email: Fix bugs with maildir creation and maildir temp files.

Fix a few issues with maildirs revealed by some high-throughput
testing.

* utils.c: If a directory is created between checking and creating it,
  return success, not failure.
* mod_smtp_delivery_local: Include specific error message to handle
  duplicate maildir temp files.
* mod_mail: Add a unique ID for maildir temp filenames to ensure
  uniqueness.

Author: InterLinked1

bbs/utils.c

@@ -802,6 +802,10 @@ int bbs_ensure_directory_exists(const char *path)
 	if (eaccess(path, R_OK | X_OK)) {
 		int derr = errno;
 		if (mkdir(path, 0700)) {
+			if (errno == EEXIST) {
+				/* Encountered a race condition in creating the directory, that's fine */
+				return 0;
+			}
 			/* The directory probably already exists, but we can't access it for some reason. */
 			bbs_warning("mkdir(%s) failed: %s\n", path, strerror(errno));
 			bbs_error("Can't access directory %s: %s\n", path, strerror(derr));

modules/mod_mail.c

@@ -991,13 +991,16 @@ int maildir_mktemp(const char *path, char *buf, size_t len, char *newbuf)
 	struct timeval tvnow;
 	struct stat st;
 	int fd;
+	static int maildir_tmp_uniqueid_suffix = 0;
 
 	for (;;) {
+		int suffix;
 #pragma GCC diagnostic ignored "-Waggregate-return"
 		tvnow = bbs_tvnow();
 #pragma GCC diagnostic pop
-		snprintf(buf, len, "%s/tmp/%lu%06lu", path, tvnow.tv_sec, tvnow.tv_usec);
-		snprintf(newbuf, len, "%s/new/%lu%06lu", path, tvnow.tv_sec, tvnow.tv_usec);
+		suffix = bbs_atomic_fetchadd_int(&maildir_tmp_uniqueid_suffix, +1); /* Use a monotonically increasing unique ID to absolutely guarantee uniqueness */
+		snprintf(buf, len, "%s/tmp/%lu%06lu_%d", path, tvnow.tv_sec, tvnow.tv_usec, suffix);
+		snprintf(newbuf, len, "%s/new/%lu%06lu_%d", path, tvnow.tv_sec, tvnow.tv_usec, suffix);
 		if (stat(buf, &st) == -1 && errno == ENOENT) {
 			/* Error means it doesn't exist. */
 			if (stat(newbuf, &st) == -1 && errno == ENOENT) {

modules/mod_smtp_delivery_local.c

@@ -190,7 +190,11 @@ static int appendmsg(struct smtp_session *smtp, struct smtp_response *resp, stru
 	}
 
 	if (rename(tmpfile, newfile)) {
-		bbs_error("rename %s -> %s failed: %s\n", tmpfile, newfile, strerror(errno));
+		if (errno == ENOENT && !bbs_file_exists(tmpfile) && bbs_file_exists(newfile)) {
+			bbs_error("%s already renamed to %s?\n", tmpfile, newfile);
+		} else {
+			bbs_error("rename %s -> %s failed: %s\n", tmpfile, newfile, strerror(errno));
+		}
 		return -1;
 	} else {
 		char maildir[256];