * Specified as `filter.shutdown`, this is an optional callback invoked
+ * when the filter is unregistered or when libgit2 is shutting down.  It
+ * will be called once at most and should release resources as needed.
+ * This may be called even if the `initialize` callback was not made.
+ *
+ * Typically this function will free the `git_filter` object itself.
+ */
+typedef void GIT_CALLBACK(git_filter_shutdown_fn)(git_filter *self);
+
+/**
+ * Callback to decide if a given source needs this filter
+ *
+ * Specified as `filter.check`, this is an optional callback that checks
+ * if filtering is needed for a given source.
+ *
+ * It should return 0 if the filter should be applied (i.e. success),
+ * GIT_PASSTHROUGH if the filter should not be applied, or an error code
+ * to fail out of the filter processing pipeline and return to the caller.
+ *
+ * The `attr_values` will be set to the values of any attributes given in
+ * the filter definition.  See `git_filter` below for more detail.
+ *
+ * The `payload` will be a pointer to a reference payload for the filter.
+ * This will start as NULL, but `check` can assign to this pointer for
+ * later use by the `stream` callback.  Note that the value should be heap
+ * allocated (not stack), so that it doesn't go away before the `stream`
+ * callback can use it.  If a filter allocates and assigns a value to the
+ * `payload`, it will need a `cleanup` callback to free the payload.
+ */
+typedef int GIT_CALLBACK(git_filter_check_fn)(
+	git_filter              *self,
+	void                   **payload, /* NULL on entry, may be set */
+	const git_filter_source *src,
+	const char             **attr_values);
+
+#ifndef GIT_DEPRECATE_HARD
+/**
+ * Callback to actually perform the data filtering
+ *
+ * Specified as `filter.apply`, this is the callback that actually filters
+ * data.  If it successfully writes the output, it should return 0.  Like
+ * `check`, it can return GIT_PASSTHROUGH to indicate that the filter
+ * doesn't want to run.  Other error codes will stop filter processing and
+ * return to the caller.
+ *
+ * The `payload` value will refer to any payload that was set by the
+ * `check` callback.  It may be read from or written to as needed.
+ *
+ * @deprecated use git_filter_stream_fn
+ */
+typedef int GIT_CALLBACK(git_filter_apply_fn)(
+	git_filter              *self,
+	void                   **payload, /* may be read and/or set */
+	git_buf                 *to,
+	const git_buf           *from,
+	const git_filter_source *src);
+#endif
+
+/**
+ * Callback to perform the data filtering.
+ *
+ * Specified as `filter.stream`, this is a callback that filters data
+ * in a streaming manner.  This function will provide a
+ * `git_writestream` that will the original data will be written to;
+ * with that data, the `git_writestream` will then perform the filter
+ * translation and stream the filtered data out to the `next` location.
+ */
+typedef int GIT_CALLBACK(git_filter_stream_fn)(
+	git_writestream        **out,
+	git_filter              *self,
+	void                   **payload,
+	const git_filter_source *src,
+	git_writestream         *next);
+
+/**
+ * Callback to clean up after filtering has been applied
+ *
+ * Specified as `filter.cleanup`, this is an optional callback invoked
+ * after the filter has been applied.  If the `check`, `apply`, or
+ * `stream` callbacks allocated a `payload` to keep per-source filter
+ * state, use this callback to free that payload and release resources
+ * as required.
+ */
+typedef void GIT_CALLBACK(git_filter_cleanup_fn)(
+	git_filter              *self,
+	void                    *payload);
+
+/**
+ * Filter structure used to register custom filters.
+ *
+ * To associate extra data with a filter, allocate extra data and put the
+ * `git_filter` struct at the start of your data buffer, then cast the
+ * `self` pointer to your larger structure when your callback is invoked.
+ */
+struct git_filter {
+	/** The `version` field should be set to `GIT_FILTER_VERSION`. */
+	unsigned int           version;
+
+ 	/**
+	 * A whitespace-separated list of attribute names to check for this
+	 * filter (e.g. "eol crlf text").  If the attribute name is bare, it
+	 * will be simply loaded and passed to the `check` callback.  If it
+	 * has a value (i.e. "name=value"), the attribute must match that
+	 * value for the filter to be applied.  The value may be a wildcard
+	 * (eg, "name=*"), in which case the filter will be invoked for any
+	 * value for the given attribute name.  See the attribute parameter
+	 * of the `check` callback for the attribute value that was specified.
+	 */
+	const char            *attributes;
+
+	/** Called when the filter is first used for any file. */
+	git_filter_init_fn     initialize;
+
+	/** Called when the filter is removed or unregistered from the system. */
+	git_filter_shutdown_fn shutdown;
+
+	/**
+	 * Called to determine whether the filter should be invoked for a
+	 * given file.  If this function returns `GIT_PASSTHROUGH` then the
+	 * `stream` or `apply` functions will not be invoked and the
+	 * contents will be passed through unmodified.
+	 */
+	git_filter_check_fn    check;
+
+#ifdef GIT_DEPRECATE_HARD
+	void *reserved;
+#else
+	/**
+	 * Provided for backward compatibility; this will apply the
+	 * filter to the given contents in a `git_buf`.  Callers should
+	 * provide a `stream` function instead.
+	 */
+	git_filter_apply_fn    apply;
+#endif
+
+	/**
+	 * Called to apply the filter, this function will provide a
+	 * `git_writestream` that will the original data will be
+	 * written to; with that data, the `git_writestream` will then
+	 * perform the filter translation and stream the filtered data
+	 * out to the `next` location.
+	 */
+	git_filter_stream_fn   stream;
+
+	/** Called when the system is done filtering for a file. */
+	git_filter_cleanup_fn  cleanup;
+};
+
+#define GIT_FILTER_VERSION 1
+#define GIT_FILTER_INIT {GIT_FILTER_VERSION}
+
+/**
+ * Initializes a `git_filter` with default values. Equivalent to
+ * creating an instance with GIT_FILTER_INIT.
+ *
+ * @param filter the `git_filter` struct to initialize.
+ * @param version Version the struct; pass `GIT_FILTER_VERSION`
+ * @return Zero on success; -1 on failure.
+ */
+GIT_EXTERN(int) git_filter_init(git_filter *filter, unsigned int version);
+
+/**
+ * Register a filter under a given name with a given priority.
+ *
+ * As mentioned elsewhere, the initialize callback will not be invoked
+ * immediately.  It is deferred until the filter is used in some way.
+ *
+ * A filter's attribute checks and `check` and `stream` (or `apply`)
+ * callbacks will be issued in order of `priority` on smudge (to
+ * workdir), and in reverse order of `priority` on clean (to odb).
+ *
+ * Two filters are preregistered with libgit2:
+ * - GIT_FILTER_CRLF with priority 0
+ * - GIT_FILTER_IDENT with priority 100
+ *
+ * Currently the filter registry is not thread safe, so any registering or
+ * deregistering of filters must be done outside of any possible usage of
+ * the filters (i.e. during application setup or shutdown).
+ *
+ * @param name A name by which the filter can be referenced.  Attempting
+ * 			to register with an in-use name will return GIT_EEXISTS.
+ * @param filter The filter definition.  This pointer will be stored as is
+ * 			by libgit2 so it must be a durable allocation (either static
+ * 			or on the heap).
+ * @param priority The priority for filter application
+ * @return 0 on successful registry, error code <0 on failure
+ */
+GIT_EXTERN(int) git_filter_register(
+	const char *name, git_filter *filter, int priority);
+
+/**
+ * Remove the filter with the given name
+ *
+ * Attempting to remove the builtin libgit2 filters is not permitted and
+ * will return an error.
+ *
+ * Currently the filter registry is not thread safe, so any registering or
+ * deregistering of filters must be done outside of any possible usage of
+ * the filters (i.e. during application setup or shutdown).
+ *
+ * @param name The name under which the filter was registered
+ * @return 0 on success, error code <0 on failure
+ */
+GIT_EXTERN(int) git_filter_unregister(const char *name);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/sys/hashsig.h b/include/git2/sys/hashsig.h
new file mode 100644
index 0000000..09c19ae
--- /dev/null
+++ b/include/git2/sys/hashsig.h
@@ -0,0 +1,106 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_hashsig_h__
+#define INCLUDE_sys_hashsig_h__
+
+#include "git2/common.h"
+
+GIT_BEGIN_DECL
+
+/**
+ * Similarity signature of arbitrary text content based on line hashes
+ */
+typedef struct git_hashsig git_hashsig;
+
+/**
+ * Options for hashsig computation
+ *
+ * The options GIT_HASHSIG_NORMAL, GIT_HASHSIG_IGNORE_WHITESPACE,
+ * GIT_HASHSIG_SMART_WHITESPACE are exclusive and should not be combined.
+ */
+typedef enum {
+	/**
+	 * Use all data
+	 */
+	GIT_HASHSIG_NORMAL = 0,
+
+	/**
+	 * Ignore whitespace
+	 */
+	GIT_HASHSIG_IGNORE_WHITESPACE = (1 << 0),
+
+	/**
+	 * Ignore \r and all space after \n
+	 */
+	GIT_HASHSIG_SMART_WHITESPACE = (1 << 1),
+
+	/**
+	 * Allow hashing of small files
+	 */
+	GIT_HASHSIG_ALLOW_SMALL_FILES = (1 << 2)
+} git_hashsig_option_t;
+
+/**
+ * Compute a similarity signature for a text buffer
+ *
+ * If you have passed the option GIT_HASHSIG_IGNORE_WHITESPACE, then the
+ * whitespace will be removed from the buffer while it is being processed,
+ * modifying the buffer in place. Sorry about that!
+ *
+ * @param out The computed similarity signature.
+ * @param buf The input buffer.
+ * @param buflen The input buffer size.
+ * @param opts The signature computation options (see above).
