common/spawn.c: add spawn_piped()

src/common/spawn.c

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0-only
 #define _POSIX_C_SOURCE 200809L
 #include <assert.h>
+#include <fcntl.h>
 #include <glib.h>
 #include <signal.h>
 #include <stdio.h>
@@ -11,6 +12,26 @@
 #include "common/spawn.h"
 #include "common/fd_util.h"
 
+static void
+reset_signals_and_limits(void)
+{
+	restore_nofile_limit();
+
+	sigset_t set;
+	sigemptyset(&set);
+	sigprocmask(SIG_SETMASK, &set, NULL);
+
+	/* Restore ignored signals */
+	signal(SIGPIPE, SIG_DFL);
+}
+
+static void
+set_cloexec(int fd)
+{
+	int flags = fcntl(fd, F_GETFD);
+	fcntl(fd, F_SETFD, flags | FD_CLOEXEC);
+}
+
 void
 spawn_async_no_shell(char const *command)
 {
@@ -39,14 +60,9 @@ spawn_async_no_shell(char const *command)
 		wlr_log(WLR_ERROR, "unable to fork()");
 		goto out;
 	case 0:
-		restore_nofile_limit();
+		reset_signals_and_limits();
 
 		setsid();
-		sigset_t set;
-		sigemptyset(&set);
-		sigprocmask(SIG_SETMASK, &set, NULL);
-		/* Restore ignored signals */
-		signal(SIGPIPE, SIG_DFL);
 		grandchild = fork();
 		if (grandchild == 0) {
 			execvp(argv[0], argv);
@@ -63,3 +79,76 @@ out:
 	g_strfreev(argv);
 }
 
+pid_t
+spawn_piped(const char *command, int *pipe_fd)
+{
+	assert(command);
+
+	int pipe_rw[2];
+	if (pipe(pipe_rw) != 0) {
+		wlr_log(WLR_ERROR, "unable to pipe()");
+		return -1;
+	}
+
+	pid_t pid = fork();
+	if (pid < 0) {
+		close(pipe_rw[0]);
+		close(pipe_rw[1]);
+		wlr_log(WLR_ERROR, "unable to fork()");
+		return pid;
+	}
+
+	if (pid == 0) {
+		/* child */
+		reset_signals_and_limits();
+
+		/*
+		 * replace stdin and stderr with /dev/null
+		 * and stdout with the write end of the pipe
+		 */
+		dup2(pipe_rw[1], STDOUT_FILENO);
+		close(pipe_rw[0]);
+		close(pipe_rw[1]);
+
+		int dev_null = open("/dev/null", O_RDWR);
+		if (dev_null < 0) {
+			wlr_log_errno(WLR_ERROR, "opening /dev/null failed");
+			/*
+			 * Just close stdin and stderr and
+			 * hope $command can deal with that.
+			 */
+			close(STDIN_FILENO);
+			close(STDERR_FILENO);
+		} else {
+			dup2(dev_null, STDIN_FILENO);
+			dup2(dev_null, STDERR_FILENO);
+			close(dev_null);
+		}
+
+		execl("/bin/sh", "sh", "-c", command, NULL);
+		/*
+		 * Our stderr points to /dev/null or is closed
+		 * at this point so logging is pretty useless.
+		 */
+		_exit(1);
+	}
+
+	/* labwc */
+	close(pipe_rw[1]);
+
+	/*
+	 * Prevent leaking the read end of the pipe to further
+	 * children forked during the lifetime of the descriptor.
+	 */
+	set_cloexec(pipe_rw[0]);
+
+	*pipe_fd = pipe_rw[0];
+	return pid;
+}
+
+void
+spawn_piped_close(pid_t pid, int pipe_fd)
+{
+	close(pipe_fd);
+	/* waitpid() is done in a generic SIGCHLD handler in src/server.c */
+}