Adding upstream version 2.1.30.upstream/2.1.30

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

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+# libyang
+
+[![BSD license](https://img.shields.io/badge/License-BSD-blue.svg)](https://opensource.org/licenses/BSD-3-Clause)
+[![Build](https://github.com/CESNET/libyang/workflows/libyang%20CI/badge.svg)](https://github.com/CESNET/libyang/actions?query=workflow%3A%22libyang+CI%22)
+[![Docs](https://img.shields.io/badge/docs-link-blue)](https://netopeer.liberouter.org/doc/libyang/)
+[![Coverity Scan Build Status](https://scan.coverity.com/projects/5259/badge.svg)](https://scan.coverity.com/projects/5259)
+[![codecov.io](https://codecov.io/github/CESNET/libyang/coverage.svg?branch=master)](https://codecov.io/github/CESNET/libyang?branch=master)
+[![Fuzzing Status](https://oss-fuzz-build-logs.storage.googleapis.com/badges/libyang.svg)](https://bugs.chromium.org/p/oss-fuzz/issues/list?sort=-opened&can=1&q=proj:libyang)
+[![Ohloh Project Status](https://www.openhub.net/p/libyang/widgets/project_thin_badge.gif)](https://www.openhub.net/p/libyang)
+
+libyang is a YANG data modelling language parser and toolkit written (and
+providing API) in C. The library is used e.g. in [libnetconf2](https://github.com/CESNET/libnetconf2),
+[Netopeer2](https://github.com/CESNET/Netopeer2) or [sysrepo](https://github.com/sysrepo/sysrepo) projects.
+
+If you are interested in future plans announcements, please subscribe to the
+[Future Plans issue](https://github.com/CESNET/libyang/issues/880).
+
+## Branches
+
+The project uses 2 main branches `master` and `devel`. Other branches should not be cloned. In `master` there are files of the
+last official *release*. Any latest improvements and changes, which were tested at least briefly are found in `devel`. On every
+new *release*, `devel` is merged into `master`.
+
+This means that when only stable official releases are to be used, either `master` can be used or specific *releases* downloaded.
+If all the latest bugfixes should be applied, `devel` branch is the  one to be used. Note that whenever **a new issue is created**
+and it occurs on the `master` branch, the **first response will likely be** to use `devel` before any further provided support.
+
+## Migration from libyang version 1 or older
+
+Look into the documentation and the section `Transition Manual`. That should help with basic migration and the
+ability to compile a project. But to actually make use of the new features, it is required to read through
+the whole documentation and the API.
+
+## Provided Features
+
+* Parsing (and validating) schemas in YANG format.
+* Parsing (and validating) schemas in YIN format.
+* Parsing, validating and printing instance data in XML format.
+* Parsing, validating and printing instance data in JSON format
+  ([RFC 7951](https://tools.ietf.org/html/rfc7951)).
+* Manipulation with the instance data.
+* Support for default values in the instance data ([RFC 6243](https://tools.ietf.org/html/rfc6243)).
+* Support for YANG extensions.
+* Support for YANG Metadata ([RFC 7952](https://tools.ietf.org/html/rfc7952)).
+* Support for YANG Schema Mount ([RFC 8528](https://tools.ietf.org/html/rfc8528)).
+* Support for YANG Structure ([RFC 8791](https://tools.ietf.org/html/rfc8791)).
+* [yanglint](#yanglint) - feature-rich YANG tool.
+
+Current implementation covers YANG 1.0 ([RFC 6020](https://tools.ietf.org/html/rfc6020))
+as well as YANG 1.1 ([RFC 7950](https://tools.ietf.org/html/rfc7950)).
+
+## Packages
+
+Binary RPM or DEB packages of the latest release can be built locally using `apkg`, look into `README` in
+the `distro` directory.
+
+## Requirements
+
+### Unix Build Requirements
+
+* C compiler
+* cmake >= 2.8.12
+* libpcre2 >= 10.21 (including devel package)
+  * note, that PCRE is supposed to be compiled with unicode support (configure's options
+    `--enable-utf` and `--enable-unicode-properties`)
+
+#### Optional
+
+* doxygen (for generating documentation)
+* cmocka >= 1.0.1 (for [tests](#Tests))
+* valgrind (for enhanced testing)
+* gcov (for code coverage)
+* lcov (for code coverage)
+* genhtml (for code coverage)
+
+### Unix Runtime Requirements
+
+* libpcre2 >= 10.21
+
+### Windows Build Requirements
+
+* Visual Studio 17 (2022)
+* cmake >= 3.22.0
+* libpcre2 (same considerations as on POSIX)
+* [`pthreads-win32`](https://sourceware.org/pthreads-win32/)
+* [`dirent`](https://github.com/tronkko/dirent)
+* [`dlfcn-win32`](https://github.com/dlfcn-win32/dlfcn-win32)
+* [`getopt-win32`](https://github.com/libimobiledevice-win32/getopt)
+
+The Windows version [does not support plugins](https://github.com/CESNET/libyang/commit/323c31221645052e13db83f7d0e6e51c3ce9d802), and the `yanglint` works in a [non-interactive mode](https://github.com/CESNET/libyang/commit/2e3f935ed6f4a47e65b31de5aeebcd8877d5a09b) only.
+On Windows, all YANG date-and-time values are first converted to UTC (if TZ offset was specified), and then returned with "unspecified timezone".
+
+## Building
+
+```
+$ mkdir build; cd build
+$ cmake ..
+$ make
+# make install
+```
+
+### Useful CMake Options
+
+#### Changing Compiler
+
+Set `CC` variable:
+
+```
+$ CC=/usr/bin/clang cmake ..
