Lock file during patching.

Closes #1106.

Author: RubenVerborgh

lib/handlers/patch.js

@@ -10,6 +10,7 @@ const $rdf = require('rdflib')
 const crypto = require('crypto')
 const overQuota = require('../utils').overQuota
 const getContentType = require('../utils').getContentType
+const { lock } = require('proper-lockfile')
 
 // Patch parsers by request body content type
 const PATCH_PARSERS = {
@@ -23,6 +24,7 @@ const DEFAULT_FOR_NEW_CONTENT_TYPE = 'text/turtle'
 async function patchHandler (req, res, next) {
   debug(`PATCH -- ${req.originalUrl}`)
   res.header('MS-Author-Via', 'SPARQL')
+  let releaseLock
   try {
     // Obtain details of the target resource
     const ldp = req.app.locals.ldp
@@ -50,22 +52,24 @@ async function patchHandler (req, res, next) {
       throw error(415, `Unsupported patch content type: ${patch.contentType}`)
     }
 
-    // Parse the target graph and the patch document,
-    // and verify permission for performing this specific patch
-    const [graph, patchObject] = await Promise.all([
-      readGraph(resource),
-      parsePatch(url, patch.uri, patch.text)
-        .then(patchObject => checkPermission(req, patchObject))
-    ])
+    // Parse the patch document and verify permissions
+    const patchObject = await parsePatch(url, patch.uri, patch.text)
+    await checkPermission(req, patchObject)
 
     // Patch the graph and write it back to the file
+    releaseLock = await lock(path, { retries: 10, realpath: false })
+    const graph = await readGraph(resource)
     await applyPatch(patchObject, graph, url)
     const result = await writeGraph(graph, resource, ldp.resourceMapper.rootPath, ldp.serverUri)
 
     // Send the result to the client
     res.send(result)
   } catch (err) {
     return next(err)
+  } finally {
+    if (releaseLock) {
+      await releaseLock()
+    }
   }
   return next()
 }