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

@@ -317,6 +317,16 @@ dep = dependency('appleframeworks', modules : 'foundation')
 
 These dependencies can never be found for non-OSX hosts.
 
+## atomic (stdatomic)
+
+*(added 1.7.0)*
+
+Provides access to the atomic operations library. This first attempts
+to look for a valid atomic external library before trying to fallback
+to what is provided by the C runtime libraries.
+
+`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 looks
+for the atomic library. If that is not found, then it will try to use what is
+provided by the libc.

mesonbuild/dependencies/__init__.py

@@ -220,6 +220,7 @@ packages.defaults.update({
     'shaderc': 'misc',
     'iconv': 'misc',
     'intl': 'misc',
+    'atomic': 'misc',
     'dl': 'misc',
     'openssl': 'misc',
     'libcrypto': 'misc',

mesonbuild/dependencies/misc.py

@@ -51,6 +51,27 @@ def netcdf_factory(env: 'Environment',
 packages['netcdf'] = netcdf_factory
 
 
+class AtomicBuiltinDependency(BuiltinDependency):
+    def __init__(self, name: str, env: Environment, kwargs: T.Dict[str, T.Any]):
+        super().__init__(name, env, kwargs)
+        self.feature_since = ('1.7.0', "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 = ('1.7.0', "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)
@@ -564,6 +585,13 @@ def shaderc_factory(env: 'Environment',
 packages['shaderc'] = shaderc_factory
 
 
+packages['atomic'] = atomic_factory = DependencyFactory(
+    'atomic',
+    [DependencyMethods.SYSTEM, DependencyMethods.BUILTIN],
+    system_class=AtomicSystemDependency,
+    builtin_class=AtomicBuiltinDependency,
+)
+
 packages['cups'] = cups_factory = DependencyFactory(
     'cups',
     [DependencyMethods.PKGCONFIG, DependencyMethods.CONFIG_TOOL, DependencyMethods.EXTRAFRAMEWORK, DependencyMethods.CMAKE],