run-command: add close_fd_above_stderr option

Add a close_fd_above_stderr flag to struct child_process.  When set,
the child closes file descriptors 3 and above between fork and exec
(skipping the child-notifier pipe), capped at sysconf(_SC_OPEN_MAX)
or 4096, whichever is smaller.  This prevents the child from
inheriting pipe endpoints or other descriptors from the parent
environment (e.g., the test harness).

Signed-off-by: Paul Tarjan <github@paulisageek.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ptarjan

run-command.c

@@ -546,6 +546,7 @@ static void atfork_parent(struct atfork_state *as)
 		"restoring signal mask");
 #endif
 }
+
 #endif /* GIT_WINDOWS_NATIVE */
 
 static inline void set_cloexec(int fd)
@@ -832,6 +833,17 @@ int start_command(struct child_process *cmd)
 			child_close(cmd->out);
 		}
 
+		if (cmd->close_fd_above_stderr) {
+			long max_fd = sysconf(_SC_OPEN_MAX);
+			int fd;
+			if (max_fd < 0 || max_fd > 4096)
+				max_fd = 4096;
+			for (fd = 3; fd < max_fd; fd++) {
+				if (fd != child_notifier)
+					close(fd);
+			}
+		}
+
 		if (cmd->dir && chdir(cmd->dir))
 			child_die(CHILD_ERR_CHDIR);
 

run-command.h

@@ -141,6 +141,15 @@ struct child_process {
 	unsigned stdout_to_stderr:1;
 	unsigned clean_on_exit:1;
 	unsigned wait_after_clean:1;
+
+	/**
+	 * Close file descriptors 3 and above in the child after forking
+	 * but before exec.  This prevents the child from inheriting
+	 * pipe endpoints or other descriptors from the parent
+	 * environment (e.g., the test harness).
+	 */
+	unsigned close_fd_above_stderr:1;
+
 	void (*clean_on_exit_handler)(struct child_process *process);
 };