Use new emscripten_errn to avoid copying strings.

PiperOrigin-RevId: 544461113
Change-Id: Iafbd6daf2d03ae18a49ea449315ee7cd6a0e615e

absl/base/internal/raw_logging.cc

@@ -206,27 +206,31 @@ void DefaultInternalLog(absl::LogSeverity severity, const char* file, int line,
 }  // namespace
 
 void AsyncSignalSafeWriteError(const char* s, size_t len) {
+  if (!len) return;
   absl::base_internal::ErrnoSaver errno_saver;
 #if defined(__EMSCRIPTEN__)
   // In WebAssembly, bypass filesystem emulation via fwrite.
-  // TODO(b/282811932): Avoid this copy if these emscripten functions can
-  // be updated to accept size directly.
+  if (s[len - 1] == '\n') {
+    // Skip a trailing newline character as emscripten_errn adds one itself.
+    len--;
+  }
+  // emscripten_errn introduced in emscripten 3.1.41
+#if ABSL_INTERNAL_EMSCRIPTEN_VERSION >= 3001041
+  emscripten_errn(s, len);
+#else
   char buf[kLogBufSize];
   if (len >= kLogBufSize) {
     len = kLogBufSize - 1;
-    size_t trunc_len = sizeof(kTruncated) - 2;
-    strncpy(buf + len - trunc_len, kTruncated, trunc_len);
+    constexpr size_t trunc_len = sizeof(kTruncated) - 2;
+    memcpy(buf + len - trunc_len, kTruncated, trunc_len);
     buf[len] = '\0';
     len -= trunc_len;
-  } else if (len && s[len - 1] == '\n') {
-    len--;
-  }
-  strncpy(buf, s, len);
-  if (len) {
+  } else {
     buf[len] = '\0';
-    // Skip a trailing newline character as emscripten_err adds one itself.
-    _emscripten_err(buf);
   }
+  memcpy(buf, s, len);
+  _emscripten_err(buf);
+#endif
 #elif defined(ABSL_HAVE_SYSCALL_WRITE)
   // We prefer calling write via `syscall` to minimize the risk of libc doing
   // something "helpful".

absl/log/internal/globals.cc

@@ -16,6 +16,7 @@
 
 #include <atomic>
 #include <cstdio>
+
 #if defined(__EMSCRIPTEN__)
 #include <emscripten/console.h>
 #endif
@@ -25,6 +26,7 @@
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/log_severity.h"
 #include "absl/strings/string_view.h"
+#include "absl/strings/strip.h"
 #include "absl/time/time.h"
 
 namespace absl {
@@ -58,16 +60,18 @@ void SetInitialized() {
 }
 
 void WriteToStderr(absl::string_view message, absl::LogSeverity severity) {
+  if (message.empty()) return;
 #if defined(__EMSCRIPTEN__)
   // In WebAssembly, bypass filesystem emulation via fwrite.
-  // TODO(b/282811932): Avoid this copy if these emscripten functions can
-  // be updated to accept size directly.
-  std::string null_terminated_message(message);
-  if (!null_terminated_message.empty() &&
-      null_terminated_message.back() == '\n') {
-    null_terminated_message.pop_back();
-  }
+  // Skip a trailing newline character as emscripten_errn adds one itself.
+  const auto message_minus_newline = absl::StripSuffix(message, "\n");
+  // emscripten_errn introduced in emscripten 3.1.41
+#if ABSL_INTERNAL_EMSCRIPTEN_VERSION >= 3001041
+  emscripten_errn(message_minus_newline.data(), message_minus_newline.size());
+#else
+  std::string null_terminated_message(message_minus_newline);
   _emscripten_err(null_terminated_message.c_str());
+#endif
 #else
   // Avoid using std::cerr from this module since we may get called during
   // exit code, and cerr may be partially or fully destroyed by then.