Export of internal Abseil changes

--
cbd86b318ce206d1fe9eb001979b8d2efe57e5fa by Derek Mauro <dmauro@google.com>:

Stop inlining `CurrentThreadIdentityIfPresent()` on Apple platforms

In some build configurations the Apple linker rejects thread_local
variables exposed in headers

Fixes #954
Fixes #965

PiperOrigin-RevId: 375930997
GitOrigin-RevId: e34600e09f748b686d20a4f729ae914ebfd9e7bf
Change-Id: I9fcd2f02bd6abdea3ed412217e1be18f2c7bf762

absl/base/internal/thread_identity.cc

@@ -120,10 +120,10 @@ void SetCurrentThreadIdentity(
     ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_CPP11
 
 // Please see the comment on `CurrentThreadIdentityIfPresent` in
-// thread_identity.h. Because DLLs cannot expose thread_local variables in
-// headers, we opt for the correct-but-slower option of placing the definition
-// of this function only in a translation unit inside DLL.
-#if defined(ABSL_BUILD_DLL) || defined(ABSL_CONSUME_DLL)
+// thread_identity.h. When we cannot expose thread_local variables in
+// headers, we opt for the correct-but-slower option of not inlining this
+// function.
+#ifndef ABSL_INTERNAL_INLINE_CURRENT_THREAD_IDENTITY_IF_PRESENT
 ThreadIdentity* CurrentThreadIdentityIfPresent() { return thread_identity_ptr; }
 #endif
 #endif

absl/base/internal/thread_identity.h

@@ -236,13 +236,18 @@ ABSL_CONST_INIT extern thread_local ThreadIdentity* thread_identity_ptr;
 #error Thread-local storage not detected on this platform
 #endif
 
-// thread_local variables cannot be in headers exposed by DLLs. However, it is
-// important for performance reasons in general that
-// `CurrentThreadIdentityIfPresent` be inlined. This is not possible across a
-// DLL boundary so, with DLLs, we opt to have the function not be inlined. Note
+// thread_local variables cannot be in headers exposed by DLLs or in certain
+// build configurations on Apple platforms. However, it is important for
+// performance reasons in general that `CurrentThreadIdentityIfPresent` be
+// inlined. In the other cases we opt to have the function not be inlined. Note
 // that `CurrentThreadIdentityIfPresent` is declared above so we can exclude
-// this entire inline definition when compiling as a DLL.
-#if !defined(ABSL_BUILD_DLL) && !defined(ABSL_CONSUME_DLL)
+// this entire inline definition.
+#if !defined(__APPLE__) && !defined(ABSL_BUILD_DLL) && \
+    !defined(ABSL_CONSUME_DLL)
+#define ABSL_INTERNAL_INLINE_CURRENT_THREAD_IDENTITY_IF_PRESENT 1
+#endif
+
+#ifdef ABSL_INTERNAL_INLINE_CURRENT_THREAD_IDENTITY_IF_PRESENT
 inline ThreadIdentity* CurrentThreadIdentityIfPresent() {
   return thread_identity_ptr;
 }