Handle transitive links to 'threads' dependencies. (#3895)

Meson already had code to propagate link dependencies from static
libraries to programs that use those static libraries.

Unfortunately, it was not handling the special cases of 'threads' and
'openmp' dependencies.

mesonbuild/backend/ninjabackend.py

@@ -2623,25 +2623,32 @@ def generate_link(self, target, outfile, outname, obj_list, linker, extra_args=[
             dependencies = target.get_dependencies()
         internal = self.build_target_link_arguments(linker, dependencies)
         commands += internal
-        # For 'automagic' deps: Boost and GTest. Also dependency('threads').
-        # pkg-config puts the thread flags itself via `Cflags:`
-        for d in target.external_deps:
-            if d.need_threads():
-                commands += linker.thread_link_flags(self.environment)
-            elif d.need_openmp():
-                commands += linker.openmp_flags()
         # Only non-static built targets need link args and link dependencies
         if not isinstance(target, build.StaticLibrary):
+            # For 'automagic' deps: Boost and GTest. Also dependency('threads').
+            # pkg-config puts the thread flags itself via `Cflags:`
+            need_threads = False
+            need_openmp = False
+
             commands += target.link_args
             # External deps must be last because target link libraries may depend on them.
             for dep in target.get_external_deps():
                 # Extend without reordering or de-dup to preserve `-L -l` sets
                 # https://github.com/mesonbuild/meson/issues/1718
                 commands.extend_direct(dep.get_link_args())
+                need_threads |= dep.need_threads()
+                need_openmp |= dep.need_openmp()
             for d in target.get_dependencies():
                 if isinstance(d, build.StaticLibrary):
                     for dep in d.get_external_deps():
+                        need_threads |= dep.need_threads()
+                        need_openmp |= dep.need_openmp()
                         commands.extend_direct(dep.get_link_args())
+            if need_openmp:
+                commands += linker.openmp_flags()
+            if need_threads:
+                commands += linker.thread_link_flags(self.environment)
+
         # Add link args for c_* or cpp_* build options. Currently this only
         # adds c_winlibs and cpp_winlibs when building for Windows. This needs
         # to be after all internal and external libraries so that unresolved

test cases/common/205 static threads/lib1.c

@@ -0,0 +1,13 @@
+#if defined _WIN32
+#include<windows.h>
+#else
+#include<pthread.h>
+#endif
+
+void *f(void) {
+#if defined _WIN32
+  return CreateThread;
+#else
+  return pthread_create;
+#endif
+}

test cases/common/205 static threads/lib2.c

@@ -0,0 +1,5 @@
+extern void *f(void);
+
+void *g(void) {
+  return f();
+}

test cases/common/205 static threads/meson.build

@@ -0,0 +1,13 @@
+project('threads', 'c')
+
+thread_dep = dependency('threads')
+
+
+lib1 = static_library('lib1', 'lib1.c',
+                      dependencies : thread_dep)
+
+lib2 = static_library('lib2', 'lib2.c',
+                       link_with : lib1)
+
+executable('prog', 'prog.c',
+           link_with : lib2)

test cases/common/205 static threads/prog.c

@@ -0,0 +1,6 @@
+extern void *g(void);
+
+int main(void) {
+  g();
+  return 0;
+}