+```
+
+#### Changing Install Path
+
+To change the prefix where the library, headers and any other files are installed,
+set `CMAKE_INSTALL_PREFIX` variable:
+```
+$ cmake -DCMAKE_INSTALL_PREFIX:PATH=/usr ..
+```
+
+Default prefix is `/usr/local`.
+
+#### Build Modes
+
+There are two build modes:
+* Release.
+  This generates library for the production use without any debug information.
+* Debug.
+  This generates library with the debug information and disables optimization
+  of the code.
+
+The `Debug` mode is currently used as the default one. to switch to the
+`Release` mode, enter at the command line:
+```
+$ cmake -D CMAKE_BUILD_TYPE:String="Release" ..
+```
+
+#### Changing Extensions Plugins Directory
+
+As for YANG extensions, libyang allows loading extension plugins. By default, the
+directory to store the plugins is LIBDIR/libyang. To change it, use the following
+cmake option with the value specifying the desired directory:
+
+```
+$ cmake -DPLUGINS_DIR:PATH=`pwd`"/src/extensions/" ..
+```
+
+The directory path can be also changed runtime via environment variable, e.g.:
+
+```
+$ LIBYANG_EXTENSIONS_PLUGINS_DIR=`pwd`/my/relative/path yanglint
+```
+
+Note that plugins are [not available on Windows](https://github.com/CESNET/libyang/commit/323c31221645052e13db83f7d0e6e51c3ce9d802).
+
+#### Optimizations
+
+Whenever the latest revision of a schema is supposed to be loaded (import without specific revision),
+it is performed in the standard way, the first time. By default, every other time when the latest
+revision of the same schema is needed, the one initially loaded is reused. If you know this can cause
+problems meaning the latest available revision of a schema can change during operation, you can force
+libyang to always search for the schema anew by:
+
+```
+$ cmake -DENABLE_LATEST_REVISIONS=OFF ..
+```
+
+### CMake Notes
+
+Note that, with CMake, if you want to change the compiler or its options after
+you already ran CMake, you need to clear its cache first - the most simple way
+to do it is to remove all content from the 'build' directory.
+
+## Usage
+
+All libyang functions are available via the main header:
+```
+#include <libyang/libyang.h>
+```
+
+To compile your program with libyang, it is necessary to link it with libyang using the
+following linker parameters:
+```
+-lyang
+```
+
+Note, that it may be necessary to call `ldconfig(8)` after library installation and if the
+library was installed into a non-standard path, the path to libyang must be specified to the
+linker. To help with setting all the compiler's options, there is `libyang.pc` file for
+`pkg-config(1)` available in the source tree. The file is installed with the library.
+
+If you are using `cmake` in you project, it is also possible to use the provided
+`FindLibYANG.cmake` file to detect presence of the libyang library in the system.
+
+## Bindings
+
+There are no bindings for other languages directly in this project but they are
+available separately.
+
+* [Python](https://github.com/CESNET/libyang-python/)
+* [C++](https://github.com/CESNET/libyang-cpp/)
+* [Rust](https://github.com/rwestphal/yang2-rs/)
+
+## yanglint
+
+libyang project includes a feature-rich tool called `yanglint(1)` for validation
+and conversion of the schemas and YANG modeled data. The source codes are
+located at [`/tools/lint`](./tools/lint) and can be used to explore how an
+application is supposed to use the libyang library. `yanglint(1)` binary as
+well as its man page are installed together with the library itself.
+
+There is also [README](./tools/lint/examples/README.md) describing some examples of
+using `yanglint`.
+
+## Tests
+
+libyang includes several tests built with [cmocka](https://cmocka.org/). The tests
+can be found in `tests` subdirectory and they are designed for checking library
+functionality after code changes. Additional regression tests done with
+a corpus of fuzzing inputs that previously caused crashes are done.
+Those are available in `tests/fuzz` and are built automatically with the
+cmocka unit tests.
+
+
+The tests are by default built in the `Debug` build mode by running
+```
+$ make
+```
+
+In case of the `Release` mode, the tests are not built by default (it requires
+additional dependency), but they can be enabled via cmake option:
+```
+$ cmake -DENABLE_TESTS=ON ..
+```
+
+Note that if the necessary [cmocka](https://cmocka.org/) headers are not present
+in the system include paths, tests are not available despite the build mode or
+cmake's options.
+
+Tests can be run by the make's `test` target:
+```
+$ make test
+```
+
+### Perf
+
+There is a performance measurement tool included that prints information about
+the time required to execute common use-cases of working with YANG instance data.
+
+To enable this test, use an option and to get representative results, enable Release build type:
+```
+$ cmake -DCMAKE_BUILD_TYPE=Release -DENABLE_PERF_TESTS=ON ..
+```
+and to run the test with seeing its output run:
+```
+$ make
+$ ctest -V -R ly_perf
+```
+
+### Code Coverage
+
+Based on the tests run, it is possible to generate code coverage report. But
+it must be enabled and these commands are needed to generate the report:
+```
+$ cmake -DENABLE_COVERAGE=ON ..
+$ make
+$ make coverage
+```
+
+## Fuzzing
+
+Multiple YANG fuzzing targets and fuzzing instructions are available in the
+`tests/fuzz` directory.
+
+All of the targets can be fuzzed with LLVM's LibFuzzer and AFL, and new targets
+can easily be added.
+Asciinema examples which describe the fuzzing setup for both AFL (https://asciinema.org/a/311060)
+and LibFuzzer (https://asciinema.org/a/311035) are available.