fix(language-service): Use module resolution cache (#32479)

This PR fixes a critical performance issue where the language
service makes a MASSIVE number of filesystem calls when performing
module resolution.
This is because there is no caching. To make matters worse, module
resolution is performed for **every** program change (which means every
few keystrokes trigger a massive number of fs calls).

PR Close #32479

packages/language-service/src/reflector_host.ts

@@ -48,11 +48,22 @@ class ReflectorModuleModuleResolutionHost implements ts.ModuleResolutionHost, Me
 }
 
 export class ReflectorHost implements StaticSymbolResolverHost {
-  private hostAdapter: ReflectorModuleModuleResolutionHost;
-  private metadataReaderCache = createMetadataReaderCache();
+  private readonly hostAdapter: ReflectorModuleModuleResolutionHost;
+  private readonly metadataReaderCache = createMetadataReaderCache();
+  private readonly moduleResolutionCache: ts.ModuleResolutionCache;
+  private readonly fakeContainingPath: string;
 
-  constructor(getProgram: () => ts.Program, private readonly serviceHost: ts.LanguageServiceHost) {
-    this.hostAdapter = new ReflectorModuleModuleResolutionHost(serviceHost, getProgram);
+  constructor(getProgram: () => ts.Program, private readonly tsLSHost: ts.LanguageServiceHost) {
+    // tsLSHost.getCurrentDirectory() returns the directory where tsconfig.json
+    // is located. This is not the same as process.cwd() because the language
+    // service host sets the "project root path" as its current directory.
+    const currentDir = tsLSHost.getCurrentDirectory();
+    this.fakeContainingPath = currentDir ? path.join(currentDir, 'fakeContainingFile.ts') : '';
+    this.hostAdapter = new ReflectorModuleModuleResolutionHost(tsLSHost, getProgram);
+    this.moduleResolutionCache = ts.createModuleResolutionCache(
+        currentDir,
+        s => s,  // getCanonicalFileName
+        tsLSHost.getCompilationSettings());
   }
 
   getMetadataFor(modulePath: string): {[key: string]: any}[]|undefined {
@@ -64,22 +75,18 @@ export class ReflectorHost implements StaticSymbolResolverHost {
       if (moduleName.startsWith('.')) {
         throw new Error('Resolution of relative paths requires a containing file.');
       }
-      // serviceHost.getCurrentDirectory() returns the directory where tsconfig.json
-      // is located. This is not the same as process.cwd() because the language
-      // service host sets the "project root path" as its current directory.
-      const currentDirectory = this.serviceHost.getCurrentDirectory();
-      if (!currentDirectory) {
+      if (!this.fakeContainingPath) {
         // If current directory is empty then the file must belong to an inferred
         // project (no tsconfig.json), in which case it's not possible to resolve
         // the module without the caller explicitly providing a containing file.
         throw new Error(`Could not resolve '${moduleName}' without a containing file.`);
       }
-      // Any containing file gives the same result for absolute imports
-      containingFile = path.join(currentDirectory, 'index.ts');
+      containingFile = this.fakeContainingPath;
     }
-    const compilerOptions = this.serviceHost.getCompilationSettings();
-    const resolved =
-        ts.resolveModuleName(moduleName, containingFile, compilerOptions, this.hostAdapter)
+    const compilerOptions = this.tsLSHost.getCompilationSettings();
+    const resolved = ts.resolveModuleName(
+                           moduleName, containingFile, compilerOptions, this.hostAdapter,
+                           this.moduleResolutionCache)
                          .resolvedModule;
     return resolved ? resolved.resolvedFileName : null;
   }

packages/language-service/test/reflector_host_spec.ts

@@ -7,8 +7,11 @@
  */
 
 import * as path from 'path';
+import * as ts from 'typescript';
 
+import {createLanguageService} from '../src/language_service';
 import {ReflectorHost} from '../src/reflector_host';
+import {TypeScriptServiceHost} from '../src/typescript_host';
 
 import {toh} from './test_data';
 import {MockTypescriptHost} from './test_utils';
@@ -19,7 +22,7 @@ describe('reflector_host_spec', () => {
   it('should be able to find angular under windows', () => {
     const originalJoin = path.join;
     const originalPosixJoin = path.posix.join;
-    let mockHost =
+    const mockHost =
         new MockTypescriptHost(['/app/main.ts', '/app/parsing-cases.ts'], toh, 'node_modules', {
           ...path,
           join: (...args: string[]) => originalJoin.apply(path, args),
@@ -37,4 +40,46 @@ describe('reflector_host_spec', () => {
     const result = reflectorHost.moduleNameToFileName('@angular/core');
     expect(result).not.toBeNull('could not find @angular/core using path.win32');
   });
+
+  it('should use module resolution cache', () => {
+    const mockHost = new MockTypescriptHost(['/app/main.ts'], toh);
+    // TypeScript relies on `ModuleResolutionHost.fileExists()` to perform
+    // module resolution, and ReflectorHost uses
+    // `LanguageServiceHost.getScriptSnapshot()` to implement `fileExists()`,
+    // so spy on this method to determine how many times it's called.
+    const spy = spyOn(mockHost, 'getScriptSnapshot').and.callThrough();
+
+    const tsLS = ts.createLanguageService(mockHost);
+
+    // First count is due to the instantiation of StaticReflector, which
+    // performs resolutions of core Angular symbols, like `NgModule`.
+    // TODO: Reduce this count to zero doing lazy instantiation.
+    const ngLSHost = new TypeScriptServiceHost(mockHost, tsLS);
+    const firstCount = spy.calls.count();
+    expect(firstCount).toBeGreaterThan(20);
+    expect(firstCount).toBeLessThan(50);
+    spy.calls.reset();
+
+    // Second count is due to resolution of the Tour of Heroes (toh) project.
+    // This resolves all Angular directives in the project.
+    ngLSHost.getAnalyzedModules();
+    const secondCount = spy.calls.count();
+    expect(secondCount).toBeGreaterThan(500);
+    expect(secondCount).toBeLessThan(600);
+    spy.calls.reset();
+
+    // Third count is due to recompution after the program changes.
+    mockHost.addCode('');  // this will mark project as dirty
+    ngLSHost.getAnalyzedModules();
+    const thirdCount = spy.calls.count();
+    expect(thirdCount).toBeGreaterThan(50);
+    expect(thirdCount).toBeLessThan(100);
+
+    // Summary
+    // |               | First Count | Second Count | Third Count |
+    // |---------------|-------------|--------------|-------------|
+    // | Without Cache | 2581        | 6291         | 257         |
+    // | With Cache    | 26          | 550          | 84          |
+    // | Improvement   | ~100x       | ~10x         | ~3x         |
+  });
 });