feat(@angular-devkit/core): add new hosts

SafeHost swallows all errors with values.
EmptyHost has no files/directories.

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

@@ -0,0 +1,42 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+import { Observable, of, throwError } from 'rxjs';
+import { FileDoesNotExistException } from '../../exception';
+import { Path, PathFragment } from '../path';
+import { FileBuffer, HostCapabilities, ReadonlyHost, Stats } from './interface';
+
+export class Empty implements ReadonlyHost {
+  readonly capabilities: HostCapabilities = {
+    synchronous: true,
+  };
+
+  read(path: Path): Observable<FileBuffer> {
+    return throwError(new FileDoesNotExistException(path));
+  }
+
+  list(path: Path): Observable<PathFragment[]> {
+    return of([]);
+  }
+
+  exists(path: Path): Observable<boolean> {
+    return of(false);
+  }
+
+  isDirectory(path: Path): Observable<boolean> {
+    return of(false);
+  }
+
+  isFile(path: Path): Observable<boolean> {
+    return of(false);
+  }
+
+  stat(path: Path): Observable<Stats<{}> | null> {
+    // We support stat() but have no file.
+    return of(null);
+  }
+}

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

@@ -8,9 +8,12 @@
 
 export * from './alias';
 export * from './buffer';
+export * from './empty';
 export * from './interface';
 export * from './memory';
 export * from './pattern';
+export * from './record';
+export * from './safe';
 export * from './scoped';
 export * from './sync';
 

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

@@ -0,0 +1,56 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+import { Observable, of } from 'rxjs';
+import { catchError } from 'rxjs/operators';
+import { Path, PathFragment } from '../path';
+import { FileBuffer, HostCapabilities, ReadonlyHost, Stats } from './interface';
+
+/**
+ * A Host that filters out errors. The only exception is `read()` which will still error out if
+ * the delegate returned an error (e.g. NodeJS will error out if the file doesn't exist).
+ */
+export class SafeReadonlyHost<StatsT extends object = {}> implements ReadonlyHost<StatsT> {
+  constructor(private _delegate: ReadonlyHost<StatsT>) {}
+
+  get capabilities(): HostCapabilities {
+    return this._delegate.capabilities;
+  }
+
+  read(path: Path): Observable<FileBuffer> {
+    return this._delegate.read(path);
+  }
+
+  list(path: Path): Observable<PathFragment[]> {
+    return this._delegate.list(path).pipe(
+      catchError(() => of([])),
+    );
+  }
+
+  exists(path: Path): Observable<boolean> {
+    return this._delegate.exists(path);
+  }
+  isDirectory(path: Path): Observable<boolean> {
+    return this._delegate.isDirectory(path).pipe(
+      catchError(() => of(false)),
+    );
+  }
+  isFile(path: Path): Observable<boolean> {
+    return this._delegate.isFile(path).pipe(
+      catchError(() => of(false)),
+    );
+  }
+
+  // Some hosts may not support stats.
+  stat(path: Path): Observable<Stats<StatsT> | null> | null {
+    const maybeStat = this._delegate.stat(path);
+
+    return maybeStat && maybeStat.pipe(
+      catchError(() => of(null)),
+    );
+  }
+}