+ * @return 0 on success, GIT_EBUFS if the buffer doesn't contain enough data to
+ * compute a valid signature (unless GIT_HASHSIG_ALLOW_SMALL_FILES is set), or
+ * error code.
+ */
+GIT_EXTERN(int) git_hashsig_create(
+	git_hashsig **out,
+	const char *buf,
+	size_t buflen,
+	git_hashsig_option_t opts);
+
+/**
+ * Compute a similarity signature for a text file
+ *
+ * This walks through the file, only loading a maximum of 4K of file data at
+ * a time. Otherwise, it acts just like `git_hashsig_create`.
+ *
+ * @param out The computed similarity signature.
+ * @param path The path to the input file.
+ * @param opts The signature computation options (see above).
+ * @return 0 on success, GIT_EBUFS if the buffer doesn't contain enough data to
+ * compute a valid signature (unless GIT_HASHSIG_ALLOW_SMALL_FILES is set), or
+ * error code.
+ */
+GIT_EXTERN(int) git_hashsig_create_fromfile(
+	git_hashsig **out,
+	const char *path,
+	git_hashsig_option_t opts);
+
+/**
+ * Release memory for a content similarity signature
+ *
+ * @param sig The similarity signature to free.
+ */
+GIT_EXTERN(void) git_hashsig_free(git_hashsig *sig);
+
+/**
+ * Measure similarity score between two similarity signatures
+ *
+ * @param a The first similarity signature to compare.
+ * @param b The second similarity signature to compare.
+ * @return [0 to 100] on success as the similarity score, or error code.
+ */
+GIT_EXTERN(int) git_hashsig_compare(
+	const git_hashsig *a,
+	const git_hashsig *b);
+
+GIT_END_DECL
+
+#endif
diff --git a/include/git2/sys/index.h b/include/git2/sys/index.h
new file mode 100644
index 0000000..1f6d93f
--- /dev/null
+++ b/include/git2/sys/index.h
@@ -0,0 +1,182 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_index_h__
+#define INCLUDE_sys_git_index_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+
+/**
+ * @file git2/sys/index.h
+ * @brief Low-level Git index manipulation routines
+ * @defgroup git_backend Git custom backend APIs
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/** Representation of a rename conflict entry in the index. */
+typedef struct git_index_name_entry {
+	char *ancestor;
+	char *ours;
+	char *theirs;
+} git_index_name_entry;
+
+/** Representation of a resolve undo entry in the index. */
+typedef struct git_index_reuc_entry {
+	uint32_t mode[3];
+	git_oid oid[3];
+	char *path;
+} git_index_reuc_entry;
+
+/** @name Conflict Name entry functions
+ *
+ * These functions work on rename conflict entries.
+ */
+/**@{*/
+
+/**
+ * Get the count of filename conflict entries currently in the index.
+ *
+ * @param index an existing index object
+ * @return integer of count of current filename conflict entries
+ */
+GIT_EXTERN(size_t) git_index_name_entrycount(git_index *index);
+
+/**
+ * Get a filename conflict entry from the index.
+ *
+ * The returned entry is read-only and should not be modified
+ * or freed by the caller.
+ *
+ * @param index an existing index object
+ * @param n the position of the entry
+ * @return a pointer to the filename conflict entry; NULL if out of bounds
+ */
+GIT_EXTERN(const git_index_name_entry *) git_index_name_get_byindex(
+	git_index *index, size_t n);
+
+/**
+ * Record the filenames involved in a rename conflict.
+ *
+ * @param index an existing index object
+ * @param ancestor the path of the file as it existed in the ancestor
+ * @param ours the path of the file as it existed in our tree
+ * @param theirs the path of the file as it existed in their tree
+ */
+GIT_EXTERN(int) git_index_name_add(git_index *index,
+	const char *ancestor, const char *ours, const char *theirs);
+
+/**
+ * Remove all filename conflict entries.
+ *
+ * @param index an existing index object
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_index_name_clear(git_index *index);
+
+/**@}*/
+
+/** @name Resolve Undo (REUC) index entry manipulation.
+ *
+ * These functions work on the Resolve Undo index extension and contains
+ * data about the original files that led to a merge conflict.
+ */
+/**@{*/
+
+/**
+ * Get the count of resolve undo entries currently in the index.
+ *
+ * @param index an existing index object
+ * @return integer of count of current resolve undo entries
+ */
+GIT_EXTERN(size_t) git_index_reuc_entrycount(git_index *index);
+
+/**
+ * Finds the resolve undo entry that points to the given path in the Git
+ * index.
+ *
+ * @param at_pos the address to which the position of the reuc entry is written (optional)
+ * @param index an existing index object
+ * @param path path to search
+ * @return 0 if found, < 0 otherwise (GIT_ENOTFOUND)
+ */
+GIT_EXTERN(int) git_index_reuc_find(size_t *at_pos, git_index *index, const char *path);
+
+/**
+ * Get a resolve undo entry from the index.
+ *
+ * The returned entry is read-only and should not be modified
+ * or freed by the caller.
+ *
+ * @param index an existing index object
+ * @param path path to search
+ * @return the resolve undo entry; NULL if not found
+ */
+GIT_EXTERN(const git_index_reuc_entry *) git_index_reuc_get_bypath(git_index *index, const char *path);
+
+/**
+ * Get a resolve undo entry from the index.
+ *
+ * The returned entry is read-only and should not be modified
+ * or freed by the caller.
+ *
+ * @param index an existing index object
+ * @param n the position of the entry
+ * @return a pointer to the resolve undo entry; NULL if out of bounds
+ */
+GIT_EXTERN(const git_index_reuc_entry *) git_index_reuc_get_byindex(git_index *index, size_t n);
+
+/**
+ * Adds a resolve undo entry for a file based on the given parameters.
+ *
+ * The resolve undo entry contains the OIDs of files that were involved
+ * in a merge conflict after the conflict has been resolved.  This allows
+ * conflicts to be re-resolved later.
+ *
+ * If there exists a resolve undo entry for the given path in the index,
+ * it will be removed.
+ *
+ * This method will fail in bare index instances.
+ *
+ * @param index an existing index object
+ * @param path filename to add
+ * @param ancestor_mode mode of the ancestor file
+ * @param ancestor_id oid of the ancestor file
+ * @param our_mode mode of our file
+ * @param our_id oid of our file
+ * @param their_mode mode of their file
+ * @param their_id oid of their file
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_index_reuc_add(git_index *index, const char *path,
+	int ancestor_mode, const git_oid *ancestor_id,
+	int our_mode, const git_oid *our_id,
+	int their_mode, const git_oid *their_id);
+
+/**
+ * Remove an resolve undo entry from the index
+ *
+ * @param index an existing index object
+ * @param n position of the resolve undo entry to remove
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_index_reuc_remove(git_index *index, size_t n);
+
+/**
+ * Remove all resolve undo entries from the index
+ *
+ * @param index an existing index object
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_index_reuc_clear(git_index *index);
+
+/**@}*/
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/sys/mempack.h b/include/git2/sys/mempack.h
new file mode 100644
index 0000000..17da590
--- /dev/null
+++ b/include/git2/sys/mempack.h
@@ -0,0 +1,87 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_odb_mempack_h__
+#define INCLUDE_sys_git_odb_mempack_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+#include "git2/oid.h"
+#include "git2/odb.h"
+#include "git2/buffer.h"
+
+/**
+ * @file git2/sys/mempack.h
+ * @brief Custom ODB backend that permits packing objects in-memory
+ * @defgroup git_backend Git custom backend APIs
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Instantiate a new mempack backend.
+ *
+ * The backend must be added to an existing ODB with the highest
+ * priority.
+ *
+ *     git_mempack_new(&mempacker);
+ *     git_repository_odb(&odb, repository);
+ *     git_odb_add_backend(odb, mempacker, 999);
+ *
+ * Once the backend has been loaded, all writes to the ODB will
+ * instead be queued in memory, and can be finalized with
+ * `git_mempack_dump`.
+ *
+ * Subsequent reads will also be served from the in-memory store
+ * to ensure consistency, until the memory store is dumped.
+ *
+ * @param out Pointer where to store the ODB backend
+ * @return 0 on success; error code otherwise
+ */
+GIT_EXTERN(int) git_mempack_new(git_odb_backend **out);
+
+/**
+ * Dump all the queued in-memory writes to a packfile.
+ *
+ * The contents of the packfile will be stored in the given buffer.
+ * It is the caller's responsibility to ensure that the generated
+ * packfile is available to the repository (e.g. by writing it
+ * to disk, or doing something crazy like distributing it across
+ * several copies of the repository over a network).
+ *
+ * Once the generated packfile is available to the repository,
+ * call `git_mempack_reset` to cleanup the memory store.
+ *
+ * Calling `git_mempack_reset` before the packfile has been
+ * written to disk will result in an inconsistent repository
+ * (the objects in the memory store won't be accessible).
+ *
+ * @param pack Buffer where to store the raw packfile
+ * @param repo The active repository where the backend is loaded
+ * @param backend The mempack backend
+ * @return 0 on success; error code otherwise
+ */
+GIT_EXTERN(int) git_mempack_dump(git_buf *pack, git_repository *repo, git_odb_backend *backend);
+
+/**
+ * Reset the memory packer by clearing all the queued objects.
+ *
+ * This assumes that `git_mempack_dump` has been called before to
+ * store all the queued objects into a single packfile.
+ *
+ * Alternatively, call `reset` without a previous dump to "undo"
+ * all the recently written objects, giving transaction-like
+ * semantics to the Git repository.
