fix(@ngtools/webpack): cleanup more resources after modules are loaded (#15292)

* feat(@angular-devkit/core): support resetting a memory host

* fix(@ngtools/webpack): cleanup more resources after modules are loaded

Followup to #12994

Author: clydin

etc/api/angular_devkit/core/src/_golden-api.d.ts

@@ -992,6 +992,7 @@ export declare class SimpleMemoryHost implements Host<{}> {
     list(path: Path): Observable<PathFragment[]>;
     read(path: Path): Observable<FileBuffer>;
     rename(from: Path, to: Path): Observable<void>;
+    reset(): void;
     stat(path: Path): Observable<Stats<{}> | null> | null;
     watch(path: Path, options?: HostWatchOptions): Observable<HostWatchEvent> | null;
     write(path: Path, content: FileBuffer): Observable<void>;

packages/angular_devkit/core/src/virtual-fs/host/memory.ts

@@ -371,4 +371,9 @@ export class SimpleMemoryHost implements Host<{}> {
   watch(path: Path, options?: HostWatchOptions): Observable<HostWatchEvent> | null {
     return this._watch(path, options);
   }
+
+  reset(): void {
+    this._cache.clear();
+    this._watchers.clear();
+  }
 }

packages/ngtools/webpack/src/angular_compiler_plugin.ts

@@ -672,6 +672,7 @@ export class AngularCompilerPlugin {
           this._program = null;
           this._transformers = [];
           this._resourceLoader = undefined;
+          this._compilerHost.reset();
         }
       });
     });

packages/ngtools/webpack/src/compiler_host.ts

@@ -20,6 +20,7 @@ const dev = Math.floor(Math.random() * 10000);
 
 export class WebpackCompilerHost implements ts.CompilerHost {
   private _syncHost: virtualFs.SyncDelegateHost;
+  private _innerMemoryHost: virtualFs.SimpleMemoryHost;
   private _memoryHost: virtualFs.SyncDelegateHost;
   private _changedFiles = new Set<string>();
   private _readResourceFiles = new Set<string>();
@@ -49,14 +50,23 @@ export class WebpackCompilerHost implements ts.CompilerHost {
     private readonly moduleResolutionCache?: ts.ModuleResolutionCache,
   ) {
     this._syncHost = new virtualFs.SyncDelegateHost(host);
-    this._memoryHost = new virtualFs.SyncDelegateHost(new virtualFs.SimpleMemoryHost());
+    this._innerMemoryHost = new virtualFs.SimpleMemoryHost();
+    this._memoryHost = new virtualFs.SyncDelegateHost(this._innerMemoryHost);
     this._basePath = normalize(basePath);
   }
 
   private get virtualFiles(): Path[] {
     return [...((this._memoryHost.delegate as {}) as { _cache: Map<Path, {}> })._cache.keys()];
   }
 
+  reset() {
+    this._innerMemoryHost.reset();
+    this._changedFiles.clear();
+    this._readResourceFiles.clear();
+    this._sourceFileCache.clear();
+    this._resourceLoader = undefined;
+  }
+
   denormalizePath(path: string) {
     return getSystemPath(normalize(path));
   }