Preserve snapshot identity across no-op updates

Author: andrewbranch

packages/typescript/src/api/async/api.ts

@@ -407,6 +407,9 @@ export class API<FromLSP extends boolean = false> implements FormatDiagnosticsHo
             snapshot: baseSnapshot.id,
             changes: toCreateSnapshotRequest(params),
         });
+        if (data.snapshot === baseSnapshot.id) {
+            return baseSnapshot;
+        }
         this.sourceFileCache.retainForSnapshot(data.snapshot, baseSnapshot.id, data.changes);
         const snapshot = new Snapshot(
             data,

packages/typescript/src/api/sync/api.ts

@@ -639,6 +639,9 @@ export class API<FromLSP extends boolean = false> implements FormatDiagnosticsHo
                     snapshot: baseSnapshot.id,
                     changes: toCreateSnapshotRequest(params),
                 });
+                if (data.snapshot === baseSnapshot.id) {
+                    return baseSnapshot;
+                }
                 owner.sourceFileCache.retainForSnapshot(data.snapshot, baseSnapshot.id, data.changes);
                 const snapshot = new Snapshot(
                     data,
@@ -666,6 +669,9 @@ export class API<FromLSP extends boolean = false> implements FormatDiagnosticsHo
                     snapshot: baseSnapshot.id,
                     changes: toCreateSnapshotRequest(params),
                 });
+                if (data.snapshot === baseSnapshot.id) {
+                    return baseSnapshot;
+                }
                 owner.sourceFileCache.retainForSnapshot(data.snapshot, baseSnapshot.id, data.changes);
                 const snapshot = new Snapshot(
                     data,

packages/typescript/test/async/api.test.ts

@@ -400,6 +400,22 @@ describe("API", () => {
         assert.equal("openedFiles" in empty.operation, false);
     });
 
+    test("snapshot.update preserves identity when the server returns the same snapshot", async () => {
+        await using api = spawnAPI();
+        const snapshot = await api.createSnapshot();
+        const client = (api as unknown as {
+            client: { apiRequest(method: string, params: unknown): Promise<unknown>; };
+        }).client;
+        const apiRequest = client.apiRequest.bind(client);
+        client.apiRequest = async (method, params) => {
+            const response = await apiRequest(method, params);
+            if (method !== "updateSnapshot" || typeof response !== "object" || response === null) return response;
+            return { ...response, snapshot: snapshot.id };
+        };
+
+        assert.strictEqual(await snapshot.update({}), snapshot);
+    });
+
     test("snapshot.update reconfigures a synthetic program", async () => {
         await using api = spawnAPI({
             "/src/a.ts": `export const a = 1;`,

packages/typescript/test/sync/api.test.ts

@@ -389,6 +389,22 @@ describe("API", () => {
         assert.equal("openedFiles" in empty.operation, false);
     });
 
+    test("snapshot.update preserves identity when the server returns the same snapshot", () => {
+        using api = spawnAPI();
+        const snapshot = api.createSnapshot();
+        const client = (api as unknown as {
+            client: { apiRequest(method: string, params: unknown): unknown; };
+        }).client;
+        const apiRequest = client.apiRequest.bind(client);
+        client.apiRequest = (method, params) => {
+            const response = apiRequest(method, params);
+            if (method !== "updateSnapshot" || typeof response !== "object" || response === null) return response;
+            return { ...response, snapshot: snapshot.id };
+        };
+
+        assert.strictEqual(snapshot.update({}), snapshot);
+    });
+
     test("snapshot.update reconfigures a synthetic program", () => {
         using api = spawnAPI({
             "/src/a.ts": `export const a = 1;`,