+ *
+ * @param backend The mempack backend
+ * @return 0 on success; error code otherwise
+ */
+GIT_EXTERN(int) git_mempack_reset(git_odb_backend *backend);
+
+GIT_END_DECL
+
+#endif
diff --git a/include/git2/sys/merge.h b/include/git2/sys/merge.h
new file mode 100644
index 0000000..ef4bc5a
--- /dev/null
+++ b/include/git2/sys/merge.h
@@ -0,0 +1,182 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_merge_h__
+#define INCLUDE_sys_git_merge_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+#include "git2/index.h"
+#include "git2/merge.h"
+
+/**
+ * @file git2/sys/merge.h
+ * @brief Git merge driver backend and plugin routines
+ * @defgroup git_merge Git merge driver APIs
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+typedef struct git_merge_driver git_merge_driver;
+
+/**
+ * Look up a merge driver by name
+ *
+ * @param name The name of the merge driver
+ * @return Pointer to the merge driver object or NULL if not found
+ */
+GIT_EXTERN(git_merge_driver *) git_merge_driver_lookup(const char *name);
+
+#define GIT_MERGE_DRIVER_TEXT   "text"
+#define GIT_MERGE_DRIVER_BINARY "binary"
+#define GIT_MERGE_DRIVER_UNION  "union"
+
+/**
+ * A merge driver source represents the file to be merged
+ */
+typedef struct git_merge_driver_source git_merge_driver_source;
+
+/** Get the repository that the source data is coming from. */
+GIT_EXTERN(git_repository *) git_merge_driver_source_repo(
+	const git_merge_driver_source *src);
+
+/** Gets the ancestor of the file to merge. */
+GIT_EXTERN(const git_index_entry *) git_merge_driver_source_ancestor(
+	const git_merge_driver_source *src);
+
+/** Gets the ours side of the file to merge. */
+GIT_EXTERN(const git_index_entry *) git_merge_driver_source_ours(
+	const git_merge_driver_source *src);
+
+/** Gets the theirs side of the file to merge. */
+GIT_EXTERN(const git_index_entry *) git_merge_driver_source_theirs(
+	const git_merge_driver_source *src);
+
+/** Gets the merge file options that the merge was invoked with */
+GIT_EXTERN(const git_merge_file_options *) git_merge_driver_source_file_options(
+	const git_merge_driver_source *src);
+
+
+/**
+ * Initialize callback on merge driver
+ *
+ * Specified as `driver.initialize`, this is an optional callback invoked
+ * before a merge driver is first used.  It will be called once at most
+ * per library lifetime.
+ *
+ * If non-NULL, the merge driver's `initialize` callback will be invoked
+ * right before the first use of the driver, so you can defer expensive
+ * initialization operations (in case libgit2 is being used in a way that
+ * doesn't need the merge driver).
+ */
+typedef int GIT_CALLBACK(git_merge_driver_init_fn)(git_merge_driver *self);
+
+/**
+ * Shutdown callback on merge driver
+ *
+ * Specified as `driver.shutdown`, this is an optional callback invoked
+ * when the merge driver is unregistered or when libgit2 is shutting down.
+ * It will be called once at most and should release resources as needed.
+ * This may be called even if the `initialize` callback was not made.
+ *
+ * Typically this function will free the `git_merge_driver` object itself.
+ */
+typedef void GIT_CALLBACK(git_merge_driver_shutdown_fn)(git_merge_driver *self);
+
+/**
+ * Callback to perform the merge.
+ *
+ * Specified as `driver.apply`, this is the callback that actually does the
+ * merge.  If it can successfully perform a merge, it should populate
+ * `path_out` with a pointer to the filename to accept, `mode_out` with
+ * the resultant mode, and `merged_out` with the buffer of the merged file
+ * and then return 0.  If the driver returns `GIT_PASSTHROUGH`, then the
+ * default merge driver should instead be run.  It can also return
+ * `GIT_EMERGECONFLICT` if the driver is not able to produce a merge result,
+ * and the file will remain conflicted.  Any other errors will fail and
+ * return to the caller.
+ *
+ * The `filter_name` contains the name of the filter that was invoked, as
+ * specified by the file's attributes.
+ *
+ * The `src` contains the data about the file to be merged.
+ */
+typedef int GIT_CALLBACK(git_merge_driver_apply_fn)(
+	git_merge_driver *self,
+	const char **path_out,
+	uint32_t *mode_out,
+	git_buf *merged_out,
+	const char *filter_name,
+	const git_merge_driver_source *src);
+
+/**
+ * Merge driver structure used to register custom merge drivers.
+ *
+ * To associate extra data with a driver, allocate extra data and put the
+ * `git_merge_driver` struct at the start of your data buffer, then cast
+ * the `self` pointer to your larger structure when your callback is invoked.
+ */
+struct git_merge_driver {
+	/** The `version` should be set to `GIT_MERGE_DRIVER_VERSION`. */
+	unsigned int                 version;
+
+	/** Called when the merge driver is first used for any file. */
+	git_merge_driver_init_fn     initialize;
+
+	/** Called when the merge driver is unregistered from the system. */
+	git_merge_driver_shutdown_fn shutdown;
+
+	/**
+	 * Called to merge the contents of a conflict.  If this function
+	 * returns `GIT_PASSTHROUGH` then the default (`text`) merge driver
+	 * will instead be invoked.  If this function returns
+	 * `GIT_EMERGECONFLICT` then the file will remain conflicted.
+	 */
+	git_merge_driver_apply_fn    apply;
+};
+
+#define GIT_MERGE_DRIVER_VERSION 1
+
+/**
+ * Register a merge driver under a given name.
+ *
+ * As mentioned elsewhere, the initialize callback will not be invoked
+ * immediately.  It is deferred until the driver is used in some way.
+ *
+ * Currently the merge driver registry is not thread safe, so any
+ * registering or deregistering of merge drivers must be done outside of
+ * any possible usage of the drivers (i.e. during application setup or
+ * shutdown).
+ *
+ * @param name The name of this driver to match an attribute.  Attempting
+ * 			to register with an in-use name will return GIT_EEXISTS.
+ * @param driver The merge driver definition.  This pointer will be stored
+ *			as is by libgit2 so it must be a durable allocation (either
+ *			static or on the heap).
+ * @return 0 on successful registry, error code <0 on failure
+ */
+GIT_EXTERN(int) git_merge_driver_register(
+	const char *name, git_merge_driver *driver);
+
+/**
+ * Remove the merge driver with the given name.
+ *
+ * Attempting to remove the builtin libgit2 merge drivers is not permitted
+ * and will return an error.
+ *
+ * Currently the merge driver registry is not thread safe, so any
+ * registering or deregistering of drivers must be done outside of any
+ * possible usage of the drivers (i.e. during application setup or shutdown).
+ *
+ * @param name The name under which the merge driver was registered
+ * @return 0 on success, error code <0 on failure
+ */
+GIT_EXTERN(int) git_merge_driver_unregister(const char *name);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/sys/midx.h b/include/git2/sys/midx.h
new file mode 100644
index 0000000..3a87484
--- /dev/null
+++ b/include/git2/sys/midx.h
@@ -0,0 +1,78 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_midx_h__
+#define INCLUDE_sys_git_midx_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+
+/**
+ * @file git2/midx.h
+ * @brief Git multi-pack-index routines
+ * @defgroup git_midx Git multi-pack-index routines
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Create a new writer for `multi-pack-index` files.
+ *
+ * @param out location to store the writer pointer.
+ * @param pack_dir the directory where the `.pack` and `.idx` files are. The
+ * `multi-pack-index` file will be written in this directory, too.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_midx_writer_new(
+		git_midx_writer **out,
+		const char *pack_dir
+#ifdef GIT_EXPERIMENTAL_SHA256
+		, git_oid_t oid_type
+#endif
+		);
+
+/**
+ * Free the multi-pack-index writer and its resources.
+ *
+ * @param w the writer to free. If NULL no action is taken.
+ */
+GIT_EXTERN(void) git_midx_writer_free(git_midx_writer *w);
+
+/**
+ * Add an `.idx` file to the writer.
+ *
+ * @param w the writer
+ * @param idx_path the path of an `.idx` file.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_midx_writer_add(
+		git_midx_writer *w,
+		const char *idx_path);
+
+/**
+ * Write a `multi-pack-index` file to a file.
+ *
+ * @param w the writer
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_midx_writer_commit(
+		git_midx_writer *w);
+
+/**
+ * Dump the contents of the `multi-pack-index` to an in-memory buffer.
+ *
+ * @param midx Buffer where to store the contents of the `multi-pack-index`.
+ * @param w the writer
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_midx_writer_dump(
+		git_buf *midx,
+		git_midx_writer *w);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/sys/odb_backend.h b/include/git2/sys/odb_backend.h
new file mode 100644
index 0000000..c42abd3
--- /dev/null
+++ b/include/git2/sys/odb_backend.h
@@ -0,0 +1,172 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_odb_backend_h__
+#define INCLUDE_sys_git_odb_backend_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+#include "git2/oid.h"
+#include "git2/odb.h"
+
+/**
+ * @file git2/sys/backend.h
+ * @brief Git custom backend implementors functions
+ * @defgroup git_backend Git custom backend APIs
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * An instance for a custom backend
+ */
+struct git_odb_backend {
+	unsigned int version;
+	git_odb *odb;
+
+	/* read and read_prefix each return to libgit2 a buffer which
+	 * will be freed later. The buffer should be allocated using
+	 * the function git_odb_backend_data_alloc to ensure that libgit2
+	 * can safely free it later. */
+	int GIT_CALLBACK(read)(
+		void **, size_t *, git_object_t *, git_odb_backend *, const git_oid *);
+
+	/* To find a unique object given a prefix of its oid.  The oid given
+	 * must be so that the remaining (GIT_OID_SHA1_HEXSIZE - len)*4 bits are 0s.
+	 */
+	int GIT_CALLBACK(read_prefix)(
+		git_oid *, void **, size_t *, git_object_t *,
+		git_odb_backend *, const git_oid *, size_t);
+
+	int GIT_CALLBACK(read_header)(
+		size_t *, git_object_t *, git_odb_backend *, const git_oid *);
+
+	/**
+	 * Write an object into the backend. The id of the object has
+	 * already been calculated and is passed in.
