This patch fixes a possible hang in programs that use the 'spawn-pipe' module
and
also spawn other child processes. It is also needed for fixing the hanging
pipe-filter-* tests on native Windows.
2022-09-09 Bruno Haible <br...@clisp.org>
spawn-pipe: Fix possible hangs in programs that spawn several children.
* lib/spawn-pipe.c (create_pipe) [Unix]: Create the ifd[] and ofd[] file
descriptors with the close-on-exec flag set.
diff --git a/lib/spawn-pipe.c b/lib/spawn-pipe.c
index f132624e98..ac803f48c4 100644
--- a/lib/spawn-pipe.c
+++ b/lib/spawn-pipe.c
@@ -193,25 +193,24 @@ create_pipe (const char *progname,
}
}
-#if ((defined _WIN32 && !defined __CYGWIN__) &&
SPAWN_PIPE_IMPL_AVOID_POSIX_SPAWN) || defined __KLIBC__
-
- /* Native Windows API.
- This uses _pipe(), dup2(), and _spawnv(). It could also be implemented
- using the low-level functions CreatePipe(), DuplicateHandle(),
- CreateProcess() and _open_osfhandle(); see the GNU make and GNU clisp
- and cvs source code. */
- char *argv_mem_to_free;
int ifd[2];
int ofd[2];
- int child;
- int nulloutfd;
- int stdinfd;
- int stdoutfd;
-
- const char **argv = prepare_spawn (prog_argv, &argv_mem_to_free);
- if (argv == NULL)
- xalloc_die ();
+ /* It is important to create the file descriptors with the close-on-exec bit
+ set.
+ * In the child process that we are about to create here, the file
+ descriptors ofd[0] -> STDIN_FILENO and ifd[1] -> STDOUT_FILENO will be
+ preserved across exec, because each dup2 call scheduled by
+ posix_spawn_file_actions_adddup2 creates a file descriptor with the
+ close-on-exec bit clear. Similarly on native Windows, where we use
+ explicit DuplicateHandle calls, and on kLIBC, where we use explicit dup2
+ calls.
+ * In the parent process, we close ofd[0] and ifd[1]; so, ofd[1] and ofd[0]
+ are still open. But if the parent process spawns another child process
+ later, if ofd[1] and ofd[0] were inherited by that child process, the
+ "end of input" / "end of output" detection would not work any more. The
+ parent or the child process would block, as long as that other child
+ process is running. */
if (pipe_stdout)
if (pipe2_safer (ifd, O_BINARY | O_CLOEXEC) < 0)
error (EXIT_FAILURE, errno, _("cannot create pipe"));
@@ -227,6 +226,23 @@ create_pipe (const char *progname,
*
*/
+#if ((defined _WIN32 && !defined __CYGWIN__) &&
SPAWN_PIPE_IMPL_AVOID_POSIX_SPAWN) || defined __KLIBC__
+
+ /* Native Windows API.
+ This uses _pipe(), dup2(), and _spawnv(). It could also be implemented
+ using the low-level functions CreatePipe(), DuplicateHandle(),
+ CreateProcess() and _open_osfhandle(); see the GNU make and GNU clisp
+ and cvs source code. */
+ char *argv_mem_to_free;
+ int child;
+ int nulloutfd;
+ int stdinfd;
+ int stdoutfd;
+
+ const char **argv = prepare_spawn (prog_argv, &argv_mem_to_free);
+ if (argv == NULL)
+ xalloc_die ();
+
child = -1;
# if (defined _WIN32 && !defined __CYGWIN__) &&
SPAWN_PIPE_IMPL_AVOID_POSIX_SPAWN
@@ -444,8 +460,6 @@ create_pipe (const char *progname,
#else
/* Unix API. */
- int ifd[2];
- int ofd[2];
sigset_t blocked_signals;
posix_spawn_file_actions_t actions;
bool actions_allocated;
@@ -454,21 +468,6 @@ create_pipe (const char *progname,
int err;
pid_t child;
- if (pipe_stdout)
- if (pipe_safer (ifd) < 0)
- error (EXIT_FAILURE, errno, _("cannot create pipe"));
- if (pipe_stdin)
- if (pipe_safer (ofd) < 0)
- error (EXIT_FAILURE, errno, _("cannot create pipe"));
-/* Data flow diagram:
- *
- * write system read
- * parent -> ofd[1] -> ofd[0] -> child if pipe_stdin
- * parent <- ifd[0] <- ifd[1] <- child if pipe_stdout
- * read system write
- *
- */
-
if (slave_process)
{
sigprocmask (SIG_SETMASK, NULL, &blocked_signals);
