[sanitizer] Allow log_path to distinguish default from explicit stderr

[tejohnson]

Summary

Split out of D89086 as suggested.

Change the default of the log_path flag to nullptr, and the code
consuming that flag (ReportFile::SetReportPath), to treat nullptr as
stderr (so no change to the behavior of existing users). This allows
code to distinguish between the log_path being specified explicitly as
stderr vs the default.

This is so the flag can be used to override the new report path variable
that will be encoded in the binary for memprof for runtime testing.

compiler-rt/lib/sanitizer_common/sanitizer_file.cpp

if (fd == kInvalidFd) {
     WriteToFile(kStderrFd, ErrorMsgPrefix, internal_strlen(ErrorMsgPrefix));
     WriteToFile(kStderrFd, full_path, internal_strlen(full_path));
     Die();
   }
   fd_pid = pid;
 }
 
 void ReportFile::SetReportPath(const char *path) {
-  if (!path)
-    return;
+  if (path) {
     uptr len = internal_strlen(path);
     if (len > sizeof(path_prefix) - 100) {
-    Report("ERROR: Path is too long: %c%c%c%c%c%c%c%c...\n",
-           path[0], path[1], path[2], path[3],
-           path[4], path[5], path[6], path[7]);
+      Report("ERROR: Path is too long: %c%c%c%c%c%c%c%c...\n", path[0], path[1],
+             path[2], path[3], path[4], path[5], path[6], path[7]);
       Die();
     }
+  }
 
   SpinMutexLock l(mu);
   if (fd != kStdoutFd && fd != kStderrFd && fd != kInvalidFd)
     CloseFile(fd);
   fd = kInvalidFd;
-  if (internal_strcmp(path, "stdout") == 0) {
-    fd = kStdoutFd;
-  } else if (internal_strcmp(path, "stderr") == 0) {
+  if (!path || internal_strcmp(path, "stderr") == 0) {
     fd = kStderrFd;
+  } else if (internal_strcmp(path, "stdout") == 0) {
+    fd = kStdoutFd;
   } else {
     internal_snprintf(path_prefix, kMaxPathLength, "%s", path);
   }
 }
 
 bool ReadFileToBuffer(const char *file_name, char **buff, uptr *buff_size,
                       uptr *read_len, uptr max_len, error_t *errno_p) {
   *buff = nullptr;

compiler-rt/lib/sanitizer_common/sanitizer_flags.inc

 //===-- sanitizer_flags.h ---------------------------------------*- 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
 //
 //===----------------------------------------------------------------------===//
 //
 // This file describes common flags available in all sanitizers.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef COMMON_FLAG
 #error "Define COMMON_FLAG prior to including this file!"
 #endif
 
 // COMMON_FLAG(Type, Name, DefaultValue, Description)
 // Supported types: bool, const char *, int, uptr.
 // Default value must be a compile-time constant.
 // Description must be a string literal.
 
 COMMON_FLAG(
     bool, symbolize, true,
     "If set, use the online symbolizer from common sanitizer runtime to turn "
     "virtual addresses to file/line locations.")
 COMMON_FLAG(
     const char *, external_symbolizer_path, nullptr,
     "Path to external symbolizer. If empty, the tool will search $PATH for "
     "the symbolizer.")
 COMMON_FLAG(
     bool, allow_addr2line, false,
     "If set, allows online symbolizer to run addr2line binary to symbolize "
     "stack traces (addr2line will only be used if llvm-symbolizer binary is "
     "unavailable.")

 COMMON_FLAG(bool, fast_unwind_on_malloc,
             !(SANITIZER_LINUX && !SANITIZER_ANDROID && SANITIZER_ARM),
             "If available, use the fast frame-pointer-based unwinder on "
             "malloc/free.")
 COMMON_FLAG(bool, handle_ioctl, false, "Intercept and handle ioctl requests.")
 COMMON_FLAG(int, malloc_context_size, 1,
             "Max number of stack frames kept for each allocation/deallocation.")
 COMMON_FLAG(
-    const char *, log_path, "stderr",
+    const char *, log_path, nullptr,
     "Write logs to \"log_path.pid\". The special values are \"stdout\" and "
-    "\"stderr\". The default is \"stderr\".")
+    "\"stderr\". If unspecified, defaults to \"stderr\".")
 COMMON_FLAG(
     bool, log_exe_name, false,
     "Mention name of executable when reporting error and "
     "append executable name to logs (as in \"log_path.exe_name.pid\").")
 COMMON_FLAG(
     bool, log_to_syslog, (bool)SANITIZER_ANDROID || (bool)SANITIZER_MAC,
     "Write all sanitizer output to syslog in addition to other means of "
     "logging.")