tempfile: a new module for handling temporary files

A lot of work went into defining the state diagram for lockfiles and
ensuring correct, race-resistant cleanup in all circumstances.

Most of that infrastructure can be applied directly to *any* temporary
file. So extract a new "tempfile" module from the "lockfile" module.
Reimplement lockfile on top of tempfile.

Subsequent commits will add more users of the new module.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 238 insertions, 0 deletions

diff --git a/tempfile.c b/tempfile.c
new file mode 100644
index 0000000000..d8358187f8
--- /dev/null
+++ b/tempfile.c
@@ -0,0 +1,238 @@
+/*
+ * State diagram and cleanup
+ * -------------------------
+ *
+ * If the program exits while a temporary file is active, we want to
+ * make sure that we remove it. This is done by remembering the active
+ * temporary files in a linked list, `tempfile_list`. An `atexit(3)`
+ * handler and a signal handler are registered, to clean up any active
+ * temporary files.
+ *
+ * Because the signal handler can run at any time, `tempfile_list` and
+ * the `tempfile` objects that comprise it must be kept in
+ * self-consistent states at all times.
+ *
+ * The possible states of a `tempfile` object are as follows:
+ *
+ * - Uninitialized. In this state the object's `on_list` field must be
+ *   zero but the rest of its contents need not be initialized. As
+ *   soon as the object is used in any way, it is irrevocably
+ *   registered in `tempfile_list`, and `on_list` is set.
+ *
+ * - Active, file open (after `create_tempfile()` or
+ *   `reopen_tempfile()`). In this state:
+ *
+ *   - the temporary file exists
+ *   - `active` is set
+ *   - `filename` holds the filename of the temporary file
+ *   - `fd` holds a file descriptor open for writing to it
+ *   - `fp` holds a pointer to an open `FILE` object if and only if
+ *     `fdopen_tempfile()` has been called on the object
+ *   - `owner` holds the PID of the process that created the file
+ *
+ * - Active, file closed (after successful `close_tempfile()`). Same
+ *   as the previous state, except that the temporary file is closed,
+ *   `fd` is -1, and `fp` is `NULL`.
+ *
+ * - Inactive (after `delete_tempfile()`, `rename_tempfile()`, a
+ *   failed attempt to create a temporary file, or a failed
+ *   `close_tempfile()`). In this state:
+ *
+ *   - `active` is unset
+ *   - `filename` is empty (usually, though there are transitory
+ *     states in which this condition doesn't hold). Client code should
+ *     *not* rely on the filename being empty in this state.
+ *   - `fd` is -1 and `fp` is `NULL`
+ *   - the object is left registered in the `tempfile_list`, and
+ *     `on_list` is set.
+ *
+ * A temporary file is owned by the process that created it. The
+ * `tempfile` has an `owner` field that records the owner's PID. This
+ * field is used to prevent a forked process from deleting a temporary
+ * file created by its parent.
+ */
+
+#include "cache.h"
+#include "tempfile.h"
+#include "sigchain.h"
+
+static struct tempfile *volatile tempfile_list;
+
+static void remove_tempfiles(int skip_fclose)
+{
+	pid_t me = getpid();
+
+	while (tempfile_list) {
+		if (tempfile_list->owner == me) {
+			/* fclose() is not safe to call in a signal handler */
+			if (skip_fclose)
+				tempfile_list->fp = NULL;
+			delete_tempfile(tempfile_list);
+		}
+		tempfile_list = tempfile_list->next;
+	}
+}
+
+static void remove_tempfiles_on_exit(void)
+{
+	remove_tempfiles(0);
+}
+
+static void remove_tempfiles_on_signal(int signo)
+{
+	remove_tempfiles(1);
+	sigchain_pop(signo);
+	raise(signo);
+}
+
+/* Make sure errno contains a meaningful value on error */
