[PATCH] Flush output streams upon process termination

[Reid Kleckner]

That's interesting.  I think we avoid this problem for most of our binaries
because they use llvm::raw_ostream.  It looks like this is only a problem
for C code that has to use C stdio.  I think we never write to C stdio from
other atexit handlers, so I think this should work fine.

I also wonder if this was the true cause of http://llvm.org/PR19744.


On Wed, Jun 4, 2014 at 1:04 PM, Zachary Turner <zturner at google.com> wrote:

> Hi rnk,
>
> Due to what can only be described as a CRT bug, stdout and amazingly even
> stderr are not always flushed upon process termination, especially when the
> system is under high threading pressure.  I have found two repros for this:
>
> 1) In lib\Support\Threading.cpp, change sys::Mutex to an
> std::recursive_mutex and run check-clang.  Usually between 30 and 40 tests
> will fail.
> 2) Add OutputDebugStrings in code that runs during static initialization
> and static shutdown.  This will sometimes generate similar failures.
>
> After a substantial amount of troubleshooting and debugging, I found that
> I could reproduce this from the command line without running check-clang.
>  Simply make the mutex change described in #1, then manually run the
> following command many times by running it once, then pressing Up -> Enter
> very quickly:
>
> D:\src\llvm\build\vs2013\Debug\bin\c-index-test.EXE
> -cursor-at=D:\src\llvm\tools\clang\test\Index\targeted-preamble.h:2:15
> D:\src\llvm\tools\clang\test\Index\targeted-cursor.c -include
> D:\src\llvm\build\vs2013\tools\clang\test\Index\Output\targeted-cursor.c.tmp.h
> -Xclang
> -error-on-deserialized-decl=NestedVar1      -Xclang
> -error-on-deserialized-decl=TopVar    |
> D:\src\llvm\build\vs2013\Debug\bin\FileCheck.EXE
> D:\src\llvm\tools\clang\test\Index\targeted-cursor.c
> -check-prefix=PREAMBLE-CURSOR1
>
> Sporadically they will fail, and attaching a debugger to a failed instance
> indicates that stdin of FileCheck.exe is empty.
>
> Note that due to the repro in #2, we can rule out a bug in the STL's mutex
> implementation, and instead conclude that this is a real flake in the
> windows test harness.
>
> http://reviews.llvm.org/D4021
>
> Files:
>   tools/c-arcmt-test/c-arcmt-test.c
>   tools/c-index-test/c-index-test.c
>
> Index: tools/c-arcmt-test/c-arcmt-test.c
> ===================================================================
> --- tools/c-arcmt-test/c-arcmt-test.c
> +++ tools/c-arcmt-test/c-arcmt-test.c
> @@ -97,14 +97,20 @@
>  void thread_runner(void *client_data_v) {
>    thread_info *client_data = client_data_v;
>    client_data->result = carcmttest_main(client_data->argc,
> client_data->argv);
> -#ifdef __CYGWIN__
> -  fflush(stdout);  /* stdout is not flushed on Cygwin. */
> -#endif
> +}
> +
> +static void flush_atexit(void) {
> +  // stdout, and surprisingly even stderr, are not always flushed on
> process
> +  // and thread exit, particularly when the system is under heavy load.
> +  fflush(stdout);
> +  fflush(stderr);
>  }
>
>  int main(int argc, const char **argv) {
>    thread_info client_data;
>
> +  atexit(flush_atexit);
> +
>  #if defined(_WIN32)
>    if (getenv("LIBCLANG_LOGGING") == NULL)
>      putenv("LIBCLANG_LOGGING=1");
> Index: tools/c-index-test/c-index-test.c
> ===================================================================
> --- tools/c-index-test/c-index-test.c
> +++ tools/c-index-test/c-index-test.c
> @@ -4116,14 +4116,20 @@
>  void thread_runner(void *client_data_v) {
>    thread_info *client_data = client_data_v;
>    client_data->result = cindextest_main(client_data->argc,
> client_data->argv);
> -#ifdef __CYGWIN__
> -  fflush(stdout);  /* stdout is not flushed on Cygwin. */
> -#endif
> +}
> +
> +static void flush_atexit(void) {
> +  // stdout, and surprisingly even stderr, are not always flushed on
> process
> +  // and thread exit, particularly when the system is under heavy load.
> +  fflush(stdout);
> +  fflush(stderr);
>  }
>
>  int main(int argc, const char **argv) {
>    thread_info client_data;
>
> +  atexit(flush_atexit);
> +
>  #ifdef CLANG_HAVE_LIBXML
>    LIBXML_TEST_VERSION
>  #endif
>
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