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/*-------------------------------------------------------------------------
*
* session.c
* Encapsulation of user session.
*
* This is intended to contain data that needs to be shared between backends
* performing work for a client session. In particular such a session is
* shared between the leader and worker processes for parallel queries. At
* some later point it might also become useful infrastructure for separating
* backends from client connections, e.g. for the purpose of pooling.
*
* Currently this infrastructure is used to share:
* - typemod registry for ephemeral row-types, i.e. BlessTupleDesc etc.
*
* Portions Copyright (c) 2017-2020, PostgreSQL Global Development Group
*
* src/backend/access/common/session.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/session.h"
#include "storage/lwlock.h"
#include "storage/shm_toc.h"
#include "utils/memutils.h"
#include "utils/typcache.h"
/* Magic number for per-session DSM TOC. */
#define SESSION_MAGIC 0xabb0fbc9
/*
* We want to create a DSA area to store shared state that has the same
* lifetime as a session. So far, it's only used to hold the shared record
* type registry. We don't want it to have to create any DSM segments just
* yet in common cases, so we'll give it enough space to hold a very small
* SharedRecordTypmodRegistry.
*/
#define SESSION_DSA_SIZE 0x30000
/*
* Magic numbers for state sharing in the per-session DSM area.
*/
#define SESSION_KEY_DSA UINT64CONST(0xFFFFFFFFFFFF0001)
#define SESSION_KEY_RECORD_TYPMOD_REGISTRY UINT64CONST(0xFFFFFFFFFFFF0002)
/* This backend's current session. */
Session *CurrentSession = NULL;
/*
* Set up CurrentSession to point to an empty Session object.
*/
void
InitializeSession(void)
{
CurrentSession = MemoryContextAllocZero(TopMemoryContext, sizeof(Session));
}
/*
* Initialize the per-session DSM segment if it isn't already initialized, and
* return its handle so that worker processes can attach to it.
*
* Unlike the per-context DSM segment, this segment and its contents are
* reused for future parallel queries.
*
* Return DSM_HANDLE_INVALID if a segment can't be allocated due to lack of
* resources.
*/
dsm_handle
GetSessionDsmHandle(void)
{
shm_toc_estimator estimator;
shm_toc *toc;
dsm_segment *seg;
size_t typmod_registry_size;
size_t size;
void *dsa_space;
void *typmod_registry_space;
dsa_area *dsa;
MemoryContext old_context;
/*
* If we have already created a session-scope DSM segment in this backend,
* return its handle. The same segment will be used for the rest of this
* backend's lifetime.
*/
if (CurrentSession->segment != NULL)
return dsm_segment_handle(CurrentSession->segment);
/* Otherwise, prepare to set one up. */
old_context = MemoryContextSwitchTo(TopMemoryContext);
shm_toc_initialize_estimator(&estimator);
/* Estimate space for the per-session DSA area. */
shm_toc_estimate_keys(&estimator, 1);
shm_toc_estimate_chunk(&estimator, SESSION_DSA_SIZE);
/* Estimate space for the per-session record typmod registry. */
typmod_registry_size = SharedRecordTypmodRegistryEstimate();
shm_toc_estimate_keys(&estimator, 1);
shm_toc_estimate_chunk(&estimator, typmod_registry_size);
/* Set up segment and TOC. */
size = shm_toc_estimate(&estimator);
seg = dsm_create(size, DSM_CREATE_NULL_IF_MAXSEGMENTS);
if (seg == NULL)
{
MemoryContextSwitchTo(old_context);
return DSM_HANDLE_INVALID;
}
toc = shm_toc_create(SESSION_MAGIC,
dsm_segment_address(seg),
size);
/* Create per-session DSA area. */
dsa_space = shm_toc_allocate(toc, SESSION_DSA_SIZE);
dsa = dsa_create_in_place(dsa_space,
SESSION_DSA_SIZE,
LWTRANCHE_PER_SESSION_DSA,
seg);
shm_toc_insert(toc, SESSION_KEY_DSA, dsa_space);
/* Create session-scoped shared record typmod registry. */
typmod_registry_space = shm_toc_allocate(toc, typmod_registry_size);
SharedRecordTypmodRegistryInit((SharedRecordTypmodRegistry *)
typmod_registry_space, seg, dsa);
shm_toc_insert(toc, SESSION_KEY_RECORD_TYPMOD_REGISTRY,
typmod_registry_space);
/*
* If we got this far, we can pin the shared memory so it stays mapped for
* the rest of this backend's life. If we don't make it this far, cleanup
* callbacks for anything we installed above (ie currently
* SharedRecordTypmodRegistry) will run when the DSM segment is detached
* by CurrentResourceOwner so we aren't left with a broken CurrentSession.
*/
dsm_pin_mapping(seg);
dsa_pin_mapping(dsa);
/* Make segment and area available via CurrentSession. */
CurrentSession->segment = seg;
CurrentSession->area = dsa;
MemoryContextSwitchTo(old_context);
return dsm_segment_handle(seg);
}
/*
* Attach to a per-session DSM segment provided by a parallel leader.
*/
void
AttachSession(dsm_handle handle)
{
dsm_segment *seg;
shm_toc *toc;
void *dsa_space;
void *typmod_registry_space;
dsa_area *dsa;
MemoryContext old_context;
old_context = MemoryContextSwitchTo(TopMemoryContext);
/* Attach to the DSM segment. */
seg = dsm_attach(handle);
if (seg == NULL)
elog(ERROR, "could not attach to per-session DSM segment");
toc = shm_toc_attach(SESSION_MAGIC, dsm_segment_address(seg));
/* Attach to the DSA area. */
dsa_space = shm_toc_lookup(toc, SESSION_KEY_DSA, false);
dsa = dsa_attach_in_place(dsa_space, seg);
/* Make them available via the current session. */
CurrentSession->segment = seg;
CurrentSession->area = dsa;
/* Attach to the shared record typmod registry. */
typmod_registry_space =
shm_toc_lookup(toc, SESSION_KEY_RECORD_TYPMOD_REGISTRY, false);
SharedRecordTypmodRegistryAttach((SharedRecordTypmodRegistry *)
typmod_registry_space);
/* Remain attached until end of backend or DetachSession(). */
dsm_pin_mapping(seg);
dsa_pin_mapping(dsa);
MemoryContextSwitchTo(old_context);
}
/*
* Detach from the current session DSM segment. It's not strictly necessary
* to do this explicitly since we'll detach automatically at backend exit, but
* if we ever reuse parallel workers it will become important for workers to
* detach from one session before attaching to another. Note that this runs
* detach hooks.
*/
void
DetachSession(void)
{
/* Runs detach hooks. */
dsm_detach(CurrentSession->segment);
CurrentSession->segment = NULL;
dsa_detach(CurrentSession->area);
CurrentSession->area = NULL;
}
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