+	 */
+	int GIT_CALLBACK(write)(
+		git_odb_backend *, const git_oid *, const void *, size_t, git_object_t);
+
+	int GIT_CALLBACK(writestream)(
+		git_odb_stream **, git_odb_backend *, git_object_size_t, git_object_t);
+
+	int GIT_CALLBACK(readstream)(
+		git_odb_stream **, size_t *, git_object_t *,
+		git_odb_backend *, const git_oid *);
+
+	int GIT_CALLBACK(exists)(
+		git_odb_backend *, const git_oid *);
+
+	int GIT_CALLBACK(exists_prefix)(
+		git_oid *, git_odb_backend *, const git_oid *, size_t);
+
+	/**
+	 * If the backend implements a refreshing mechanism, it should be exposed
+	 * through this endpoint. Each call to `git_odb_refresh()` will invoke it.
+	 *
+	 * The odb layer will automatically call this when needed on failed
+	 * lookups (ie. `exists()`, `read()`, `read_header()`).
+	 */
+	int GIT_CALLBACK(refresh)(git_odb_backend *);
+
+	int GIT_CALLBACK(foreach)(
+		git_odb_backend *, git_odb_foreach_cb cb, void *payload);
+
+	int GIT_CALLBACK(writepack)(
+		git_odb_writepack **, git_odb_backend *, git_odb *odb,
+		git_indexer_progress_cb progress_cb, void *progress_payload);
+
+	/**
+	 * If the backend supports pack files, this will create a
+	 * `multi-pack-index` file which will contain an index of all objects
+	 * across all the `.pack` files.
+	 */
+	int GIT_CALLBACK(writemidx)(git_odb_backend *);
+
+	/**
+	 * "Freshens" an already existing object, updating its last-used
+	 * time.  This occurs when `git_odb_write` was called, but the
+	 * object already existed (and will not be re-written).  The
+	 * underlying implementation may want to update last-used timestamps.
+	 *
+	 * If callers implement this, they should return `0` if the object
+	 * exists and was freshened, and non-zero otherwise.
+	 */
+	int GIT_CALLBACK(freshen)(git_odb_backend *, const git_oid *);
+
+	/**
+	 * Frees any resources held by the odb (including the `git_odb_backend`
+	 * itself). An odb backend implementation must provide this function.
+	 */
+	void GIT_CALLBACK(free)(git_odb_backend *);
+};
+
+#define GIT_ODB_BACKEND_VERSION 1
+#define GIT_ODB_BACKEND_INIT {GIT_ODB_BACKEND_VERSION}
+
+/**
+ * Initializes a `git_odb_backend` with default values. Equivalent to
+ * creating an instance with GIT_ODB_BACKEND_INIT.
+ *
+ * @param backend the `git_odb_backend` struct to initialize.
+ * @param version Version the struct; pass `GIT_ODB_BACKEND_VERSION`
+ * @return Zero on success; -1 on failure.
+ */
+GIT_EXTERN(int) git_odb_init_backend(
+	git_odb_backend *backend,
+	unsigned int version);
+
+/**
+ * Allocate data for an ODB object.  Custom ODB backends may use this
+ * to provide data back to the ODB from their read function.  This
+ * memory should not be freed once it is returned to libgit2.  If a
+ * custom ODB uses this function but encounters an error and does not
+ * return this data to libgit2, then they should use the corresponding
+ * git_odb_backend_data_free function.
+ *
+ * @param backend the ODB backend that is allocating this memory
+ * @param len the number of bytes to allocate
+ * @return the allocated buffer on success or NULL if out of memory
+ */
+GIT_EXTERN(void *) git_odb_backend_data_alloc(git_odb_backend *backend, size_t len);
+
+/**
+ * Frees custom allocated ODB data.  This should only be called when
+ * memory allocated using git_odb_backend_data_alloc is not returned
+ * to libgit2 because the backend encountered an error in the read
+ * function after allocation and did not return this data to libgit2.
+ *
+ * @param backend the ODB backend that is freeing this memory
+ * @param data the buffer to free
+ */
+GIT_EXTERN(void) git_odb_backend_data_free(git_odb_backend *backend, void *data);
+
+
+/*
+ * Users can avoid deprecated functions by defining `GIT_DEPRECATE_HARD`.
+ */
+#ifndef GIT_DEPRECATE_HARD
+
+/**
+ * Allocate memory for an ODB object from a custom backend.  This is
+ * an alias of `git_odb_backend_data_alloc` and is preserved for
+ * backward compatibility.
+ *
+ * This function is deprecated, but there is no plan to remove this
+ * function at this time.
+ *
+ * @deprecated git_odb_backend_data_alloc
+ * @see git_odb_backend_data_alloc
+ */
+GIT_EXTERN(void *) git_odb_backend_malloc(git_odb_backend *backend, size_t len);
+
+#endif
+
+GIT_END_DECL
+
+#endif
diff --git a/include/git2/sys/openssl.h b/include/git2/sys/openssl.h
new file mode 100644
index 0000000..b41c55c
--- /dev/null
+++ b/include/git2/sys/openssl.h
@@ -0,0 +1,38 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_git_openssl_h__
+#define INCLUDE_git_openssl_h__
+
+#include "git2/common.h"
+
+GIT_BEGIN_DECL
+
+/**
+ * Initialize the OpenSSL locks
+ *
+ * OpenSSL requires the application to determine how it performs
+ * locking.
+ *
+ * This is a last-resort convenience function which libgit2 provides for
+ * allocating and initializing the locks as well as setting the
+ * locking function to use the system's native locking functions.
+ *
+ * The locking function will be cleared and the memory will be freed
+ * when you call git_threads_sutdown().
+ *
+ * If your programming language has an OpenSSL package/bindings, it
+ * likely sets up locking. You should very strongly prefer that over
+ * this function.
+ *
+ * @return 0 on success, -1 if there are errors or if libgit2 was not
+ * built with OpenSSL and threading support.
+ */
+GIT_EXTERN(int) git_openssl_set_locking(void);
+
+GIT_END_DECL
+#endif
+
diff --git a/include/git2/sys/path.h b/include/git2/sys/path.h
new file mode 100644
index 0000000..2a0c7e0
--- /dev/null
+++ b/include/git2/sys/path.h
@@ -0,0 +1,64 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#ifndef INCLUDE_sys_git_path_h__
+#define INCLUDE_sys_git_path_h__
+
+#include "git2/common.h"
+
+GIT_BEGIN_DECL
+
+/**
+ * The kinds of git-specific files we know about.
+ *
+ * The order needs to stay the same to not break the `gitfiles`
+ * array in path.c
+ */
+typedef enum {
+	/** Check for the .gitignore file */
+	GIT_PATH_GITFILE_GITIGNORE,
+	/** Check for the .gitmodules file */
+	GIT_PATH_GITFILE_GITMODULES,
+	/** Check for the .gitattributes file */
+	GIT_PATH_GITFILE_GITATTRIBUTES
+} git_path_gitfile;
+
+/**
+ * The kinds of checks to perform according to which filesystem we are trying to
+ * protect.
+ */
+typedef enum {
+	/** Do both NTFS- and HFS-specific checks */
+	GIT_PATH_FS_GENERIC,
+	/** Do NTFS-specific checks only */
+	GIT_PATH_FS_NTFS,
+	/** Do HFS-specific checks only */
+	GIT_PATH_FS_HFS
+} git_path_fs;
+
+/**
+ * Check whether a path component corresponds to a .git$SUFFIX
+ * file.
+ *
+ * As some filesystems do special things to filenames when
+ * writing files to disk, you cannot always do a plain string
+ * comparison to verify whether a file name matches an expected
+ * path or not. This function can do the comparison for you,
+ * depending on the filesystem you're on.
+ *
+ * @param path the path component to check
+ * @param pathlen the length of `path` that is to be checked
+ * @param gitfile which file to check against
+ * @param fs which filesystem-specific checks to use
+ * @return 0 in case the file does not match, a positive value if
+ *         it does; -1 in case of an error
+ */
+GIT_EXTERN(int) git_path_is_gitfile(const char *path, size_t pathlen, git_path_gitfile gitfile, git_path_fs fs);
+
+GIT_END_DECL
+
+#endif	/* INCLUDE_sys_git_path */
diff --git a/include/git2/sys/refdb_backend.h b/include/git2/sys/refdb_backend.h
new file mode 100644
index 0000000..c31e26d
--- /dev/null
+++ b/include/git2/sys/refdb_backend.h
@@ -0,0 +1,361 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_refdb_backend_h__
+#define INCLUDE_sys_git_refdb_backend_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+#include "git2/oid.h"
+
+/**
+ * @file git2/refdb_backend.h
+ * @brief Git custom refs backend functions
+ * @defgroup git_refdb_backend Git custom refs backend API
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+
+/**
+ * Every backend's iterator must have a pointer to itself as the first
+ * element, so the API can talk to it. You'd define your iterator as
+ *
+ *     struct my_iterator {
+ *             git_reference_iterator parent;
+ *             ...
+ *     }
+ *
+ * and assign `iter->parent.backend` to your `git_refdb_backend`.
+ */
+struct git_reference_iterator {
+	git_refdb *db;
+
+	/**
+	 * Return the current reference and advance the iterator.
+	 */
+	int GIT_CALLBACK(next)(
+		git_reference **ref,
+		git_reference_iterator *iter);
+
+	/**
+	 * Return the name of the current reference and advance the iterator
+	 */
+	int GIT_CALLBACK(next_name)(
+		const char **ref_name,
+		git_reference_iterator *iter);
+
+	/**
+	 * Free the iterator
+	 */
+	void GIT_CALLBACK(free)(
+		git_reference_iterator *iter);
+};
+
+/** An instance for a custom backend */
+struct git_refdb_backend {
+	unsigned int version; /**< The backend API version */
+
+	/**
+	 * Queries the refdb backend for the existence of a reference.
