Gracefully handle symlinks to ., .., etc in require.context

Summary:
`matchFilesWithContext` powers `require.context`, returning all paths under a given root matching a given `regex`.

This is originally an established [Webpack feature](https://webpack.js.org/guides/dependency-management/#requirecontext), and we aim to match behaviour as closely as we can. Webpack *does* follow symlinks when enumerating paths prior to matching, so that for example in the following file layout:

```
 - link-to-foo -> foo
 - foo
 - - bar.js
```

`link-to-foo/bar.js` is enumerated, so that `require.context(myRoot, true, /link-to-foo\/bar.js/)` matches.

Webpack's implementation of this uses recursive `fs.readdir` calls, following symlinks, to enumerate all reachable paths.

This raises the question of handling a kind of cycle, for example:

```
 - foo -> .
 - bar.js
```

Would theoretically match `./bar.js`, `./foo/bar.js`, `./foo/foo/bar.js` and so on.

Webpack actually dies on this, throwing `ELOOP` from `fs`. Similarly, currently, Metro will crash with call stack size exceeded inside `_pathIterator`.

This diff makes the (arguably arbitrary, but reasonablish) choice to follow symlinks at most once per path, by tracking "seen" links in a set during enumeration.

```
 - foo -> .
 - baz -> .
 - bar.js
```
Expands to:
```
  bar.js
  foo/bar.js
  foo/baz/bar.js
  baz/bar.js
  baz/foo/bar.js
```

Changelog:
```
 - **[Experimental]**: Fix crash when `require.context` is used on a directory with infinite path expansions.

Reviewed By: motiz88

Differential Revision: D46767544

fbshipit-source-id: ed3d17b3afcf27189aa442627a31b6225fd873bf

Author: robhogan

packages/metro-file-map/src/lib/TreeFS.js

@@ -458,6 +458,7 @@ export default class TreeFS implements MutableFileSystem {
       subtreeOnly: boolean,
     }>,
     pathPrefix: string = '',
+    followedLinks: $ReadOnlySet<FileMetaData> = new Set(),
   ): Iterable<Path> {
     const pathSep = opts.alwaysYieldPosix ? '/' : path.sep;
     const prefixWithSep = pathPrefix === '' ? pathPrefix : pathPrefix + pathSep;
@@ -498,17 +499,27 @@ export default class TreeFS implements MutableFileSystem {
             // Symlink goes nowhere, nothing to report.
             continue;
           }
-          if (!(resolved.node instanceof Map)) {
+          const target = resolved.node;
+          if (!(target instanceof Map)) {
             // Symlink points to a file, just yield the path of the symlink.
             yield nodePath;
-          } else if (opts.recursive && opts.follow) {
+          } else if (
+            opts.recursive &&
+            opts.follow &&
+            !followedLinks.has(node)
+          ) {
             // Symlink points to a directory - iterate over its contents using
             // the path where we found the symlink as a prefix.
-            yield* this._pathIterator(resolved.node, opts, nodePath);
+            yield* this._pathIterator(
+              target,
+              opts,
+              nodePath,
+              new Set([...followedLinks, node]),
+            );
           }
         }
       } else if (opts.recursive) {
-        yield* this._pathIterator(node, opts, nodePath);
+        yield* this._pathIterator(node, opts, nodePath, followedLinks);
       }
     }
   }

packages/metro-file-map/src/lib/__tests__/TreeFS-test.js

@@ -32,11 +32,13 @@ describe.each([['win32'], ['posix']])('TreeFS on %s', platform => {
       rootDir: p('/project'),
       files: new Map([
         [p('foo/another.js'), ['', 123, 0, 0, '', '', 0]],
+        [p('foo/owndir'), ['', 0, 0, 0, '', '', '.']],
         [p('foo/link-to-bar.js'), ['', 0, 0, 0, '', '', p('../bar.js')]],
         [p('foo/link-to-another.js'), ['', 0, 0, 0, '', '', p('another.js')]],
         [p('../outside/external.js'), ['', 0, 0, 0, '', '', 0]],
         [p('bar.js'), ['', 234, 0, 0, '', '', 0]],
-        [p('link-to-foo'), ['', 456, 0, 0, '', '', p('./foo')]],
+        [p('link-to-foo'), ['', 456, 0, 0, '', '', p('././abnormal/../foo')]],
+        [p('abs-link-out'), ['', 456, 0, 0, '', '', p('/outside/./baz/..')]],
         [p('root'), ['', 0, 0, 0, '', '', '..']],
         [p('link-to-nowhere'), ['', 123, 0, 0, '', '', p('./nowhere')]],
         [p('link-to-self'), ['', 123, 0, 0, '', '', p('./link-to-self')]],
@@ -129,10 +131,12 @@ describe.each([['win32'], ['posix']])('TreeFS on %s', platform => {
           [p('link-to-self'), ['', 123, 0, 0, '', '', 0]],
         ]),
         removedFiles: new Set([
+          p('foo/owndir'),
           p('foo/link-to-bar.js'),
           p('foo/link-to-another.js'),
           p('../outside/external.js'),
           p('bar.js'),
+          p('abs-link-out'),
           p('root'),
         ]),
       });
@@ -162,6 +166,9 @@ describe.each([['win32'], ['posix']])('TreeFS on %s', platform => {
         }),
       ).toEqual([
         p('/project/foo/another.js'),
+        p('/project/foo/owndir/another.js'),
+        p('/project/foo/owndir/link-to-bar.js'),
+        p('/project/foo/owndir/link-to-another.js'),
         p('/project/foo/link-to-bar.js'),
         p('/project/foo/link-to-another.js'),
       ]);
@@ -186,12 +193,19 @@ describe.each([['win32'], ['posix']])('TreeFS on %s', platform => {
         }),
       ).toEqual([
         p('/project/foo/another.js'),
+        p('/project/foo/owndir/another.js'),
+        p('/project/foo/owndir/link-to-bar.js'),
+        p('/project/foo/owndir/link-to-another.js'),
         p('/project/foo/link-to-bar.js'),
         p('/project/foo/link-to-another.js'),
         p('/project/bar.js'),
         p('/project/link-to-foo/another.js'),
+        p('/project/link-to-foo/owndir/another.js'),
+        p('/project/link-to-foo/owndir/link-to-bar.js'),
+        p('/project/link-to-foo/owndir/link-to-another.js'),
         p('/project/link-to-foo/link-to-bar.js'),
         p('/project/link-to-foo/link-to-another.js'),
+        p('/project/abs-link-out/external.js'),
         p('/project/root/outside/external.js'),
       ]);
     });
@@ -220,7 +234,9 @@ describe.each([['win32'], ['posix']])('TreeFS on %s', platform => {
         }),
       ).toEqual([
         p('/project/foo/another.js'),
+        p('/project/foo/owndir/another.js'),
         p('/project/link-to-foo/another.js'),
+        p('/project/link-to-foo/owndir/another.js'),
       ]);
     });
   });