storage.js: check fsync capability from return code rather than using `process.platform` heuristics

- silently accept (in)capability to fsync directories based on actual storage backend/driver behavior and capabilities; evaluate error codes rather than `process.platform` heuristics
- make byline test run with windows line endings to validate storage behavior on windows
- add shebang in EMFILE exhaustion test

lib/storage.js

@@ -50,15 +50,25 @@ storage.flushToStorage = (options, callback) => {
     flags = options.isDir ? 'r' : 'r+'
   }
 
-  // Windows can't fsync (FlushFileBuffers) directories. We can live with this as it cannot cause 100% dataloss
-  // except in the very rare event of the first time database is loaded and a crash happens
-  if (flags === 'r' && (process.platform === 'win32' || process.platform === 'win64')) return callback(null)
+  /**
+   * Some OSes and/or storage backends (augmented node fs) do not support fsync (FlushFileBuffers) directories,
+   * or calling open() on directories at all. Flushing fails silently in this case, supported by following heuristics:
+   *  + isDir === true
+   *  |-- open(<dir>) -> (err.code === 'EISDIR'): can't call open() on directories (eg. BrowserFS)
+   *  `-- fsync(<dir>) -> (errFS.code === 'EPERM' || errFS.code === 'EISDIR'): can't fsync directory: permissions are checked
+   *        on open(); EPERM error should only occur on fsync incapability and not for general lack of permissions (e.g. Windows)
+   *
+   * We can live with this as it cannot cause 100% dataloss except in the very rare event of the first time
+   * database is loaded and a crash happens.
+   */
 
   fs.open(filename, flags, (err, fd) => {
-    if (err) return callback(err)
+    if (err) {
+      return callback((err.code === 'EISDIR' && options.isDir) ? null : err)
+    }
     fs.fsync(fd, errFS => {
       fs.close(fd, errC => {
-        if (errFS || errC) {
+        if ((errFS || errC) && !((errFS.code === 'EPERM' || errFS.code === 'EISDIR') && options.isDir)) {
           const e = new Error('Failed to flush to storage')
           e.errorOnFsync = errFS
           e.errorOnClose = errC

test/byline.test.js

@@ -37,7 +37,7 @@ describe('byline', function () {
     lineStream.pipe(output)
     output.on('close', function () {
       const out = fs.readFileSync(localPath('test.txt'), 'utf8')
-      const in_ = fs.readFileSync(localPath('empty.txt'), 'utf8').replace(/\n/g, '')
+      const in_ = fs.readFileSync(localPath('empty.txt'), 'utf8').replace(/\r?\n/g, '')
       assert.equal(in_, out)
       fs.unlinkSync(localPath('test.txt'))
       done()

test_lac/openFdsLaunch.sh

@@ -1,2 +1,3 @@
+#!/bin/sh
 ulimit -n 128
 node ./test_lac/openFds.test.js