+	 *
+	 * A refdb implementation must provide this function.
+	 *
+	 * @arg exists The implementation shall set this to `0` if a ref does
+	 *             not exist, otherwise to `1`.
+	 * @arg ref_name The reference's name that should be checked for
+	 *               existence.
+	 * @return `0` on success, a negative error value code.
+	 */
+	int GIT_CALLBACK(exists)(
+		int *exists,
+		git_refdb_backend *backend,
+		const char *ref_name);
+
+	/**
+	 * Queries the refdb backend for a given reference.
+	 *
+	 * A refdb implementation must provide this function.
+	 *
+	 * @arg out The implementation shall set this to the allocated
+	 *          reference, if it could be found, otherwise to `NULL`.
+	 * @arg ref_name The reference's name that should be checked for
+	 *               existence.
+	 * @return `0` on success, `GIT_ENOTFOUND` if the reference does
+	 *         exist, otherwise a negative error code.
+	 */
+	int GIT_CALLBACK(lookup)(
+		git_reference **out,
+		git_refdb_backend *backend,
+		const char *ref_name);
+
+	/**
+	 * Allocate an iterator object for the backend.
+	 *
+	 * A refdb implementation must provide this function.
+	 *
+	 * @arg out The implementation shall set this to the allocated
+	 *          reference iterator. A custom structure may be used with an
+	 *          embedded `git_reference_iterator` structure. Both `next`
+	 *          and `next_name` functions of `git_reference_iterator` need
+	 *          to be populated.
+	 * @arg glob A pattern to filter references by. If given, the iterator
+	 *           shall only return references that match the glob when
+	 *           passed to `wildmatch`.
+	 * @return `0` on success, otherwise a negative error code.
+	 */
+	int GIT_CALLBACK(iterator)(
+		git_reference_iterator **iter,
+		struct git_refdb_backend *backend,
+		const char *glob);
+
+	/**
+	 * Writes the given reference to the refdb.
+	 *
+	 * A refdb implementation must provide this function.
+	 *
+	 * @arg ref The reference to persist. May either be a symbolic or
+	 *          direct reference.
+	 * @arg force Whether to write the reference if a reference with the
+	 *            same name already exists.
+	 * @arg who The person updating the reference. Shall be used to create
+	 *          a reflog entry.
+	 * @arg message The message detailing what kind of reference update is
+	 *              performed. Shall be used to create a reflog entry.
+	 * @arg old If not `NULL` and `force` is not set, then the
+	 *          implementation needs to ensure that the reference is currently at
+	 *          the given OID before writing the new value. If both `old`
+	 *          and `old_target` are `NULL`, then the reference should not
+	 *          exist at the point of writing.
+	 * @arg old_target If not `NULL` and `force` is not set, then the
+	 *                 implementation needs to ensure that the symbolic
+	 *                 reference is currently at the given target before
+	 *                 writing the new value. If both `old` and
+	 *                 `old_target` are `NULL`, then the reference should
+	 *                 not exist at the point of writing.
+	 * @return `0` on success, otherwise a negative error code.
+	 */
+	int GIT_CALLBACK(write)(git_refdb_backend *backend,
+		     const git_reference *ref, int force,
+		     const git_signature *who, const char *message,
+		     const git_oid *old, const char *old_target);
+
+	/**
+	 * Rename a reference in the refdb.
+	 *
+	 * A refdb implementation must provide this function.
+	 *
+	 * @arg out The implementation shall set this to the newly created
+	 *          reference or `NULL` on error.
+	 * @arg old_name The current name of the reference that is to be renamed.
+	 * @arg new_name The new name that the old reference shall be renamed to.
+	 * @arg force Whether to write the reference if a reference with the
+	 *            target name already exists.
+	 * @arg who The person updating the reference. Shall be used to create
+	 *          a reflog entry.
+	 * @arg message The message detailing what kind of reference update is
+	 *              performed. Shall be used to create a reflog entry.
+	 * @return `0` on success, otherwise a negative error code.
+	 */
+	int GIT_CALLBACK(rename)(
+		git_reference **out, git_refdb_backend *backend,
+		const char *old_name, const char *new_name, int force,
+		const git_signature *who, const char *message);
+
+	/**
+	 * Deletes the given reference from the refdb.
+	 *
+	 * If it exists, its reflog should be deleted as well.
+	 *
+	 * A refdb implementation must provide this function.
+	 *
+	 * @arg ref_name The name of the reference name that shall be deleted.
+	 * @arg old_id If not `NULL` and `force` is not set, then the
+	 *             implementation needs to ensure that the reference is currently at
+	 *             the given OID before writing the new value.
+	 * @arg old_target If not `NULL` and `force` is not set, then the
+	 *                 implementation needs to ensure that the symbolic
+	 *                 reference is currently at the given target before
+	 *                 writing the new value.
+	 * @return `0` on success, otherwise a negative error code.
+	 */
+	int GIT_CALLBACK(del)(git_refdb_backend *backend, const char *ref_name, const git_oid *old_id, const char *old_target);
+
+	/**
+	 * Suggests that the given refdb compress or optimize its references.
+	 *
+	 * This mechanism is implementation specific. For on-disk reference
+	 * databases, this may pack all loose references.
+	 *
+	 * A refdb implementation may provide this function; if it is not
+	 * provided, nothing will be done.
+	 *
+	 * @return `0` on success a negative error code otherwise
+	 */
+	int GIT_CALLBACK(compress)(git_refdb_backend *backend);
+
+	/**
+	 * Query whether a particular reference has a log (may be empty)
+	 *
+	 * Shall return 1 if it has a reflog, 0 it it doesn't and negative in
+	 * case an error occurred.
+	 *
+	 * A refdb implementation must provide this function.
+	 *
+	 * @return `0` on success, `1` if the reflog for the given reference
+	 *         exists, a negative error code otherwise
+	 */
+	int GIT_CALLBACK(has_log)(git_refdb_backend *backend, const char *refname);
+
+	/**
+	 * Make sure a particular reference will have a reflog which
+	 * will be appended to on writes.
+	 *
+	 * A refdb implementation must provide this function.
+	 *
+	 * @return `0` on success, a negative error code otherwise
+	 */
+	int GIT_CALLBACK(ensure_log)(git_refdb_backend *backend, const char *refname);
+
+	/**
+	 * Frees any resources held by the refdb (including the `git_refdb_backend`
+	 * itself).
+	 *
+	 * A refdb backend implementation must provide this function.
+	 */
+	void GIT_CALLBACK(free)(git_refdb_backend *backend);
+
+	/**
+	 * Read the reflog for the given reference name.
+	 *
+	 * A refdb implementation must provide this function.
+	 *
+	 * @return `0` on success, a negative error code otherwise
+	 */
+	int GIT_CALLBACK(reflog_read)(git_reflog **out, git_refdb_backend *backend, const char *name);
+
+	/**
+	 * Write a reflog to disk.
+	 *
+	 * A refdb implementation must provide this function.
+	 *
+	 * @arg reflog The complete reference log for a given reference. Note
+	 *             that this may contain entries that have already been
+	 *             written to disk.
+	 * @return `0` on success, a negative error code otherwise
+	 */
+	int GIT_CALLBACK(reflog_write)(git_refdb_backend *backend, git_reflog *reflog);
+
+	/**
+	 * Rename a reflog.
+	 *
+	 * A refdb implementation must provide this function.
+	 *
+	 * @arg old_name The name of old reference whose reflog shall be renamed from.
+	 * @arg new_name The name of new reference whose reflog shall be renamed to.
+	 * @return `0` on success, a negative error code otherwise
+	 */
+	int GIT_CALLBACK(reflog_rename)(git_refdb_backend *_backend, const char *old_name, const char *new_name);
+
+	/**
+	 * Remove a reflog.
+	 *
+	 * A refdb implementation must provide this function.
+	 *
+	 * @arg name The name of the reference whose reflog shall be deleted.
+	 * @return `0` on success, a negative error code otherwise
+	 */
+	int GIT_CALLBACK(reflog_delete)(git_refdb_backend *backend, const char *name);
+
+	/**
+	 * Lock a reference.
+	 *
+	 * A refdb implementation may provide this function; if it is not
+	 * provided, the transaction API will fail to work.
+	 *
+	 * @arg payload_out Opaque parameter that will be passed verbosely to
+	 *                  `unlock`.
+	 * @arg refname Reference that shall be locked.
+	 * @return `0` on success, a negative error code otherwise
+	 */
+	int GIT_CALLBACK(lock)(void **payload_out, git_refdb_backend *backend, const char *refname);
+
+	/**
+	 * Unlock a reference.
+	 *
+	 * Only one of target or symbolic_target will be set.
+	 * `success` will be true if the reference should be update, false if
+	 * the lock must be discarded.
+	 *
+	 * A refdb implementation must provide this function if a `lock`
+	 * implementation is provided.
+	 *
+	 * @arg payload The payload returned by `lock`.
+	 * @arg success `1` if a reference should be updated, `2` if
+	 *              a reference should be deleted, `0` if the lock must be
+	 *              discarded.
+	 * @arg update_reflog `1` in case the reflog should be updated, `0`
+	 *                    otherwise.
+	 * @arg ref The reference which should be unlocked.
+	 * @arg who The person updating the reference. Shall be used to create
+	 *          a reflog entry in case `update_reflog` is set.
+	 * @arg message The message detailing what kind of reference update is
+	 *              performed. Shall be used to create a reflog entry in
+	 *              case `update_reflog` is set.
+	 * @return `0` on success, a negative error code otherwise
+	 */
+	int GIT_CALLBACK(unlock)(git_refdb_backend *backend, void *payload, int success, int update_reflog,
+		      const git_reference *ref, const git_signature *sig, const char *message);
+};
+
+#define GIT_REFDB_BACKEND_VERSION 1
+#define GIT_REFDB_BACKEND_INIT {GIT_REFDB_BACKEND_VERSION}
+
+/**
+ * Initializes a `git_refdb_backend` with default values. Equivalent to
+ * creating an instance with GIT_REFDB_BACKEND_INIT.
