[SRE] Applying the Elm Corrupt Cache fix

index.js

@@ -6,6 +6,9 @@ var temp = require("temp").track();
 var loaderUtils = require("loader-utils");
 var elmCompiler = require("node-elm-compiler");
 
+let elmCompilerSingleton = Promise.resolve();
+
+
 var getFiles = function(options) {
     var files = options && options.files;
 
@@ -124,9 +127,23 @@ module.exports = function() {
         promises.push(dependencies);
     }
 
-    var compilation = compile(files, options)
-        .then(function(v) { return { kind: "success", result: v }; })
-        .catch(function(v) { return { kind: "error", error: v }; });
+
+    var compilation = aquireLock()
+        .then(releaseLock => 
+            new Promise((resolve, reject) => 
+                compile(files, options)
+                        .then(v => resolve({v, releaseLock}))
+                        .catch(e => reject(e))
+                )
+        )
+        .then(function({v, releaseLock}) { 
+            releaseLock();
+            return { kind: "success", result: v }; 
+        })
+        .catch(function({v, releaseLock}) { 
+            releaseLock()
+            return { kind: "error", error: v }; 
+        });
 
     promises.push(compilation);
 
@@ -216,3 +233,26 @@ function remove(condemned, items) {
         return item !== condemned;
     });
 }
+
+
+// CONCURRENCY STUFF??
+
+/**
+ * The Elm compiler may only run once in the same folder at any given moment.
+ * If it runs multiple time, it will corrupt the elm-stuff folder for the other Elm compiler
+ * processes.
+ *
+ * This lock guarantees the sequential compilation of all Elm files.
+ */
+function aquireLock() {
+    let release;
+    const before = elmCompilerSingleton;
+    elmCompilerSingleton = new Promise((resolve) => {
+        release = resolve;
+    });
+    return new Promise((resolve, reject) => 
+        before
+            .then(() => resolve(release))
+            .catch(e => reject(e))
+    )
+}