utils: add run_command

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

src/lxc/utils.c

@@ -2246,3 +2246,65 @@ int lxc_unstack_mountpoint(const char *path, bool lazy)
 
 	return umounts;
 }
+
+int run_command(char *buf, size_t buf_size, int (*child_fn)(void *), void *args)
+{
+	pid_t child;
+	int ret, fret, pipefd[2];
+	ssize_t bytes;
+
+	/* Make sure our callers do not receive unitialized memory. */
+	if (buf_size > 0 && buf)
+		buf[0] = '\0';
+
+	if (pipe(pipefd) < 0) {
+		SYSERROR("failed to create pipe");
+		return -1;
+	}
+
+	child = fork();
+	if (child < 0) {
+		close(pipefd[0]);
+		close(pipefd[1]);
+		SYSERROR("failed to create new process");
+		return -1;
+	}
+
+	if (child == 0) {
+		/* Close the read-end of the pipe. */
+		close(pipefd[0]);
+
+		/* Redirect std{err,out} to write-end of the
+		 * pipe.
+		 */
+		ret = dup2(pipefd[1], STDOUT_FILENO);
+		if (ret >= 0)
+			ret = dup2(pipefd[1], STDERR_FILENO);
+
+		/* Close the write-end of the pipe. */
+		close(pipefd[1]);
+
+		if (ret < 0) {
+			SYSERROR("failed to duplicate std{err,out} file descriptor");
+			exit(EXIT_FAILURE);
+		}
+
+		/* Does not return. */
+		child_fn(args);
+		ERROR("failed to exec command");
+		exit(EXIT_FAILURE);
+	}
+
+	/* close the write-end of the pipe */
+	close(pipefd[1]);
+
+	bytes = read(pipefd[0], buf, (buf_size > 0) ? (buf_size - 1) : 0);
+	if (bytes > 0)
+		buf[bytes - 1] = '\0';
+
+	fret = wait_for_pid(child);
+	/* close the read-end of the pipe */
+	close(pipefd[0]);
+
+	return fret;
+}

src/lxc/utils.h

@@ -355,4 +355,18 @@ int lxc_prepare_loop_dev(const char *source, char *loop_dev, int flags);
  */
 int lxc_unstack_mountpoint(const char *path, bool lazy);
 
+/*
+ * run_command runs a command and collect it's std{err,out} output in buf.
+ *
+ * @param[out] buf     The buffer where the commands std{err,out] output will be
+ *                     read into. If no output was produced, buf will be memset
+ *                     to 0.
+ * @param[in] buf_size The size of buf. This function will reserve one byte for
+ *                     \0-termination.
+ * @param[in] child_fn The function to be run in the child process. This
+ *                     function must exec.
+ * @param[in] args     Arguments to be passed to child_fn.
+ */
+int run_command(char *buf, size_t buf_size, int (*child_fn)(void *), void *args);
+
 #endif /* __LXC_UTILS_H */