Make FML_LOG safe from static initialization (#42219)
I ran into this while trying to get some printing going for places where we're creating thread local keys.
Supposedly, just including `<iostream>` should statically initialize `std::cout/cerr`, but it gets really hard to reason about whether your statically initialized code is going to be initialized before or after that happens. I tried making sure that the TU for `fml/logging.cc` did that initialization statically, but that also failed in the verison of the test included here (it passed in some other iterations that modified run_all_unittests.cc). We _could_ make sure it happens each and every time we touch `std::cerr` but ... we could also just use `fprintf(stderr, ...)` and it works just fine.
/cc @flar who ran into problems around this a little while back and was asking about it.
diff --git a/fml/logging.cc b/fml/logging.cc
index 5a0280e..61b4b5c 100644
--- a/fml/logging.cc
+++ b/fml/logging.cc
@@ -123,8 +123,9 @@
fx_logger_log_with_source(fx_log_get_logger(), fx_severity, nullptr, file_,
line_, stream_.str().c_str());
#else
- std::cerr << stream_.str();
- std::cerr.flush();
+ // Don't use std::cerr here, because it may not be initialized properly yet.
+ fprintf(stderr, "%s", stream_.str().c_str());
+ fflush(stderr);
#endif
if (severity_ >= LOG_FATAL) {
diff --git a/fml/logging_unittests.cc b/fml/logging_unittests.cc
index a64d33e..b418397 100644
--- a/fml/logging_unittests.cc
+++ b/fml/logging_unittests.cc
@@ -2,6 +2,8 @@
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
+#include <signal.h>
+
#include "flutter/fml/build_config.h"
#include "flutter/fml/log_settings.h"
#include "flutter/fml/logging.h"
@@ -19,6 +21,41 @@
namespace fml {
namespace testing {
+#ifndef OS_FUCHSIA
+class MakeSureFmlLogDoesNotSegfaultWhenStaticallyCalled {
+ public:
+ MakeSureFmlLogDoesNotSegfaultWhenStaticallyCalled() {
+ SegfaultCatcher catcher;
+ // If this line causes a segfault, FML is using a method of logging that is
+ // not safe from static initialization on your platform.
+ FML_LOG(INFO)
+ << "This log exists to verify that static logging from FML works.";
+ }
+
+ private:
+ struct SegfaultCatcher {
+ typedef void (*sighandler_t)(int);
+
+ SegfaultCatcher() {
+ handler = ::signal(SIGSEGV, SegfaultHandler);
+ FML_CHECK(handler != SIG_ERR);
+ }
+
+ ~SegfaultCatcher() { FML_CHECK(::signal(SIGSEGV, handler) != SIG_ERR); }
+
+ static void SegfaultHandler(int signal) {
+ fprintf(stderr,
+ "FML failed to handle logging from static initialization.\n");
+ exit(signal);
+ }
+
+ sighandler_t handler;
+ };
+};
+
+static MakeSureFmlLogDoesNotSegfaultWhenStaticallyCalled fml_log_static_check_;
+#endif // !defined(OS_FUCHSIA)
+
int UnreachableScopeWithoutReturnDoesNotMakeCompilerMad() {
KillProcess();
// return 0; <--- Missing but compiler is fine.