+ *
+ * @param backend the `git_refdb_backend` struct to initialize
+ * @param version Version of struct; pass `GIT_REFDB_BACKEND_VERSION`
+ * @return Zero on success; -1 on failure.
+ */
+GIT_EXTERN(int) git_refdb_init_backend(
+	git_refdb_backend *backend,
+	unsigned int version);
+
+/**
+ * Constructors for default filesystem-based refdb backend
+ *
+ * Under normal usage, this is called for you when the repository is
+ * opened / created, but you can use this to explicitly construct a
+ * filesystem refdb backend for a repository.
+ *
+ * @param backend_out Output pointer to the git_refdb_backend object
+ * @param repo Git repository to access
+ * @return 0 on success, <0 error code on failure
+ */
+GIT_EXTERN(int) git_refdb_backend_fs(
+	git_refdb_backend **backend_out,
+	git_repository *repo);
+
+/**
+ * Sets the custom backend to an existing reference DB
+ *
+ * The `git_refdb` will take ownership of the `git_refdb_backend` so you
+ * should NOT free it after calling this function.
+ *
+ * @param refdb database to add the backend to
+ * @param backend pointer to a git_refdb_backend instance
+ * @return 0 on success; error code otherwise
+ */
+GIT_EXTERN(int) git_refdb_set_backend(
+	git_refdb *refdb,
+	git_refdb_backend *backend);
+
+GIT_END_DECL
+
+#endif
diff --git a/include/git2/sys/reflog.h b/include/git2/sys/reflog.h
new file mode 100644
index 0000000..c9d0041
--- /dev/null
+++ b/include/git2/sys/reflog.h
@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_reflog_h__
+#define INCLUDE_sys_git_reflog_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+#include "git2/oid.h"
+
+GIT_BEGIN_DECL
+
+GIT_EXTERN(git_reflog_entry *) git_reflog_entry__alloc(void);
+GIT_EXTERN(void) git_reflog_entry__free(git_reflog_entry *entry);
+
+GIT_END_DECL
+
+#endif
diff --git a/include/git2/sys/refs.h b/include/git2/sys/refs.h
new file mode 100644
index 0000000..d2ce2e0
--- /dev/null
+++ b/include/git2/sys/refs.h
@@ -0,0 +1,49 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_refdb_h__
+#define INCLUDE_sys_git_refdb_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+#include "git2/oid.h"
+
+/**
+ * @file git2/sys/refs.h
+ * @brief Low-level Git ref creation
+ * @defgroup git_backend Git custom backend APIs
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Create a new direct reference from an OID.
+ *
+ * @param name the reference name
+ * @param oid the object id for a direct reference
+ * @param peel the first non-tag object's OID, or NULL
+ * @return the created git_reference or NULL on error
+ */
+GIT_EXTERN(git_reference *) git_reference__alloc(
+	const char *name,
+	const git_oid *oid,
+	const git_oid *peel);
+
+/**
+ * Create a new symbolic reference.
+ *
+ * @param name the reference name
+ * @param target the target for a symbolic reference
+ * @return the created git_reference or NULL on error
+ */
+GIT_EXTERN(git_reference *) git_reference__alloc_symbolic(
+	const char *name,
+	const char *target);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/sys/remote.h b/include/git2/sys/remote.h
new file mode 100644
index 0000000..0eae923
--- /dev/null
+++ b/include/git2/sys/remote.h
@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#ifndef INCLUDE_sys_git_remote_h
+#define INCLUDE_sys_git_remote_h
+
+#include "git2/remote.h"
+
+/**
+ * @file git2/sys/remote.h
+ * @brief Low-level remote functionality for custom transports
+ * @defgroup git_remote Low-level remote functionality
+ * @ingroup Git
+ * @{
+*/
+
+GIT_BEGIN_DECL
+
+typedef enum {
+	/** Remote supports fetching an advertised object by ID. */
+	GIT_REMOTE_CAPABILITY_TIP_OID = (1 << 0),
+
+	/** Remote supports fetching an individual reachable object. */
+	GIT_REMOTE_CAPABILITY_REACHABLE_OID = (1 << 1),
+} git_remote_capability_t;
+
+/**
+ * Disposes libgit2-initialized fields from a git_remote_connect_options.
+ * This should only be used for git_remote_connect_options returned by
+ * git_transport_remote_connect_options.
+ *
+ * Note that this does not free the `git_remote_connect_options` itself, just
+ * the memory pointed to by it.
+ *
+ * @param opts The `git_remote_connect_options` struct to dispose.
+ */
+GIT_EXTERN(void) git_remote_connect_options_dispose(
+		git_remote_connect_options *opts);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/sys/repository.h b/include/git2/sys/repository.h
new file mode 100644
index 0000000..892be66
--- /dev/null
+++ b/include/git2/sys/repository.h
@@ -0,0 +1,180 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_repository_h__
+#define INCLUDE_sys_git_repository_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+
+/**
+ * @file git2/sys/repository.h
+ * @brief Git repository custom implementation routines
+ * @defgroup git_backend Git custom backend APIs
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Create a new repository with neither backends nor config object
+ *
+ * Note that this is only useful if you wish to associate the repository
+ * with a non-filesystem-backed object database and config store.
+ *
+ * Caveats: since this repository has no physical location, some systems
+ * can fail to function properly: locations under $GIT_DIR, $GIT_COMMON_DIR,
+ * or $GIT_INFO_DIR are impacted.
+ *
+ * @param out The blank repository
+ * @return 0 on success, or an error code
+ */
+GIT_EXTERN(int) git_repository_new(git_repository **out);
+
+/**
+ * Reset all the internal state in a repository.
+ *
+ * This will free all the mapped memory and internal objects
+ * of the repository and leave it in a "blank" state.
+ *
+ * There's no need to call this function directly unless you're
+ * trying to aggressively cleanup the repo before its
+ * deallocation. `git_repository_free` already performs this operation
+ * before deallocating the repo.
+ *
+ * @param repo The repository to clean up
+ * @return 0 on success, or an error code
+ */
+GIT_EXTERN(int) git_repository__cleanup(git_repository *repo);
+
+/**
+ * Update the filesystem config settings for an open repository
+ *
+ * When a repository is initialized, config values are set based on the
+ * properties of the filesystem that the repository is on, such as
+ * "core.ignorecase", "core.filemode", "core.symlinks", etc.  If the
+ * repository is moved to a new filesystem, these properties may no
+ * longer be correct and API calls may not behave as expected.  This
+ * call reruns the phase of repository initialization that sets those
+ * properties to compensate for the current filesystem of the repo.
+ *
+ * @param repo A repository object
+ * @param recurse_submodules Should submodules be updated recursively
+ * @return 0 on success, < 0 on error
+ */
+GIT_EXTERN(int) git_repository_reinit_filesystem(
+	git_repository *repo,
+	int recurse_submodules);
+
+/**
+ * Set the configuration file for this repository
+ *
+ * This configuration file will be used for all configuration
+ * queries involving this repository.
+ *
+ * The repository will keep a reference to the config file;
+ * the user must still free the config after setting it
+ * to the repository, or it will leak.
+ *
+ * @param repo A repository object
+ * @param config A Config object
+ * @return 0 on success, or an error code
+ */
+GIT_EXTERN(int) git_repository_set_config(git_repository *repo, git_config *config);
+
+/**
+ * Set the Object Database for this repository
+ *
+ * The ODB will be used for all object-related operations
+ * involving this repository.
+ *
+ * The repository will keep a reference to the ODB; the user
+ * must still free the ODB object after setting it to the
+ * repository, or it will leak.
+ *
+ * @param repo A repository object
+ * @param odb An ODB object
+ * @return 0 on success, or an error code
+ */
+GIT_EXTERN(int) git_repository_set_odb(git_repository *repo, git_odb *odb);
+
+/**
+ * Set the Reference Database Backend for this repository
+ *
+ * The refdb will be used for all reference related operations
+ * involving this repository.
+ *
+ * The repository will keep a reference to the refdb; the user
+ * must still free the refdb object after setting it to the
+ * repository, or it will leak.
+ *
+ * @param repo A repository object
+ * @param refdb An refdb object
+ * @return 0 on success, or an error code
+ */
+GIT_EXTERN(int) git_repository_set_refdb(git_repository *repo, git_refdb *refdb);
+
+/**
+ * Set the index file for this repository
+ *
+ * This index will be used for all index-related operations
+ * involving this repository.
+ *
+ * The repository will keep a reference to the index file;
+ * the user must still free the index after setting it
+ * to the repository, or it will leak.
+ *
+ * @param repo A repository object
+ * @param index An index object
+ * @return 0 on success, or an error code
+ */
+GIT_EXTERN(int) git_repository_set_index(git_repository *repo, git_index *index);
+
+/**
+ * Set a repository to be bare.
+ *
+ * Clear the working directory and set core.bare to true.  You may also
+ * want to call `git_repository_set_index(repo, NULL)` since a bare repo
+ * typically does not have an index, but this function will not do that
+ * for you.
+ *
+ * @param repo Repo to make bare
+ * @return 0 on success, <0 on failure
+ */
+GIT_EXTERN(int) git_repository_set_bare(git_repository *repo);
+
+/**
+ * Load and cache all submodules.
+ *
+ * Because the `.gitmodules` file is unstructured, loading submodules is an
+ * O(N) operation.  Any operation (such as `git_rebase_init`) that requires
+ * accessing all submodules is O(N^2) in the number of submodules, if it
+ * has to look each one up individually.  This function loads all submodules
+ * and caches them so that subsequent calls to `git_submodule_lookup` are O(1).
+ *
+ * @param repo the repository whose submodules will be cached.
