raw_logging: Rename SafeWriteToStderr to indicate what about it is safe (answer: it's async-signal-safe).

Also, preserve errno across calls to make it actually signal-safe.

PiperOrigin-RevId: 446620926
Change-Id: I875fbec02b909e8424ddf763303b0d6007f8548f

absl/base/BUILD.bazel

@@ -75,6 +75,7 @@ cc_library(
         ":atomic_hook",
         ":config",
         ":core_headers",
+        ":errno_saver",
         ":log_severity",
     ],
 )

absl/base/CMakeLists.txt

@@ -66,6 +66,7 @@ absl_cc_library(
     absl::atomic_hook
     absl::config
     absl::core_headers
+    absl::errno_saver
     absl::log_severity
   COPTS
     ${ABSL_DEFAULT_COPTS}

absl/base/internal/raw_logging.cc

@@ -24,6 +24,7 @@
 #include "absl/base/attributes.h"
 #include "absl/base/config.h"
 #include "absl/base/internal/atomic_hook.h"
+#include "absl/base/internal/errno_saver.h"
 #include "absl/base/log_severity.h"
 
 // We know how to perform low-level writes to stderr in POSIX and Windows.  For
@@ -169,7 +170,7 @@ void RawLogVA(absl::LogSeverity severity, const char* file, int line,
     } else {
       DoRawLog(&buf, &size, "%s", kTruncated);
     }
-    SafeWriteToStderr(buffer, strlen(buffer));
+    AsyncSignalSafeWriteToStderr(buffer, strlen(buffer));
   }
 #else
   static_cast<void>(format);
@@ -196,8 +197,11 @@ void DefaultInternalLog(absl::LogSeverity severity, const char* file, int line,
 
 }  // namespace
 
-void SafeWriteToStderr(const char *s, size_t len) {
+void AsyncSignalSafeWriteToStderr(const char* s, size_t len) {
+  absl::base_internal::ErrnoSaver errno_saver;
 #if defined(ABSL_HAVE_SYSCALL_WRITE)
+  // We prefer calling write via `syscall` to minimize the risk of libc doing
+  // something "helpful".
   syscall(SYS_write, STDERR_FILENO, s, len);
 #elif defined(ABSL_HAVE_POSIX_WRITE)
   write(STDERR_FILENO, s, len);

absl/base/internal/raw_logging.h

@@ -109,12 +109,9 @@ namespace raw_logging_internal {
 void RawLog(absl::LogSeverity severity, const char* file, int line,
             const char* format, ...) ABSL_PRINTF_ATTRIBUTE(4, 5);
 
-// Writes the provided buffer directly to stderr, in a safe, low-level manner.
-//
-// In POSIX this means calling write(), which is async-signal safe and does
-// not malloc.  If the platform supports the SYS_write syscall, we invoke that
-// directly to side-step any libc interception.
-void SafeWriteToStderr(const char *s, size_t len);
+// Writes the provided buffer directly to stderr, in a signal-safe, low-level
+// manner.
+void AsyncSignalSafeWriteToStderr(const char* s, size_t len);
 
 // compile-time function to get the "base" filename, that is, the part of
 // a filename after the last "/" or "\" path separator.  The search starts at

absl/debugging/BUILD.bazel

@@ -143,7 +143,6 @@ cc_library(
         "//absl/base",
         "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/base:errno_saver",
         "//absl/base:raw_logging_internal",
     ],
 )

absl/debugging/CMakeLists.txt

@@ -126,7 +126,6 @@ absl_cc_library(
     absl::base
     absl::config
     absl::core_headers
-    absl::errno_saver
     absl::raw_logging_internal
   PUBLIC
 )

absl/debugging/failure_signal_handler.cc

@@ -42,7 +42,6 @@
 #include <ctime>
 
 #include "absl/base/attributes.h"
-#include "absl/base/internal/errno_saver.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/sysinfo.h"
 #include "absl/debugging/internal/examine_stack.h"
@@ -217,8 +216,7 @@ static void InstallOneFailureHandler(FailureSignalData* data,
 #endif
 
 static void WriteToStderr(const char* data) {
-  absl::base_internal::ErrnoSaver errno_saver;
-  absl::raw_logging_internal::SafeWriteToStderr(data, strlen(data));
+  absl::raw_logging_internal::AsyncSignalSafeWriteToStderr(data, strlen(data));
 }
 
 static void WriteSignalMessage(int signo, int cpu,