fix: handle concurrent writes on windows

Windows can return 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

@@ -4,7 +4,7 @@ const fs = require('fs')
 const glob = require('glob')
 const mkdirp = require('mkdirp')
 const promisify = require('util').promisify
-const writeFile = promisify(require('fast-write-atomic'))
+const writeAtomic = promisify(require('fast-write-atomic'))
 const path = require('path')
 
 const filter = require('interface-datastore').utils.filter
@@ -22,6 +22,26 @@ const fsUnlink = promisify(fs.unlink || noop)
 const Key = IDatastore.Key
 const Errors = IDatastore.Errors
 
+async function writeFile (path, contents) {
+  try {
+    await writeAtomic(path, contents)
+  } catch (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
+      await fsAccess(path, fs.constants.F_OK | fs.constants.W_OK)
+
+      // The file was created by another context - this means there were
+      // attempts to write the same block by two different function calls
+      return
+    }
+
+    throw err
+  }
+}
+
 /**
  * A datastore backed by the file system.
  *

test/index.spec.js

@@ -172,4 +172,19 @@ describe('FsDatastore', () => {
       }
     })
   })
+
+  it('can survive concurrent writes', async () => {
+    const dir = utils.tmpdir()
+    const fstore = new FsStore(dir)
+    const key = new Key('CIQGFTQ7FSI2COUXWWLOQ45VUM2GUZCGAXLWCTOKKPGTUWPXHBNIVOY')
+    const value = Buffer.from('Hello world')
+
+    await Promise.all(
+      new Array(100).fill(0).map(() => fstore.put(key, value))
+    )
+
+    const res = await fstore.get(key)
+
+    expect(res).to.deep.equal(value)
+  })
 })