spawn: New posix_spawn()-like API for exec

1 files changed, 158 insertions, 0 deletions

diff --git a/spawn.c b/spawn.c
new file mode 100644
index 0000000..82bce11
--- /dev/null
+++ b/spawn.c
@@ -0,0 +1,158 @@
+/****************************************************************************
+ * bfs                                                                      *
+ * Copyright (C) 2018 Tavian Barnes <tavianator@tavianator.com>             *
+ *                                                                          *
+ * Permission to use, copy, modify, and/or distribute this software for any *
+ * purpose with or without fee is hereby granted.                           *
+ *                                                                          *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES *
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF         *
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR  *
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES   *
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN    *
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF  *
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.           *
+ ****************************************************************************/
+
+#include "spawn.h"
+#include "util.h"
+#include <errno.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+/**
+ * Types of spawn actions.
+ */
+enum bfs_spawn_op {
+	BFS_SPAWN_FCHDIR,
+};
+
+/**
+ * A spawn action.
+ */
+struct bfs_spawn_action {
+	struct bfs_spawn_action *next;
+
+	enum bfs_spawn_op op;
+	int fd;
+};
+
+int bfs_spawn_init(struct bfs_spawn *ctx) {
+	ctx->flags = 0;
+	ctx->actions = NULL;
+	ctx->tail = &ctx->actions;
+	return 0;
+}
+
+int bfs_spawn_destroy(struct bfs_spawn *ctx) {
+	struct bfs_spawn_action *action = ctx->actions;
+	while (action) {
+		struct bfs_spawn_action *next = action->next;
+		free(action);
+		action = next;
+	}
+	return 0;
+}
+
+int bfs_spawn_setflags(struct bfs_spawn *ctx, enum bfs_spawn_flags flags) {
+	ctx->flags = flags;
+	return 0;
+}
+
+static struct bfs_spawn_action *bfs_spawn_add(struct bfs_spawn *ctx, enum bfs_spawn_op op) {
+	struct bfs_spawn_action *action = malloc(sizeof(*action));
+	if (action) {
+		action->next = NULL;
+		action->op = op;
+		action->fd = -1;
+
+		*ctx->tail = action;
+		ctx->tail = &action->next;
+	}
+	return action;
+}
+
+int bfs_spawn_addfchdir(struct bfs_spawn *ctx, int fd) {
+	if (fd < 0) {
+		errno = EBADF;
+		return -1;
+	}
+
+	struct bfs_spawn_action *action = bfs_spawn_add(ctx, BFS_SPAWN_FCHDIR);
+	if (action) {
+		action->fd = fd;
+		return 0;
+	} else {
+		return -1;
+	}
+}
+
+static void bfs_spawn_exec(const char *file, const struct bfs_spawn *ctx, char **argv, char **envp, int pipefd[2]) {
+	int error;
+	enum bfs_spawn_flags flags = ctx ? ctx->flags : 0;
+	const struct bfs_spawn_action *actions = ctx ? ctx->actions : NULL;
+
+	close(pipefd[0]);
+
+	for (const struct bfs_spawn_action *action = actions; action; action = action->next) {
+		switch (action->op) {
+		case BFS_SPAWN_FCHDIR:
+			if (fchdir(action->fd) != 0) {
+				goto fail;
+			}
+			break;
+		}
+	}
+
+	if (flags & BFS_SPAWN_USEPATH) {
+		execvpe(file, argv, envp);
+	} else {
+		execve(file, argv, envp);
+	}
+
+fail:
+	error = errno;
+	while (write(pipefd[1], &error, sizeof(error)) < sizeof(error));
+	close(pipefd[1]);
+	_Exit(127);
+}
+
+pid_t bfs_spawn(const char *file, const struct bfs_spawn *ctx, char **argv, char **envp) {
+	// Use a pipe to report errors from the child
+	int pipefd[2];
+	if (pipe_cloexec(pipefd) != 0) {
+		return -1;
+	}
+
+	int error;
+	pid_t pid = fork();
+
+	if (pid < 0) {
+		error = errno;
+		close(pipefd[1]);
+		close(pipefd[0]);
+		errno = error;
+		return -1;
+	} else if (pid == 0) {
+		// Child
+		bfs_spawn_exec(file, ctx, argv, envp, pipefd);
+	}
+
+	// Parent
+	close(pipefd[1]);
+
+	ssize_t nbytes = read(pipefd[0], &error, sizeof(error));
+	close(pipefd[0]);
+	if (nbytes == sizeof(error)) {
+		int wstatus;
+		waitpid(pid, &wstatus, 0);
+		errno = error;
+		return -1;
+	}
+
+	return pid;
+}