[Tarantool-patches] [PATCH] popen: fix 'may be clobbered' compiler warning

[Tarantool-patches] [PATCH] popen: fix 'may be clobbered' compiler warning

[Alexander Turenko]

I guess a compiler assumes that the code inside vfork may use the stack
slot that corresponds the variable and so clobber it. The recent commit
07a07b3cc7b85375d20b3fc6ca1e5060304f337b ('popen: decouple logger fd
from stderr') adds read from this variable after vfork() in the parent
process.

The warning is produced on RelWithDebInfo build with LTO enabled on GCC
9.2.0 (locally) and on GCC 8.3.0 (in CI).

Part of #4031
---

We have -Werror enabled on some jobs in CI, so the warnings fails the CI
status. See https://gitlab.com/tarantool/tarantool/-/jobs/506891936

I verified the build locally and in CI.

https://github.com/tarantool/tarantool/tree/Totktonada/gh-4031-popen-fix-lto-build-full-ci

 src/lib/core/popen.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/lib/core/popen.c b/src/lib/core/popen.c
index 5d0587e00..df7f797b9 100644
--- a/src/lib/core/popen.c
+++ b/src/lib/core/popen.c
@@ -784,10 +784,11 @@ struct popen_handle *
 popen_new(struct popen_opts *opts)
 {
 	/*
-	 * Without volatile compiler claims that
-	 * handle might be clobbered from vfork.
+	 * Without volatile compiler claims that those
+	 * variables might be clobbered from vfork.
 	 */
 	struct popen_handle * volatile handle;
+	int volatile log_fd = -1;
 
 	int pfd[POPEN_FLAG_FD_STDEND_BIT][2] = {
 		{-1, -1}, {-1, -1}, {-1, -1},
@@ -853,7 +854,6 @@ popen_new(struct popen_opts *opts)
 	 * reached a file descriptor limit.
 	 */
 	int old_log_fd = log_get_fd();
-	int log_fd = -1;
 	if (old_log_fd >= 0) {
 		log_fd = dup_not_std_streams(old_log_fd);
 		if (log_fd < 0)
-- 
2.25.0

Re: [Tarantool-patches] [PATCH] popen: fix 'may be clobbered' compiler warning

[Cyrill Gorcunov]

On Sat, Apr 11, 2020 at 01:28:07AM +0300, Alexander Turenko wrote:
> I guess a compiler assumes that the code inside vfork may use the stack
> slot that corresponds the variable and so clobber it. The recent commit
> 07a07b3cc7b85375d20b3fc6ca1e5060304f337b ('popen: decouple logger fd
> from stderr') adds read from this variable after vfork() in the parent
> process.
> 
> The warning is produced on RelWithDebInfo build with LTO enabled on GCC
> 9.2.0 (locally) and on GCC 8.3.0 (in CI).
> 
> Part of #4031
Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>

Re: [Tarantool-patches] [PATCH] popen: fix 'may be clobbered' compiler warning

[Alexander Turenko]

On Sat, Apr 11, 2020 at 10:30:26AM +0300, Cyrill Gorcunov wrote:
> On Sat, Apr 11, 2020 at 01:28:07AM +0300, Alexander Turenko wrote:
> > I guess a compiler assumes that the code inside vfork may use the stack
> > slot that corresponds the variable and so clobber it. The recent commit
> > 07a07b3cc7b85375d20b3fc6ca1e5060304f337b ('popen: decouple logger fd
> > from stderr') adds read from this variable after vfork() in the parent
> > process.
> > 
> > The warning is produced on RelWithDebInfo build with LTO enabled on GCC
> > 9.2.0 (locally) and on GCC 8.3.0 (in CI).
> > 
> > Part of #4031
> Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>

Pushed to master.

WBR, Alexander Turenko.