+int create_tempfile(struct tempfile *tempfile, const char *path)
+{
+	size_t pathlen = strlen(path);
+
+	if (!tempfile_list) {
+		/* One-time initialization */
+		sigchain_push_common(remove_tempfiles_on_signal);
+		atexit(remove_tempfiles_on_exit);
+	}
+
+	if (tempfile->active)
+		die("BUG: create_tempfile called for active object");
+	if (!tempfile->on_list) {
+		/* Initialize *tempfile and add it to tempfile_list: */
+		tempfile->fd = -1;
+		tempfile->fp = NULL;
+		tempfile->active = 0;
+		tempfile->owner = 0;
+		strbuf_init(&tempfile->filename, pathlen);
+		tempfile->next = tempfile_list;
+		tempfile_list = tempfile;
+		tempfile->on_list = 1;
+	} else if (tempfile->filename.len) {
+		/* This shouldn't happen, but better safe than sorry. */
+		die("BUG: create_tempfile called for improperly-reset object");
+	}
+
+	strbuf_add_absolute_path(&tempfile->filename, path);
+	tempfile->fd = open(tempfile->filename.buf, O_RDWR | O_CREAT | O_EXCL, 0666);
+	if (tempfile->fd < 0) {
+		strbuf_reset(&tempfile->filename);
+		return -1;
+	}
+	tempfile->owner = getpid();
+	tempfile->active = 1;
+	if (adjust_shared_perm(tempfile->filename.buf)) {
+		int save_errno = errno;
+		error("cannot fix permission bits on %s", tempfile->filename.buf);
+		delete_tempfile(tempfile);
+		errno = save_errno;
+		return -1;
+	}
+	return tempfile->fd;
+}
+
+FILE *fdopen_tempfile(struct tempfile *tempfile, const char *mode)
+{
+	if (!tempfile->active)
+		die("BUG: fdopen_tempfile() called for inactive object");
+	if (tempfile->fp)
+		die("BUG: fdopen_tempfile() called for open object");
+
+	tempfile->fp = fdopen(tempfile->fd, mode);
+	return tempfile->fp;
+}
+
+const char *get_tempfile_path(struct tempfile *tempfile)
+{
+	if (!tempfile->active)
+		die("BUG: get_tempfile_path() called for inactive object");
+	return tempfile->filename.buf;
+}
+
+int get_tempfile_fd(struct tempfile *tempfile)
+{
+	if (!tempfile->active)
+		die("BUG: get_tempfile_fd() called for inactive object");
+	return tempfile->fd;
+}
+
+FILE *get_tempfile_fp(struct tempfile *tempfile)
+{
+	if (!tempfile->active)
+		die("BUG: get_tempfile_fp() called for inactive object");
+	return tempfile->fp;
+}
+
+int close_tempfile(struct tempfile *tempfile)
+{
+	int fd = tempfile->fd;
+	FILE *fp = tempfile->fp;
+	int err;
+
+	if (fd < 0)
+		return 0;
+
+	tempfile->fd = -1;
+	if (fp) {
+		tempfile->fp = NULL;
+
+		/*
+		 * Note: no short-circuiting here; we want to fclose()
+		 * in any case!
+		 */
+		err = ferror(fp) | fclose(fp);
+	} else {
+		err = close(fd);
+	}
+
+	if (err) {
+		int save_errno = errno;
+		delete_tempfile(tempfile);
+		errno = save_errno;
+		return -1;
+	}
+
+	return 0;
+}
+
+int reopen_tempfile(struct tempfile *tempfile)
+{
+	if (0 <= tempfile->fd)
+		die("BUG: reopen_tempfile called for an open object");
+	if (!tempfile->active)
+		die("BUG: reopen_tempfile called for an inactive object");
+	tempfile->fd = open(tempfile->filename.buf, O_WRONLY);
+	return tempfile->fd;
+}
+
+int rename_tempfile(struct tempfile *tempfile, const char *path)
+{
+	if (!tempfile->active)
+		die("BUG: rename_tempfile called for inactive object");
+
+	if (close_tempfile(tempfile))
+		return -1;
+
+	if (rename(tempfile->filename.buf, path)) {
+		int save_errno = errno;
+		delete_tempfile(tempfile);
+		errno = save_errno;
+		return -1;
+	}
+
+	tempfile->active = 0;
+	strbuf_reset(&tempfile->filename);
+	return 0;
+}
+
+void delete_tempfile(struct tempfile *tempfile)
+{
+	if (!tempfile->active)
+		return;
+
+	if (!close_tempfile(tempfile)) {
+		unlink_or_warn(tempfile->filename.buf);
+		tempfile->active = 0;
+		strbuf_reset(&tempfile->filename);
+	}
+}