dependencies: add custom atomic dependency

Almost exactly the same as how the dl dependency works. On certain
systems (like BSDs that use clang), stdatomic is provided by compiler-rt
and doesn't need a separate library explictly linked. On a typical
GNU/LINUX system, atomic is a separate library that must be explictly
found and linked against. So just add a builtin and system method for
these two use cases.

docs/markdown/Dependencies.md

@@ -315,6 +315,16 @@ dep = dependency('appleframeworks', modules : 'foundation')
 
 These dependencies can never be found for non-OSX hosts.
 
+## atomic (stdatomic)
+
+*(added 1.2.0)*
+
+Provides access to the atomic operations library. On systems where
+this is not provided by compiler-rt (i.e. gcc), this tries to find
+an external library instead.
+
+`method` may be `auto`, `builtin` or `system`.
+
 ## Blocks
 
 Enable support for Clang's blocks extension.

docs/markdown/snippets/atomic-dependency.md

@@ -0,0 +1,9 @@
+## New custom dependency for atomic
+
+```
+dependency('atomic')
+```
+
+checks for the availability of the atomic operation library. First, it checks
+if stdatomic is provided by compiler-rt (e.g. when using clang). If not, it
+looks for the atomic library specifically.

mesonbuild/dependencies/__init__.py

@@ -36,7 +36,7 @@
     BlocksDependency, OpenMPDependency, cups_factory, curses_factory, gpgme_factory,
     libgcrypt_factory, libwmf_factory, netcdf_factory, pcap_factory,
     shaderc_factory, threads_factory, ThreadDependency, iconv_factory, intl_factory,
-    dl_factory, openssl_factory, libcrypto_factory, libssl_factory,
+    atomic_factory, dl_factory, openssl_factory, libcrypto_factory, libssl_factory,
 )
 from .platform import AppleFrameworks
 from .python import python_factory as python3_factory, pybind11_factory
@@ -260,6 +260,7 @@ def __init__(self, name: str, environment: 'Environment', kwargs: T.Dict[str, T.
     'shaderc': shaderc_factory,
     'iconv': iconv_factory,
     'intl': intl_factory,
+    'atomic': atomic_factory,
     'dl': dl_factory,
     'openssl': openssl_factory,
     'libcrypto': libcrypto_factory,

mesonbuild/dependencies/misc.py

@@ -58,6 +58,27 @@ def netcdf_factory(env: 'Environment',
     return candidates
 
 
+class AtomicBuiltinDependency(BuiltinDependency):
+    def __init__(self, name: str, env: 'Environment', kwargs: T.Dict[str, T.Any]):
+        super().__init__(name, env, kwargs)
+        self.feature_since = ('1.2.0', "consider checking for `atomic_flag_clear` with and without `find_library('atomic')`")
+
+        if self.clib_compiler.has_function('atomic_flag_clear', '#include <stdatomic.h>', env)[0]:
+            self.is_found = True
+
+
+class AtomicSystemDependency(SystemDependency):
+    def __init__(self, name: str, env: 'Environment', kwargs: T.Dict[str, T.Any]):
+        super().__init__(name, env, kwargs)
+        self.feature_since = ('1.2.0', "consider checking for `atomic_flag_clear` with and without `find_library('atomic')`")
+
+        h = self.clib_compiler.has_header('stdatomic.h', '', env)
+        self.link_args = self.clib_compiler.find_library('atomic', env, [], self.libtype)
+
+        if h[0] and self.link_args:
+            self.is_found = True
+
+
 class DlBuiltinDependency(BuiltinDependency):
     def __init__(self, name: str, env: 'Environment', kwargs: T.Dict[str, T.Any]):
         super().__init__(name, env, kwargs)
@@ -526,6 +547,13 @@ def shaderc_factory(env: 'Environment',
     return candidates
 
 
+atomic_factory = DependencyFactory(
+    'atomic',
+    [DependencyMethods.BUILTIN, DependencyMethods.SYSTEM],
+    builtin_class=AtomicBuiltinDependency,
+    system_class=AtomicSystemDependency,
+)
+
 cups_factory = DependencyFactory(
     'cups',
     [DependencyMethods.PKGCONFIG, DependencyMethods.CONFIG_TOOL, DependencyMethods.EXTRAFRAMEWORK, DependencyMethods.CMAKE],