copyFileSync from one file system to another

src/__tests__/union.test.ts

@@ -183,6 +183,24 @@ describe('union', () => {
           expect(() => ufs.readdirSync('/bar')).toThrow();
         });
       });
+
+      describe('copyFileSync()', () => {
+        it('can copy from one filesystem to another', async () => {
+          const vol1 = Volume.fromJSON({
+            '/foo/bar': 'bar',
+          });
+          const vol2 = Volume.fromJSON({
+            '/bar/baz': 'baz',
+          });
+
+          const ufs = new Union();
+          ufs.use(vol1 as any).use(vol2 as any);
+
+          ufs.copyFileSync('/foo/bar', '/bar/barCopy');
+          expect(vol2.readFileSync('/bar/barCopy').toString()).toEqual('bar');
+          expect(vol1.existsSync('/bar/barCopy')).toBeFalsy();
+        });
+      });
     });
     describe('async methods', () => {
       it('Basic one file system', done => {

src/fs.ts

@@ -32,6 +32,7 @@ type FSMethods =
   | 'readFileSync'
   | 'writeFileSync'
   | 'appendFileSync'
+  | 'copyFileSync'
   | 'existsSync'
   | 'accessSync'
   | 'createReadStream'

src/union.ts

@@ -1,6 +1,8 @@
 import { FSWatcher, Dirent } from 'fs';
 import { IFS } from './fs';
 import { Readable, Writable } from 'stream';
+import { dirname } from 'path';
+import { ENOENT } from 'constants';
 const { fsAsyncMethods, fsSyncMethods } = require('fs-monkey/lib/util/lists');
 
 export interface IUnionFsError extends Error {
@@ -13,6 +15,7 @@ const SPECIAL_METHODS = new Set([
   'existsSync',
   'readdir',
   'readdirSync',
+  'copyFileSync',
   'createReadStream',
   'createWriteStream',
   'watch',
@@ -245,6 +248,42 @@ export class Union {
     return this.sortedArrayFromReaddirResult(result);
   };
 
+  public copyFileSync(src: string, dest: string, flags?: number) {
+    const destDir = dirname(dest);
+
+    // CASE 1: dest is a filename. Don't treat, standard copy (opinionated)
+    if (destDir.length == 0) {
+      return this.syncMethod('copyFileSync', [src, dest, flags]);
+    }
+
+    // CASE 2: src doesn't exist in any fs.
+    const srcFS = this.fss.find(fs => {
+      try { return fs.existsSync(src) }
+      catch (error) { return false }; 
+    })
+
+    if (!srcFS) {
+      const error = new Error(`no such source file: ${src}`) as NodeJS.ErrnoException;
+      error.errno = ENOENT;
+      throw error;
+    }
+
+    // CASE 3: dest dir does not exist or is in the same fs.
+    // To protect ourselves, simply launch standard copy, whatever the policies
+    // about destination dir doesn't exist.
+    const destFS = this.fss.find(fs => {
+      try { return fs.existsSync(destDir) }
+      catch (error) { return false };
+    });
+
+    if (!destFS || srcFS == destFS) {
+      return this.syncMethod('copyFileSync', [src, dest, flags]);
+    }
+
+    // CASE 4: different FS. Workaround: read src, write dest.
+    destFS.writeFileSync(dest, srcFS.readFileSync(src));
+  }
+
   public readdirPromise = async (...args): Promise<Array<readdirEntry>> => {
     let lastError: IUnionFsError | null = null;
     let result = new Map<string, readdirEntry>();