gvfs-helper-client: clean up server process (#756)

On Linux, the following command would cause the terminal to be stuck
waiting:

```
  git fetch origin foobar
```

The issue would be that the fetch would fail with the error

```
  fatal: couldn't find remote ref foobar
```

but the underlying `git-gvfs-helper` process wouldn't die. The
`subprocess_exit_handler()` method would close its stdin and stdout, but
that wouldn't be enough to cause the process to end.

This PR addresses that by skipping the `finish_command()` call of the
`clean_on_exit_handler` and instead lets `cleanup_children()` send a
SIGTERM to terminate those spawned child processes.

Author: dscho

gvfs-helper-client.c

@@ -335,6 +335,36 @@ static void gh_client__choose_odb(void)
 	}
 }
 
+/*
+ * Custom exit handler for the `gvfs-helper` subprocesses.
+ *
+ * These helper subprocesses will keep waiting for input until they are
+ * stopped. The default `subprocess_exit_handler()` will instead wait for
+ * the subprocess to exit, which is not what we want: In case of a fatal
+ * error, the Git process will exit and the `gvfs-helper` subprocesses will
+ * need to be stopped explicitly.
+ *
+ * The default behavior of `cleanup_children()` does, however, terminate
+ * the process after calling the `clean_on_exit_handler`. So that's exactly
+ * what we do here: reproduce the exact same code as
+ * `subprocess_exit_handler()` modulo waiting for the process that won't
+ * ever terminate on its own.
+ */
+static void gh_client__subprocess_exit_handler(struct child_process *process)
+{
+	sigchain_push(SIGPIPE, SIG_IGN);
+	/* Closing the pipe signals the subprocess to initiate a shutdown. */
+	close(process->in);
+	close(process->out);
+	sigchain_pop(SIGPIPE);
+	/*
+	 * In contrast to subprocess_exit_handler(), do _not_ wait for the
+	 * process to finish on its own accord: It needs to be terminated via
+	 * a signal, which is what `cleanup_children()` will do after this
+	 * function returns.
+	 */
+}
+
 static struct gh_server__process *gh_client__find_long_running_process(
 	unsigned int cap_needed)
 {
@@ -386,6 +416,9 @@ static struct gh_server__process *gh_client__find_long_running_process(
 					  &entry->subprocess, 1,
 					  &argv, gh_client__start_fn))
 			FREE_AND_NULL(entry);
+		else
+			entry->subprocess.process.clean_on_exit_handler =
+				gh_client__subprocess_exit_handler;
 	}
 
 	if (entry &&

t/t9210-scalar.sh

@@ -426,6 +426,10 @@ test_expect_success '`scalar clone` with GVFS-enabled server' '
 	)
 '
 
+test_expect_success 'fetch <non-existent> does not hang in gvfs-helper' '
+	test_must_fail git -C using-gvfs/src fetch origin does-not-exist
+'
+
 test_expect_success '`scalar clone --no-gvfs-protocol` skips gvfs/config' '
 	# the fake cache server requires fake authentication &&
 	git config --global core.askPass true &&