[sanitizer] Allow log_path to distinguish default from explicit stderr

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.

Differential Revision: https://reviews.llvm.org/D89629

compiler-rt/lib/sanitizer_common/sanitizer_file.cpp

@@ -69,24 +69,23 @@ void ReportFile::ReopenIfNecessary() {
 }
 
 void ReportFile::SetReportPath(const char *path) {
-  if (!path)
-    return;
-  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]);
-    Die();
+  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]);
+      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);
   }

compiler-rt/lib/sanitizer_common/sanitizer_flags.inc

@@ -52,9 +52,9 @@ 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 "