[libc][NFC] Refactor internal errno.

[sivachandra]

Summary

This is in preparation for the transition to a solution to make libc tests
hermetic with respect to their use of errno. The implementation of strdup
has been switched over to libc_errno as an example of what the code looks
like in the new way.

See #61037 for more information.

libc/src/errno/CMakeLists.txt

-if (LLVM_LIBC_FULL_BUILD)
 add_object_library(
   errno
   SRCS
-    errno.cpp
+    libc_errno.cpp
   HDRS
-    llvmlibc_errno.h
+    libc_errno.h     # Include this
+    llvmlibc_errno.h # DEPRECATED: Will be removed soon
+  DEPENDS
+    libc.include.errno
 )
-else()
-add_object_library(
-  errno
-  SRCS
-    dummy_errno.cpp
-  HDRS
-    dummy_errno.h
-)
-endif()

libc/src/errno/dummy_errno.h

-//===-- An empty file to be used for mixed mode builds ----------*- C++ -*-===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//

libc/src/errno/dummy_errno.cpp

-//===-- An empty file to be used for mixed mode builds --------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//

libc/src/errno/errno.cpp

-//===-- Implementation of __errno_location --------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-thread_local int __llvmlibc_errno = 0;

libc/src/errno/libc_errno.h

+//===-- Implementation header for errno -------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_ERRNO_LLVMLIBC_ERRNO_H
+#define LLVM_LIBC_SRC_ERRNO_LLVMLIBC_ERRNO_H
+
+#include <errno.h>
+
+// All of the libc runtime and test code should use the "libc_errno" macro. They
+// should not refer to the "errno" macro directly.
+#ifdef LIBC_COPT_PUBLIC_PACKAGING
+// This macro will resolve to errno from the errno.h file included above. Under
+// full build, this will be LLVM libc's errno. In overlay build, it will be
+// system libc's errno.
+#define libc_errno errno
+#else
+namespace __llvm_libc {
+
+extern "C" {
+extern thread_local int __llvmlibc_internal_errno;
+} // extern "C"
+
+// TODO: After all of libc/src and libc/test are switched over to use
+// libc_errno, this header file will be "shipped" via an add_entrypoint_object
+// target. At which point libc_errno, should point to __llvmlibc_internal_errno
+// if LIBC_COPT_PUBLIC_PACKAGING is not defined.
+#define libc_errno errno
+
+} // namespace __llvm_libc
+#endif
+
+#endif // LLVM_LIBC_SRC_ERRNO_LLVMLIBC_ERRNO_H

libc/src/errno/libc_errno.cpp

+//===-- Implementation of errno -------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+namespace __llvm_libc {
+
+extern "C" {
+// TODO: Declare __llvmlibc_errno only under LIBC_COPT_PUBLIC_PACKAGING and
+// __llvmlibc_internal_errno otherwise.
+//
+// In overlay mode, this will be an unused thread local variable as libc_errno
+// will resolve to errno from the system libc's errno.h. In full build mode
+// however, libc_errno will resolve to this thread local variable via the errno
+// macro defined in LLVM libc's public errno.h header file.
+// TODO: Use a macro to distinguish full build and overlay build which can be
+//       used to exclude __llvmlibc_errno under overlay build.
+thread_local int __llvmlibc_errno;
+thread_local int __llvmlibc_internal_errno;
+} // extern "C"
+
+} // namespace __llvm_libc

libc/src/errno/llvmlibc_errno.h

 //===-- Implementation header for errno -------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIBC_SRC_ERRNO_LLVMLIBC_ERRNO_H
 #define LLVM_LIBC_SRC_ERRNO_LLVMLIBC_ERRNO_H
 
-// Internal code should use this and not use the errno macro from the
-// public header.
-extern thread_local int __llvmlibc_errno;
-#define llvmlibc_errno __llvmlibc_errno
+#include <errno.h>
+
+// DEPRECATED: Use libc_errno from libc_errno.h instead. This macro is only
+// present to facilitate gradual transition (as in, in multiple simple patches)
+// to libc_errno.
+// TODO: After all of libc/src and libc/test is switched over to use libc_errno,
+// remove this macro and header file.
+#define llvmlibc_errno errno
 
 #endif // LLVM_LIBC_SRC_ERRNO_LLVMLIBC_ERRNO_H

libc/src/string/CMakeLists.txt

add_entrypoint_object(
   strdup
   SRCS
     strdup.cpp
   HDRS
     strdup.h
   DEPENDS
     .memory_utils.memcpy_implementation
     .string_utils
-    libc.include.errno
     libc.include.stdlib
     libc.src.errno.errno
 )
 
 add_entrypoint_object(
   strerror
   SRCS
     strerror.cpp

libc/src/string/strdup.cpp

 //===-- Implementation of strdup ------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "src/string/strdup.h"
+#include "src/errno/libc_errno.h"
 #include "src/string/allocating_string_utils.h"
 #include "src/string/memory_utils/memcpy_implementations.h"
 
 #include "src/__support/common.h"
 
-#include <errno.h>
 #include <stdlib.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(char *, strdup, (const char *src)) {
   auto dup = internal::strdup(src);
   if (dup)
     return *dup;
   if (src != nullptr)
-    errno = ENOMEM;
+    libc_errno = ENOMEM;
   return nullptr;
 }
 
 } // namespace __llvm_libc