Always update LS state through Project (microsoft#56356)

Co-authored-by: Daniel Rosenwasser <DanielRosenwasser@users.noreply.github.com>

Author: sheetalkamat

src/server/project.ts

@@ -1330,6 +1330,14 @@ export abstract class Project implements LanguageServiceHost, ModuleResolutionHo
         this.hasAddedOrRemovedSymlinks = true;
     }
 
+    /** @internal */
+    updateFromProjectInProgress = false;
+
+    /** @internal */
+    updateFromProject() {
+        updateProjectIfDirty(this);
+    }
+
     /**
      * Updates set of files that contribute to this project
      * @returns: true if set of files in the project stays the same and false - otherwise.
@@ -1523,8 +1531,10 @@ export abstract class Project implements LanguageServiceHost, ModuleResolutionHo
         this.hasInvalidatedResolutions = hasInvalidatedResolutions;
         this.hasInvalidatedLibResolutions = hasInvalidatedLibResolutions;
         this.resolutionCache.startCachingPerDirectoryResolution();
-        this.program = this.languageService.getProgram(); // TODO: GH#18217
         this.dirty = false;
+        this.updateFromProjectInProgress = true;
+        this.program = this.languageService.getProgram(); // TODO: GH#18217
+        this.updateFromProjectInProgress = false;
         tracing?.push(tracing.Phase.Session, "finishCachingPerDirectoryResolution");
         this.resolutionCache.finishCachingPerDirectoryResolution(this.program, oldProgram);
         tracing?.pop();

src/services/services.ts

@@ -1620,6 +1620,15 @@ export function createLanguageService(
     }
 
     function synchronizeHostData(): void {
+        if (host.updateFromProject && !host.updateFromProjectInProgress) {
+            host.updateFromProject();
+        }
+        else {
+            synchronizeHostDataWorker();
+        }
+    }
+
+    function synchronizeHostDataWorker(): void {
         Debug.assert(languageServiceMode !== LanguageServiceMode.Syntactic);
         // perform fast check if host supports it
         if (host.getProjectVersion) {

src/services/types.ts

@@ -316,6 +316,9 @@ export interface IncompleteCompletionsCache {
 export interface LanguageServiceHost extends GetEffectiveTypeRootsHost, MinimalResolutionCacheHost {
     getCompilationSettings(): CompilerOptions;
     getNewLine?(): string;
+    /** @internal */ updateFromProject?(): void;
+    /** @internal */ updateFromProjectInProgress?: boolean;
+
     getProjectVersion?(): string;
     getScriptFileNames(): string[];
     getScriptKind?(fileName: string): ScriptKind;

src/testRunner/unittests/tsserver/plugins.ts

@@ -28,6 +28,8 @@ describe("unittests:: tsserver:: plugins:: loading", () => {
                     create(info: ts.server.PluginCreateInfo) {
                         info.session?.addProtocolHandler(testProtocolCommand, request => {
                             session.logger.log(`addProtocolHandler: ${jsonToReadableText(request)}`);
+                            // Assume this one needs program
+                            info.languageService.getProgram();
                             return {
                                 response: testProtocolCommandResponse,
                             };
@@ -101,6 +103,41 @@ describe("unittests:: tsserver:: plugins:: loading", () => {
         baselineTsserverLogs("plugins", "With session and custom protocol message", session);
     });
 
+    it("when plugins use LS to get program and update is pending", () => {
+        const pluginName = "some-plugin";
+        const aTs: File = {
+            path: "/user/username/projects/project/a.ts",
+            content: `/// <reference path="./b.ts"/>`,
+        };
+        const tsconfig: File = {
+            path: "/user/username/projects/project/tsconfig.json",
+            content: jsonToReadableText({
+                compilerOptions: {
+                    plugins: [
+                        { name: pluginName },
+                    ],
+                },
+            }),
+        };
+
+        const { session, host } = createHostWithPlugin([aTs, tsconfig, libFile]);
+
+        openFilesForSession([aTs], session);
+        // Write the missing file (referenced by 'a.ts') to schedule an update.
+        host.writeFile("/user/username/projects/project/b.ts", "const y = 10;");
+
+        // This should update the language service with a new program.
+        session.executeCommandSeq({
+            command: testProtocolCommand,
+            arguments: testProtocolCommandRequest,
+        });
+
+        // This results in a program update.
+        host.runQueuedTimeoutCallbacks();
+
+        baselineTsserverLogs("plugins", "when plugins use LS to get program and update is pending", session);
+    });
+
     it("gets external files with config file reload", () => {
         const aTs: File = { path: `/user/username/projects/myproject/a.ts`, content: `export const x = 10;` };
         const tsconfig: File = {