file: fix concurrent load/commit race condition

file.c

@@ -816,15 +816,15 @@ static void uci_file_commit(struct uci_context *ctx, struct uci_package **packag
 done:
 	free(name);
 	free(path);
-	uci_close_stream(f1);
 	if (do_rename) {
 		path = realpath(p->path, NULL);
 		if (!path || stat(path, &statbuf) || chmod(filename, statbuf.st_mode) || rename(filename, path)) {
 			unlink(filename);
-			UCI_THROW(ctx, UCI_ERR_IO);
+			ctx->err = UCI_ERR_IO;
 		}
 		free(path);
 	}
+	uci_close_stream(f1);
 	if (ctx->err)
 		UCI_THROW(ctx, ctx->err);
 }

util.c

@@ -218,9 +218,32 @@ __private FILE *uci_open_stream(struct uci_context *ctx, const char *filename, c
 	if (fd < 0)
 		goto error;
 
-	ret = flock(fd, (write ? LOCK_EX : LOCK_SH));
-	if ((ret < 0) && (errno != ENOSYS))
-		goto error_close;
+	if (write) {
+		ret = flock(fd, LOCK_EX);
+		if ((ret < 0) && (errno != ENOSYS))
+			goto error_close;
+	} else {
+		while (1) {
+			ret = flock(fd, LOCK_SH | LOCK_NB);
+			if (ret == 0 || errno == ENOSYS)
+				break;
+
+			if (errno == EINTR)
+				continue;
+
+			if (errno == EWOULDBLOCK || errno == EAGAIN) {
+				/* Reopen to follow a potentially replaced inode while writer holds lock */
+				close(fd);
+				usleep(1000);
+				fd = open(filename, flags, mode);
+				if (fd < 0)
+					goto error;
+				continue;
+			}
+
+			goto error_close;
+		}
+	}
 
 	ret = lseek(fd, 0, pos);