fix: handle concurrent writes on windows

Windows returns EPERM errors when trying to rename temp files to
files that already exist. In our case a file with a given name
will always have the same content so if it's created while we
are trying to also create it, we can reasonably assume it's ok
to use.

If we want to be more thorough we could hash the contents of the
new file.

src/index.js

@@ -10,7 +10,7 @@ const setImmediate = require('async/setImmediate')
 const waterfall = require('async/series')
 const each = require('async/each')
 const mkdirp = require('mkdirp')
-const writeFile = require('fast-write-atomic')
+const writeAtomic = require('fast-write-atomic')
 const path = require('path')
 
 const asyncFilter = require('interface-datastore').utils.asyncFilter
@@ -19,6 +19,32 @@ const IDatastore = require('interface-datastore')
 const Key = IDatastore.Key
 const Errors = IDatastore.Errors
 
+function writeFile (path, contents, callback) {
+  writeAtomic(path, contents, (err) => {
+    if (err) {
+      if (err.code === 'EPERM' && err.syscall === 'rename') {
+        // fast-write-atomic writes a file to a temp location before renaming it.
+        // On Windows, if the final file already exists this error is thrown.
+        // No such error is thrown on Linux/Mac
+        // Make sure we can read & write to this file
+        return fs.access(path, fs.constants.F_OK | fs.constants.W_OK, (err) => {
+          if (err) {
+            return callback(err)
+          }
+
+          // The file was created by another context - this means there were
+          // attempts to write the same block by two different function calls
+          return callback()
+        })
+      }
+
+      return callback(err)
+    }
+
+    callback()
+  })
+}
+
 /* :: export type FsInputOptions = {
   createIfMissing?: bool,
   errorIfExists?: bool,

test/index.spec.js

@@ -153,4 +153,29 @@ describe('FsDatastore', () => {
       }
     })
   })
+
+  it('can survive concurrent writes', (done) => {
+    const dir = utils.tmpdir()
+    const fstore = new FsStore(dir)
+    const key = new Key('CIQGFTQ7FSI2COUXWWLOQ45VUM2GUZCGAXLWCTOKKPGTUWPXHBNIVOY')
+    const value = Buffer.from('Hello world')
+
+    parallel(
+      new Array(100).fill(0).map(() => {
+        return (cb) => {
+          fstore.put(key, value, cb)
+        }
+      }),
+      (err) => {
+        expect(err).to.not.exist()
+
+        fstore.get(key, (err, res) => {
+          expect(err).to.not.exist()
+          expect(res).to.deep.equal(value)
+
+          done()
+        })
+      }
+    )
+  })
 })