fix(coding-agent): resolved hashline snapshot tag collisions using exact matches

- Guard against 16-bit snapshot tag collisions by requiring live text to be byte-identical to the retained snapshot. - Transition base text resolution to query exact matches via `snapshots.byHashExact`. - Prevent applying incorrect preview edits when live file contents drift to a colliding state.
2026-07-02 01:51:10 +02:00
parent e12a8f161a
commit ac9e85564f
2 changed files with 70 additions and 2 deletions
@@ -298,6 +298,63 @@ describe("computeHashlineDiff", () => {
 			expect(result.error).toContain('internal scheme "local://"');
 		}
 	});
+
+	// A 16-bit snapshot tag can collide across two different file states. The
+	// preview must mirror Patcher's apply-time guard: hash equality alone never
+	// proves the live text IS the snapshot the anchors were minted against.
+	test("rejects the no-drift path when the live text is a colliding ambiguous tag", async () => {
+		// Both texts hash to `1D84` (pinned in hashline's collision tests).
+		const SNAPSHOT_TEXT = "line one 263\nline two 4471\n";
+		const LIVE_TEXT = "line one 410\nline two 6970\n";
+		const sourcePath = path.join(tempDir, "source.txt");
+		await Bun.write(sourcePath, LIVE_TEXT);
+
+		const snapshotStore = new InMemorySnapshotStore();
+		// Anchors were minted against SNAPSHOT_TEXT; the live file is the
+		// colliding LIVE_TEXT, also retained (e.g. read after an external
+		// write). The tag is ambiguous — previewing the SWAP against live
+		// would show the model's payload landing on unrelated content.
+		const tag = snapshotStore.record(sourcePath, SNAPSHOT_TEXT);
+		snapshotStore.record(sourcePath, LIVE_TEXT);
+
+		const result = await computeHashlineDiff(
+			{ input: `${formatHashlineHeader(sourcePath, tag)}\nSWAP 2.=2:\n+edited from snapshot` },
+			tempDir,
+			snapshotStore,
+		);
+
+		expect("error" in result).toBe(true);
+		if ("error" in result) {
+			expect(result.error).toContain("file changed between read and edit");
+		}
+	});
+
+	test("rejects the no-drift path when the live text collides with the single retained snapshot", async () => {
+		const SNAPSHOT_TEXT = "line one 263\nline two 4471\n";
+		const LIVE_TEXT = "line one 410\nline two 6970\n";
+		const sourcePath = path.join(tempDir, "source.txt");
+		await Bun.write(sourcePath, LIVE_TEXT);
+
+		const snapshotStore = new InMemorySnapshotStore();
+		// Only SNAPSHOT_TEXT is retained; live drifted to a colliding text the
+		// store never saw. computeFileHash(live) === tag, but the retained
+		// text differs — recovery (3-way merge from SNAPSHOT_TEXT) must run
+		// instead of anchoring directly onto the collider. Here the merge
+		// cannot apply (every line differs), so the preview surfaces the
+		// drift error rather than a bogus diff.
+		const tag = snapshotStore.record(sourcePath, SNAPSHOT_TEXT);
+
+		const result = await computeHashlineDiff(
+			{ input: `${formatHashlineHeader(sourcePath, tag)}\nSWAP 2.=2:\n+edited from snapshot` },
+			tempDir,
+			snapshotStore,
+		);
+
+		expect("error" in result).toBe(true);
+		if ("error" in result) {
+			expect(result.error).toContain("file changed between read and edit");
+		}
+	});
 });

 describe("computeEditDiff", () => {