Use the exclusive move for save rename and widen its copy fallback

Fall back to copying on any link error except EEXIST, since filesystems
without hard links report it with several codes (EISDIR on exFAT under
Windows, ENOSYS on some FUSE mounts). Rename now goes through the same
move so it also gets the fallback and the rollback. Only report a
missing source save on transfer when the source is actually missing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/host/src/Host.ts

@@ -42,8 +42,9 @@ async function moveSaveExclusive(sourcePath: string, targetPath: string) {
 	try {
 		await fs.link(sourcePath, targetPath);
 	} catch (err: any) {
-		// Hard links are not possible across filesystems and on some filesystems.
-		if (!["EXDEV", "EPERM", "ENOTSUP"].includes(err.code)) {
+		// Hard links fail across filesystems and on some filesystems with
+		// varying codes, the copy is still exclusive.
+		if (err.code === "EEXIST") {
 			throw err;
 		}
 		await copySaveExclusive(sourcePath, targetPath);
@@ -1054,13 +1055,10 @@ export default class Host extends lib.Link {
 		checkRequestSaveName(newName);
 		let instance = this.getRequestInstance(instanceId);
 		try {
-			// Link save in the new location.
-			await fs.link(
+			await moveSaveExclusive(
 				path.join(instance.path, "saves", oldName),
 				path.join(instance.path, "saves", newName),
 			);
-			// Unlink save in old location.
-			await fs.unlink(path.join(instance.path, "saves", oldName));
 		} catch (err: any) {
 			if (err.code === "EEXIST") {
 				throw new lib.RequestError(`${newName} already exists`);
@@ -1125,14 +1123,16 @@ export default class Host extends lib.Link {
 
 		// For consistency with remote transfer initiated through pullSave the
 		// target is renamed if it already exists.
+		const sourcePath = path.join(sourceInstance.path, "saves", sourceName);
 		try {
 			targetName = await this._moveOrCopySave(
-				path.join(sourceInstance.path, "saves", sourceName),
+				sourcePath,
 				path.join(targetInstance.path, "saves", targetName),
 				copy,
 			);
 		} catch (err: any) {
-			if (err.code === "ENOENT") {
+			// ENOENT may also come from a missing target saves folder.
+			if (err.code === "ENOENT" && !await fs.access(sourcePath).then(() => true, () => false)) {
 				throw new lib.RequestError(`${sourceName} does not exist`);
 			}
 			throw err;