[libc][pthread] fix -Wmissing-field-initializers (#126314)

Fixes:


llvm-project/libc/test/integration/src/pthread/pthread_rwlock_test.cpp:59:29:
    warning: missing field '__preference' initializer
    [-Wmissing-field-initializers]
       59 |   pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER;
          |                             ^

Also, add a test that demonstrates the same issue for
PTHREAD_MUTEX_INITIALIZER, and fix that, too.

PTHREAD_ONCE_INIT does not have this issue and does have test coverage.

libc/include/llvm-libc-macros/CMakeLists.txt

@@ -328,6 +328,8 @@ add_macro_header(
   pthread_macros
   HDR
     pthread-macros.h
+  DEPENDS
+    .null_macro
 )
 
 add_macro_header(

libc/include/llvm-libc-macros/pthread-macros.h

@@ -9,6 +9,8 @@
 #ifndef LLVM_LIBC_MACROS_PTHREAD_MACRO_H
 #define LLVM_LIBC_MACROS_PTHREAD_MACRO_H
 
+#include "null-macro.h"
+
 #define PTHREAD_CREATE_JOINABLE 0
 #define PTHREAD_CREATE_DETACHED 1
 
@@ -25,8 +27,34 @@
 #define PTHREAD_PROCESS_PRIVATE 0
 #define PTHREAD_PROCESS_SHARED 1
 
-#define PTHREAD_MUTEX_INITIALIZER {0}
-#define PTHREAD_RWLOCK_INITIALIZER {0}
+#ifdef __linux__
+#define PTHREAD_MUTEX_INITIALIZER                                              \
+  {                                                                            \
+      /* .__timed = */ 0,      /* .__recursive = */ 0,                         \
+      /* .__robust = */ 0,     /* .__owner = */ NULL,                          \
+      /* .__lock_count = */ 0, /* .__futex_word = */ {0},                      \
+  }
+#else
+#define PTHREAD_MUTEX_INITIALIZER                                              \
+  {                                                                            \
+      /* .__timed = */ 0,      /* .__recursive = */ 0,                         \
+      /* .__robust = */ 0,     /* .__owner = */ NULL,                          \
+      /* .__lock_count = */ 0,                                                 \
+  }
+#endif
+
+#define PTHREAD_RWLOCK_INITIALIZER                                             \
+  {                                                                            \
+      /* .__is_pshared = */ 0,                                                 \
+      /* .__preference = */ 0,                                                 \
+      /* .__state = */ 0,                                                      \
+      /* .__write_tid = */ 0,                                                  \
+      /* .__wait_queue_mutex = */ {0},                                         \
+      /* .__pending_readers = */ {0},                                          \
+      /* .__pending_writers = */ {0},                                          \
+      /* .__reader_serialization = */ {0},                                     \
+      /* .__writer_serialization = */ {0},                                     \
+  }
 
 // glibc extensions
 #define PTHREAD_STACK_MIN (1 << 14) // 16KB

libc/test/integration/src/pthread/pthread_mutex_test.cpp

@@ -186,6 +186,10 @@ void multiple_waiters() {
   LIBC_NAMESPACE::pthread_mutex_destroy(&counter_lock);
 }
 
+// Test the initializer
+[[maybe_unused]]
+static pthread_mutex_t test_initializer = PTHREAD_MUTEX_INITIALIZER;
+
 TEST_MAIN() {
   relay_counter();
   wait_and_step();