run-command: optionally kill children on exit

When we spawn a helper process, it should generally be done
and finish_command called before we exit. However, if we
exit abnormally due to an early return or a signal, the
helper may continue to run in our absence.

In the best case, this may simply be wasted CPU cycles or a
few stray messages on a terminal. But it could also mean a
process that the user thought was aborted continues to run
to completion (e.g., a push's pack-objects helper will
complete the push, even though you killed the push process).

This patch provides infrastructure for run-command to keep
track of PIDs to be killed, and clean them on signal
reception or input, just as we do with tempfiles. PIDs can
be added in two ways:

  1. If NO_PTHREADS is defined, async helper processes are
     automatically marked. By definition this code must be
     ready to die when the parent dies, since it may be
     implemented as a thread of the parent process.

  2. If the run-command caller specifies the "clean_on_exit"
     option. This is not the default, as there are cases
     where it is OK for the child to outlive us (e.g., when
     spawning a pager).

PIDs are cleared from the kill-list automatically during
wait_or_whine, which is called from finish_command and
finish_async.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -1,8 +1,66 @@
 #include "cache.h"
 #include "run-command.h"
 #include "exec_cmd.h"
+#include "sigchain.h"
 #include "argv-array.h"
 
+struct child_to_clean {
+	pid_t pid;
+	struct child_to_clean *next;
+};
+static struct child_to_clean *children_to_clean;
+static int installed_child_cleanup_handler;
+
+static void cleanup_children(int sig)
+{
+	while (children_to_clean) {
+		struct child_to_clean *p = children_to_clean;
+		children_to_clean = p->next;
+		kill(p->pid, sig);
+		free(p);
+	}
+}
+
+static void cleanup_children_on_signal(int sig)
+{
+	cleanup_children(sig);
+	sigchain_pop(sig);
+	raise(sig);
+}
+
+static void cleanup_children_on_exit(void)
+{
+	cleanup_children(SIGTERM);
+}
+
+static void mark_child_for_cleanup(pid_t pid)
+{
+	struct child_to_clean *p = xmalloc(sizeof(*p));
+	p->pid = pid;
+	p->next = children_to_clean;
+	children_to_clean = p;
+
+	if (!installed_child_cleanup_handler) {
+		atexit(cleanup_children_on_exit);
+		sigchain_push_common(cleanup_children_on_signal);
+		installed_child_cleanup_handler = 1;
+	}
+}
+
+static void clear_child_for_cleanup(pid_t pid)
+{
+	struct child_to_clean **last, *p;
+
+	last = &children_to_clean;
+	for (p = children_to_clean; p; p = p->next) {
+		if (p->pid == pid) {
+			*last = p->next;
+			free(p);
+			return;
+		}
+	}
+}
+
 static inline void close_pair(int fd[2])
 {
 	close(fd[0]);
@@ -130,6 +188,9 @@ static int wait_or_whine(pid_t pid, const char *argv0, int silent_exec_failure)
 	} else {
 		error("waitpid is confused (%s)", argv0);
 	}
+
+	clear_child_for_cleanup(pid);
+
 	errno = failed_errno;
 	return code;
 }
@@ -292,6 +353,8 @@ fail_pipe:
 	if (cmd->pid < 0)
 		error("cannot fork() for %s: %s", cmd->argv[0],
 			strerror(failed_errno = errno));
+	else if (cmd->clean_on_exit)
+		mark_child_for_cleanup(cmd->pid);
 
 	/*
 	 * Wait for child's execvp. If the execvp succeeds (or if fork()
@@ -312,6 +375,7 @@ fail_pipe:
 		cmd->pid = -1;
 	}
 	close(notify_pipe[0]);
+
 }
 #else
 {
@@ -356,6 +420,8 @@ fail_pipe:
 	failed_errno = errno;
 	if (cmd->pid < 0 && (!cmd->silent_exec_failure || errno != ENOENT))
 		error("cannot spawn %s: %s", cmd->argv[0], strerror(errno));
+	if (cmd->clean_on_exit && cmd->pid >= 0)
+		mark_child_for_cleanup(cmd->pid);
 
 	if (cmd->env)
 		free_environ(env);
@@ -540,6 +606,8 @@ int start_async(struct async *async)
 		exit(!!async->proc(proc_in, proc_out, async->data));
 	}
 
+	mark_child_for_cleanup(async->pid);
+
 	if (need_in)
 		close(fdin[0]);
 	else if (async->in)

run-command.h

@@ -38,6 +38,7 @@ struct child_process {
 	unsigned silent_exec_failure:1;
 	unsigned stdout_to_stderr:1;
 	unsigned use_shell:1;
+	unsigned clean_on_exit:1;
 	void (*preexec_cb)(void);
 };