From 955f77bb0b82ca4f9740376e5ff84990310cc3d7 Mon Sep 17 00:00:00 2001
From: Nicolas Meienberger <github@thisprops.com>
Date: Sat, 4 Apr 2026 14:51:04 +0200
Subject: [PATCH] fix: serialize mirror copy operations per destination
 repository

---
 .../__tests__/backups.execution.test.ts       | 56 +++++++++++++++++++
 .../backups/helpers/backup-maintenance.ts     |  2 +-
 2 files changed, 57 insertions(+), 1 deletion(-)

diff --git a/app/server/modules/backups/__tests__/backups.execution.test.ts b/app/server/modules/backups/__tests__/backups.execution.test.ts
index feb444ee..2aafe12c 100644
--- a/app/server/modules/backups/__tests__/backups.execution.test.ts
+++ b/app/server/modules/backups/__tests__/backups.execution.test.ts
@@ -525,4 +525,60 @@ describe("mirror operations", () => {
 		// assert
 		expect(resticForgetMock).not.toHaveBeenCalled();
 	});
+
+	test("should serialize mirror copies for schedules that share the same mirror repository", async () => {
+		const { resticCopyMock } = setup();
+		const sourceRepository = await createTestRepository();
+		const mirrorRepository = await createTestRepository();
+		const firstVolume = await createTestVolume();
+		const secondVolume = await createTestVolume();
+		const firstSchedule = await createTestBackupSchedule({
+			volumeId: firstVolume.id,
+			repositoryId: sourceRepository.id,
+		});
+		const secondSchedule = await createTestBackupSchedule({
+			volumeId: secondVolume.id,
+			repositoryId: sourceRepository.id,
+		});
+
+		await createTestBackupScheduleMirror(firstSchedule.id, mirrorRepository.id);
+		await createTestBackupScheduleMirror(secondSchedule.id, mirrorRepository.id);
+
+		let releaseFirstCopy = () => {};
+		let resolveFirstCopyStarted = () => {};
+		const firstCopyStarted = new Promise<void>((resolve) => {
+			resolveFirstCopyStarted = resolve;
+		});
+
+		resticCopyMock.mockImplementationOnce(
+			() =>
+				new Promise((resolve) => {
+					resolveFirstCopyStarted();
+					releaseFirstCopy = () => resolve({ success: true, output: "" });
+				}),
+		);
+		resticCopyMock.mockImplementation(() => Promise.resolve({ success: true, output: "" }));
+
+		const firstCopyPromise = backupsExecutionService.copyToMirrors(firstSchedule.id, sourceRepository, null);
+		await firstCopyStarted;
+
+		const secondCopyPromise = backupsExecutionService.copyToMirrors(secondSchedule.id, sourceRepository, null);
+
+		try {
+			const secondCopyState = await Promise.race<"resolved" | "timeout">([
+				secondCopyPromise.then(() => "resolved"),
+				new Promise((resolve) => {
+					setTimeout(() => resolve("timeout"), 50);
+				}),
+			]);
+
+			expect(secondCopyState).toBe("timeout");
+			expect(resticCopyMock).toHaveBeenCalledTimes(1);
+		} finally {
+			releaseFirstCopy();
+			await Promise.all([firstCopyPromise, secondCopyPromise]);
+		}
+
+		expect(resticCopyMock).toHaveBeenCalledTimes(2);
+	});
 });
diff --git a/app/server/modules/backups/helpers/backup-maintenance.ts b/app/server/modules/backups/helpers/backup-maintenance.ts
index 77f6c797..6ab4d7a5 100644
--- a/app/server/modules/backups/helpers/backup-maintenance.ts
+++ b/app/server/modules/backups/helpers/backup-maintenance.ts
@@ -93,7 +93,7 @@ async function copyToSingleMirror(
 		});
 
 		const releaseSource = await repoMutex.acquireShared(sourceRepository.id, `mirror_source:${scheduleId}`);
-		const releaseMirror = await repoMutex.acquireShared(mirror.repository.id, `mirror:${scheduleId}`);
+		const releaseMirror = await repoMutex.acquireExclusive(mirror.repository.id, `mirror:${scheduleId}`);
 
 		try {
 			await restic.copy(sourceRepository.config, mirror.repository.config, { tag: schedule.shortId, organizationId });