+ */
+GIT_EXTERN(int) git_repository_submodule_cache_all(
+	git_repository *repo);
+
+/**
+ * Clear the submodule cache.
+ *
+ * Clear the submodule cache populated by `git_repository_submodule_cache_all`.
+ * If there is no cache, do nothing.
+ *
+ * The cache incorporates data from the repository's configuration, as well
+ * as the state of the working tree, the index, and HEAD.  So any time any
+ * of these has changed, the cache might become invalid.
+ *
+ * @param repo the repository whose submodule cache will be cleared
+ */
+GIT_EXTERN(int) git_repository_submodule_cache_clear(
+	git_repository *repo);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/sys/stream.h b/include/git2/sys/stream.h
new file mode 100644
index 0000000..3d28d09
--- /dev/null
+++ b/include/git2/sys/stream.h
@@ -0,0 +1,152 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_sys_git_stream_h__
+#define INCLUDE_sys_git_stream_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+#include "git2/proxy.h"
+
+GIT_BEGIN_DECL
+
+#define GIT_STREAM_VERSION 1
+
+/**
+ * Every stream must have this struct as its first element, so the
+ * API can talk to it. You'd define your stream as
+ *
+ *     struct my_stream {
+ *             git_stream parent;
+ *             ...
+ *     }
+ *
+ * and fill the functions
+ */
+typedef struct git_stream {
+	int version;
+
+	int encrypted : 1,
+	    proxy_support : 1;
+
+	/**
+	 * Timeout for read and write operations; can be set to `0` to
+	 * block indefinitely.
+	 */
+	int timeout;
+
+	/**
+	 * Timeout to connect to the remote server; can be set to `0`
+	 * to use the system defaults. This can be shorter than the
+	 * system default - often 75 seconds - but cannot be longer.
+	 */
+	int connect_timeout;
+
+	int GIT_CALLBACK(connect)(struct git_stream *);
+	int GIT_CALLBACK(certificate)(git_cert **, struct git_stream *);
+	int GIT_CALLBACK(set_proxy)(struct git_stream *, const git_proxy_options *proxy_opts);
+	ssize_t GIT_CALLBACK(read)(struct git_stream *, void *, size_t);
+	ssize_t GIT_CALLBACK(write)(struct git_stream *, const char *, size_t, int);
+	int GIT_CALLBACK(close)(struct git_stream *);
+	void GIT_CALLBACK(free)(struct git_stream *);
+} git_stream;
+
+typedef struct {
+	/** The `version` field should be set to `GIT_STREAM_VERSION`. */
+	int version;
+
+	/**
+	 * Called to create a new connection to a given host.
+	 *
+	 * @param out The created stream
+	 * @param host The hostname to connect to; may be a hostname or
+	 *             IP address
+	 * @param port The port to connect to; may be a port number or
+	 *             service name
+	 * @return 0 or an error code
+	 */
+	int GIT_CALLBACK(init)(git_stream **out, const char *host, const char *port);
+
+	/**
+	 * Called to create a new connection on top of the given stream.  If
+	 * this is a TLS stream, then this function may be used to proxy a
+	 * TLS stream over an HTTP CONNECT session.  If this is unset, then
+	 * HTTP CONNECT proxies will not be supported.
+	 *
+	 * @param out The created stream
+	 * @param in An existing stream to add TLS to
+	 * @param host The hostname that the stream is connected to,
+	 *             for certificate validation
+	 * @return 0 or an error code
+	 */
+	int GIT_CALLBACK(wrap)(git_stream **out, git_stream *in, const char *host);
+} git_stream_registration;
+
+/**
+ * The type of stream to register.
+ */
+typedef enum {
+	/** A standard (non-TLS) socket. */
+	GIT_STREAM_STANDARD = 1,
+
+	/** A TLS-encrypted socket. */
+	GIT_STREAM_TLS = 2
+} git_stream_t;
+
+/**
+ * Register stream constructors for the library to use
+ *
+ * If a registration structure is already set, it will be overwritten.
+ * Pass `NULL` in order to deregister the current constructor and return
+ * to the system defaults.
+ *
+ * The type parameter may be a bitwise AND of types.
+ *
+ * @param type the type or types of stream to register
+ * @param registration the registration data
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_stream_register(
+	git_stream_t type, git_stream_registration *registration);
+
+#ifndef GIT_DEPRECATE_HARD
+
+/** @name Deprecated TLS Stream Registration Functions
+ *
+ * These functions are retained for backward compatibility.  The newer
+ * versions of these values should be preferred in all new code.
+ *
+ * There is no plan to remove these backward compatibility values at
+ * this time.
+ */
+/**@{*/
+
+/**
+ * @deprecated Provide a git_stream_registration to git_stream_register
+ * @see git_stream_registration
+ */
+typedef int GIT_CALLBACK(git_stream_cb)(git_stream **out, const char *host, const char *port);
+
+/**
+ * Register a TLS stream constructor for the library to use.  This stream
+ * will not support HTTP CONNECT proxies.  This internally calls
+ * `git_stream_register` and is preserved for backward compatibility.
+ *
+ * This function is deprecated, but there is no plan to remove this
+ * function at this time.
+ *
+ * @deprecated Provide a git_stream_registration to git_stream_register
+ * @see git_stream_register
+ */
+GIT_EXTERN(int) git_stream_register_tls(git_stream_cb ctor);
+
+/**@}*/
+
+#endif
+
+GIT_END_DECL
+
+#endif
diff --git a/include/git2/sys/transport.h b/include/git2/sys/transport.h
new file mode 100644
index 0000000..370ca45
--- /dev/null
+++ b/include/git2/sys/transport.h
@@ -0,0 +1,465 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#ifndef INCLUDE_sys_git_transport_h
+#define INCLUDE_sys_git_transport_h
+
+#include "git2/net.h"
+#include "git2/oidarray.h"
+#include "git2/proxy.h"
+#include "git2/remote.h"
+#include "git2/strarray.h"
+#include "git2/transport.h"
+#include "git2/types.h"
+
+/**
+ * @file git2/sys/transport.h
+ * @brief Git custom transport registration interfaces and functions
+ * @defgroup git_transport Git custom transport registration
+ * @ingroup Git
+ * @{
+ */
+
+GIT_BEGIN_DECL
+
+typedef struct {
+	const git_remote_head * const *refs;
+	size_t refs_len;
+	git_oid *shallow_roots;
+	size_t shallow_roots_len;
+	int depth;
+} git_fetch_negotiation;
+
+struct git_transport {
+	unsigned int version; /**< The struct version */
+
+	/**
+	 * Connect the transport to the remote repository, using the given
+	 * direction.
+	 */
+	int GIT_CALLBACK(connect)(
+		git_transport *transport,
+		const char *url,
+		int direction,
+		const git_remote_connect_options *connect_opts);
+
+	/**
+	 * Resets the connect options for the given transport.  This
+	 * is useful for updating settings or callbacks for an already
+	 * connected transport.
+	 */
+	int GIT_CALLBACK(set_connect_opts)(
+		git_transport *transport,
+		const git_remote_connect_options *connect_opts);
+
+	/**
+	 * Gets the capabilities for this remote repository.
+	 *
+	 * This function may be called after a successful call to
+	 * `connect()`.
+	 */
+	int GIT_CALLBACK(capabilities)(
+		unsigned int *capabilities,
+		git_transport *transport);
+
+#ifdef GIT_EXPERIMENTAL_SHA256
+	/**
+	 * Gets the object type for the remote repository.
+	 *
+	 * This function may be called after a successful call to
+	 * `connect()`.
+	 */
+	int GIT_CALLBACK(oid_type)(
+		git_oid_t *object_type,
+		git_transport *transport);
+#endif
+
+	/**
+	 * Get the list of available references in the remote repository.
+	 *
+	 * This function may be called after a successful call to
+	 * `connect()`. The array returned is owned by the transport and
+	 * must be kept valid until the next call to one of its functions.
+	 */
+	int GIT_CALLBACK(ls)(
+		const git_remote_head ***out,
+		size_t *size,
+		git_transport *transport);
+
+	/** Executes the push whose context is in the git_push object. */
+	int GIT_CALLBACK(push)(
+		git_transport *transport,
+		git_push *push);
+
+	/**
+	 * Negotiate a fetch with the remote repository.
+	 *
+	 * This function may be called after a successful call to `connect()`,
+	 * when the direction is GIT_DIRECTION_FETCH. The function performs a
+	 * negotiation to calculate the `wants` list for the fetch.
+	 */
+	int GIT_CALLBACK(negotiate_fetch)(
+		git_transport *transport,
+		git_repository *repo,
+		const git_fetch_negotiation *fetch_data);
+
+	/**
+	 * Return the shallow roots of the remote.
+	 *
+	 * This function may be called after a successful call to
+	 * `negotiate_fetch`.
+	 */
+	int GIT_CALLBACK(shallow_roots)(
+		git_oidarray *out,
+		git_transport *transport);
+
+	/**
+	 * Start downloading the packfile from the remote repository.
+	 *
+	 * This function may be called after a successful call to
+	 * negotiate_fetch(), when the direction is GIT_DIRECTION_FETCH.
+	 */
+	int GIT_CALLBACK(download_pack)(
+		git_transport *transport,
+		git_repository *repo,
+		git_indexer_progress *stats);
+
+	/** Checks to see if the transport is connected */
+	int GIT_CALLBACK(is_connected)(git_transport *transport);
+
+	/** Cancels any outstanding transport operation */
+	void GIT_CALLBACK(cancel)(git_transport *transport);
+
+	/**
+	 * Close the connection to the remote repository.
+	 *
+	 * This function is the reverse of connect() -- it terminates the
+	 * connection to the remote end.
