Add createFileSystemFromSnapshot(oldSnapshot, extraFSLayer)

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

@@ -678,6 +678,13 @@ export class API<FromLSP extends boolean = false> implements FormatDiagnosticsHo
 
     private prepareCreateSnapshotParams(params: CreateSnapshotParams | undefined): ProtocolCreateSnapshotParams {
         params ??= {};
+        if (params.fileSystem?.kind === "snapshot") {
+            const sourceId = params.fileSystem.snapshot;
+            const source = sourceId === undefined ? undefined : this.activeSnapshots.get(sourceId);
+            if (!source || source.isDisposed()) {
+                throw new Error("Cannot use an inactive snapshot filesystem");
+            }
+        }
         const prepareOptions = (options: CreateProgramOptions | undefined): ProtocolCreateProgramOptions | undefined => {
             if (!options) return undefined;
             const { moduleResolver, projectReferences, ...rest } = options;
@@ -1109,6 +1116,12 @@ export class Snapshot {
         return this.getProject(projectId)?.program;
     }
 
+    /**
+     * Creates a snapshot using this snapshot's projects and incremental state.
+     * Use `createFileSystemFromSnapshot(source)` from the filesystem API as `fileSystem`
+     * to overlay another snapshot's retained request entries without replaying
+     * intermediate updates. Changes and program reuse are relative to this snapshot.
+     */
     update<
         const CreatePrograms extends CreateSnapshotParams["createPrograms"] = undefined,
         const OpenFiles extends CreateSnapshotParams["openFiles"] = undefined,

packages/typescript/src/api/fs.ts

@@ -5,6 +5,7 @@ import type {
     RootedFilePath,
     RootedPath,
 } from "../ast/index.ts";
+import type { Snapshot } from "./async/api.ts";
 import { normalizePath } from "./path.ts";
 import type {
     RequestDirectoryEntries,
@@ -169,6 +170,26 @@ export function createFileSystemLayer(
     return createRequestFileSystem("layer", files, options);
 }
 
+/**
+ * Reuses a snapshot's retained request filesystem without resending its contents.
+ * Entries take precedence over the receiving snapshot's filesystem; a full source
+ * filesystem remains total. Host and editor contents are not copied.
+ * An optional layer or full filesystem is applied over the reused filesystem.
+ * The source must remain active until the snapshot operation completes.
+ */
+export function createFileSystemFromSnapshot(
+    snapshot: Pick<Snapshot, "id" | "isDisposed">,
+    fileSystem?: RequestFileSystem | undefined,
+): RequestFileSystem {
+    if (snapshot.isDisposed()) {
+        throw new Error("Cannot create a filesystem from a disposed snapshot");
+    }
+    if (fileSystem?.kind === "snapshot") {
+        throw new Error("Only a layer or full filesystem can be applied on top of a snapshot filesystem");
+    }
+    return { kind: "snapshot", snapshot: snapshot.id, files: {}, fileSystem };
+}
+
 function createRequestFileSystem(
     kind: RequestFileSystem["kind"],
     files: RequestFileEntries,

packages/typescript/src/api/proto.generated.ts

@@ -308,7 +308,9 @@ export interface CreateSnapshotParams extends SnapshotRequestChangesParams {
     /**
      * FileSystem supplies file contents and directory listings for the new snapshot.
      * A full filesystem is canonical and total. A filesystem layer is checked
-     * before falling back to the base snapshot or host filesystem.
+     * before falling back to the base snapshot or host filesystem. A snapshot
+     * filesystem overlays retained request entries from another active snapshot,
+     * then applies an optional layer or full filesystem.
      */
     fileSystem?: RequestFileSystem | undefined;
 }
@@ -1570,7 +1572,11 @@ export interface FileNotifications {
  * for a request that creates a snapshot.
  */
 export interface RequestFileSystem {
-    kind: "full" | "layer";
+    kind: "full" | "layer" | "snapshot";
+    /** Snapshot identifies the retained snapshot supplying request entries for KindSnapshot. */
+    snapshot?: number | undefined;
+    /** FileSystem is an optional layer or full filesystem applied on top of KindSnapshot. */
+    fileSystem?: RequestFileSystem | undefined;
     /** Files maps file names to their complete contents. */
     files: Record<string, string>;
     /**

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

@@ -1052,6 +1052,13 @@ export class API<FromLSP extends boolean = false> implements FormatDiagnosticsHo
 
     private prepareCreateSnapshotParams(params: CreateSnapshotParams | undefined): ProtocolCreateSnapshotParams {
         params ??= {};
+        if (params.fileSystem?.kind === "snapshot") {
+            const sourceId = params.fileSystem.snapshot;
+            const source = sourceId === undefined ? undefined : this.activeSnapshots.get(sourceId);
+            if (!source || source.isDisposed()) {
+                throw new Error("Cannot use an inactive snapshot filesystem");
+            }
+        }
         const prepareOptions = (options: CreateProgramOptions | undefined): ProtocolCreateProgramOptions | undefined => {
             if (!options) return undefined;
             const { moduleResolver, projectReferences, ...rest } = options;
@@ -1737,6 +1744,12 @@ export class Snapshot {
         return this.getProject(projectId)?.program;
     }
 
+    /**
+     * Creates a snapshot using this snapshot's projects and incremental state.
+     * Use `createFileSystemFromSnapshot(source)` from the filesystem API as `fileSystem`
+     * to overlay another snapshot's retained request entries without replaying
+     * intermediate updates. Changes and program reuse are relative to this snapshot.
+     */
     get update(): {
         <const CreatePrograms extends CreateSnapshotParams["createPrograms"] = undefined, const OpenFiles extends CreateSnapshotParams["openFiles"] = undefined>(params: SnapshotOperationParams<CreateSnapshotParams, CreatePrograms, OpenFiles>): SnapshotForOperationResults<CreatePrograms, OpenFiles>;
         (params: CreateSnapshotParams): Snapshot;