+	 */
+	int GIT_CALLBACK(close)(git_transport *transport);
+
+	/** Frees/destructs the git_transport object. */
+	void GIT_CALLBACK(free)(git_transport *transport);
+};
+
+#define GIT_TRANSPORT_VERSION 1
+#define GIT_TRANSPORT_INIT {GIT_TRANSPORT_VERSION}
+
+/**
+ * Initializes a `git_transport` with default values. Equivalent to
+ * creating an instance with GIT_TRANSPORT_INIT.
+ *
+ * @param opts the `git_transport` struct to initialize
+ * @param version Version of struct; pass `GIT_TRANSPORT_VERSION`
+ * @return Zero on success; -1 on failure.
+ */
+GIT_EXTERN(int) git_transport_init(
+	git_transport *opts,
+	unsigned int version);
+
+/**
+ * Function to use to create a transport from a URL. The transport database
+ * is scanned to find a transport that implements the scheme of the URI (i.e.
+ * git:// or http://) and a transport object is returned to the caller.
+ *
+ * @param out The newly created transport (out)
+ * @param owner The git_remote which will own this transport
+ * @param url The URL to connect to
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_new(git_transport **out, git_remote *owner, const char *url);
+
+/**
+ * Create an ssh transport with custom git command paths
+ *
+ * This is a factory function suitable for setting as the transport
+ * callback in a remote (or for a clone in the options).
+ *
+ * The payload argument must be a strarray pointer with the paths for
+ * the `git-upload-pack` and `git-receive-pack` at index 0 and 1.
+ *
+ * @param out the resulting transport
+ * @param owner the owning remote
+ * @param payload a strarray with the paths
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_ssh_with_paths(git_transport **out, git_remote *owner, void *payload);
+
+/**
+ * Add a custom transport definition, to be used in addition to the built-in
+ * set of transports that come with libgit2.
+ *
+ * The caller is responsible for synchronizing calls to git_transport_register
+ * and git_transport_unregister with other calls to the library that
+ * instantiate transports.
+ *
+ * @param prefix The scheme (ending in "://") to match, i.e. "git://"
+ * @param cb The callback used to create an instance of the transport
+ * @param param A fixed parameter to pass to cb at creation time
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_register(
+	const char *prefix,
+	git_transport_cb cb,
+	void *param);
+
+/**
+ * Unregister a custom transport definition which was previously registered
+ * with git_transport_register.
+ *
+ * The caller is responsible for synchronizing calls to git_transport_register
+ * and git_transport_unregister with other calls to the library that
+ * instantiate transports.
+ *
+ * @param prefix From the previous call to git_transport_register
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_unregister(
+	const char *prefix);
+
+/* Transports which come with libgit2 (match git_transport_cb). The expected
+ * value for "param" is listed in-line below. */
+
+/**
+ * Create an instance of the dummy transport.
+ *
+ * @param out The newly created transport (out)
+ * @param owner The git_remote which will own this transport
+ * @param payload You must pass NULL for this parameter.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_dummy(
+	git_transport **out,
+	git_remote *owner,
+	/* NULL */ void *payload);
+
+/**
+ * Create an instance of the local transport.
+ *
+ * @param out The newly created transport (out)
+ * @param owner The git_remote which will own this transport
+ * @param payload You must pass NULL for this parameter.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_local(
+	git_transport **out,
+	git_remote *owner,
+	/* NULL */ void *payload);
+
+/**
+ * Create an instance of the smart transport.
+ *
+ * @param out The newly created transport (out)
+ * @param owner The git_remote which will own this transport
+ * @param payload A pointer to a git_smart_subtransport_definition
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_smart(
+	git_transport **out,
+	git_remote *owner,
+	/* (git_smart_subtransport_definition *) */ void *payload);
+
+/**
+ * Call the certificate check for this transport.
+ *
+ * @param transport a smart transport
+ * @param cert the certificate to pass to the caller
+ * @param valid whether we believe the certificate is valid
+ * @param hostname the hostname we connected to
+ * @return the return value of the callback: 0 for no error, GIT_PASSTHROUGH
+ *         to indicate that there is no callback registered (or the callback
+ *         refused to validate the certificate and callers should behave as
+ *         if no callback was set), or < 0 for an error
+ */
+GIT_EXTERN(int) git_transport_smart_certificate_check(git_transport *transport, git_cert *cert, int valid, const char *hostname);
+
+/**
+ * Call the credentials callback for this transport
+ *
+ * @param out the pointer where the creds are to be stored
+ * @param transport a smart transport
+ * @param user the user we saw on the url (if any)
+ * @param methods available methods for authentication
+ * @return the return value of the callback: 0 for no error, GIT_PASSTHROUGH
+ *         to indicate that there is no callback registered (or the callback
+ *         refused to provide credentials and callers should behave as if no
+ *         callback was set), or < 0 for an error
+ */
+GIT_EXTERN(int) git_transport_smart_credentials(git_credential **out, git_transport *transport, const char *user, int methods);
+
+/**
+ * Get a copy of the remote connect options
+ *
+ * All data is copied and must be freed by the caller by calling
+ * `git_remote_connect_options_dispose`.
+ *
+ * @param out options struct to fill
+ * @param transport the transport to extract the data from.
+ */
+GIT_EXTERN(int) git_transport_remote_connect_options(
+		git_remote_connect_options *out,
+		git_transport *transport);
+
+/*
+ *** End of base transport interface ***
+ *** Begin interface for subtransports for the smart transport ***
+ */
+
+/** Actions that the smart transport can ask a subtransport to perform */
+typedef enum {
+	GIT_SERVICE_UPLOADPACK_LS = 1,
+	GIT_SERVICE_UPLOADPACK = 2,
+	GIT_SERVICE_RECEIVEPACK_LS = 3,
+	GIT_SERVICE_RECEIVEPACK = 4
+} git_smart_service_t;
+
+typedef struct git_smart_subtransport git_smart_subtransport;
+typedef struct git_smart_subtransport_stream git_smart_subtransport_stream;
+
+/**
+ * A stream used by the smart transport to read and write data
+ * from a subtransport.
+ *
+ * This provides a customization point in case you need to
+ * support some other communication method.
+ */
+struct git_smart_subtransport_stream {
+	git_smart_subtransport *subtransport; /**< The owning subtransport */
+
+	/**
+	 * Read available data from the stream.
+	 *
+	 * The implementation may read less than requested.
+	 */
+	int GIT_CALLBACK(read)(
+		git_smart_subtransport_stream *stream,
+		char *buffer,
+		size_t buf_size,
+		size_t *bytes_read);
+
+	/**
+	 * Write data to the stream
+	 *
+	 * The implementation must write all data or return an error.
+	 */
+	int GIT_CALLBACK(write)(
+		git_smart_subtransport_stream *stream,
+		const char *buffer,
+		size_t len);
+
+	/** Free the stream */
+	void GIT_CALLBACK(free)(
+		git_smart_subtransport_stream *stream);
+};
+
+/**
+ * An implementation of a subtransport which carries data for the
+ * smart transport
+ */
+struct git_smart_subtransport {
+	/**
+	 * Setup a subtransport stream for the requested action.
+	 */
+	int GIT_CALLBACK(action)(
+			git_smart_subtransport_stream **out,
+			git_smart_subtransport *transport,
+			const char *url,
+			git_smart_service_t action);
+
+	/**
+	 * Close the subtransport.
+	 *
+	 * Subtransports are guaranteed a call to close() between
+	 * calls to action(), except for the following two "natural" progressions
+	 * of actions against a constant URL:
+	 *
+	 * - UPLOADPACK_LS -> UPLOADPACK
+	 * - RECEIVEPACK_LS -> RECEIVEPACK
+	 */
+	int GIT_CALLBACK(close)(git_smart_subtransport *transport);
+
+	/** Free the subtransport */
+	void GIT_CALLBACK(free)(git_smart_subtransport *transport);
+};
+
+/** A function which creates a new subtransport for the smart transport */
+typedef int GIT_CALLBACK(git_smart_subtransport_cb)(
+	git_smart_subtransport **out,
+	git_transport *owner,
+	void *param);
+
+/**
+ * Definition for a "subtransport"
+ *
+ * The smart transport knows how to speak the git protocol, but it has no
+ * knowledge of how to establish a connection between it and another endpoint,
+ * or how to move data back and forth. For this, a subtransport interface is
+ * declared, and the smart transport delegates this work to the subtransports.
+ *
+ * Three subtransports are provided by libgit2: ssh, git, http(s).
+ *
+ * Subtransports can either be RPC = 0 (persistent connection) or RPC = 1
+ * (request/response). The smart transport handles the differences in its own
+ * logic. The git subtransport is RPC = 0, while http is RPC = 1.
+ */
+typedef struct git_smart_subtransport_definition {
+	/** The function to use to create the git_smart_subtransport */
+	git_smart_subtransport_cb callback;
+
+	/**
+	 * True if the protocol is stateless; false otherwise. For example,
+	 * http:// is stateless, but git:// is not.
+	 */
+	unsigned rpc;
+
+	/** User-specified parameter passed to the callback */
+	void *param;
+} git_smart_subtransport_definition;
+
+/* Smart transport subtransports that come with libgit2 */
+
+/**
+ * Create an instance of the http subtransport.
+ *
+ * This subtransport also supports https.
+ *
+ * @param out The newly created subtransport
+ * @param owner The smart transport to own this subtransport
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_smart_subtransport_http(
+	git_smart_subtransport **out,
+	git_transport *owner,
+	void *param);
+
+/**
+ * Create an instance of the git subtransport.
+ *
+ * @param out The newly created subtransport
+ * @param owner The smart transport to own this subtransport
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_smart_subtransport_git(
+	git_smart_subtransport **out,
+	git_transport *owner,
+	void *param);
+
+/**
+ * Create an instance of the ssh subtransport.
+ *
+ * @param out The newly created subtransport
+ * @param owner The smart transport to own this subtransport
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_smart_subtransport_ssh(
+	git_smart_subtransport **out,
+	git_transport *owner,
+	void *param);
+
+/** @} */
+GIT_END_DECL
+#endif