Invalidate cache when dependencies included in parent change (#514)

Some dependencies are pre-compiled into their parent assets, e.g. stylus imports. But we still need to invalidate the cache if any of those files change.

Now the FSCache looks for deps with `includedInParent: true`, and stores an mtime in the cache file. When reading from the cache, those deps are checked to see if they changed and if so, the parent asset is invalidated.

src/FSCache.js

@@ -3,7 +3,6 @@ const path = require('path');
 const md5 = require('./utils/md5');
 const objectHash = require('./utils/objectHash');
 const pkg = require('../package.json');
-const json5 = require('json5');
 
 // These keys can affect the output, so if they differ, the cache should not match
 const OPTION_KEYS = ['publicURL', 'minify', 'hmr'];
@@ -30,16 +29,42 @@ class FSCache {
     return path.join(this.dir, hash + '.json');
   }
 
+  async writeDepMtimes(data) {
+    // Write mtimes for each dependent file that is already compiled into this asset
+    for (let dep of data.dependencies) {
+      if (dep.includedInParent) {
+        let stats = await fs.stat(dep.name);
+        dep.mtime = stats.mtime.getTime();
+      }
+    }
+  }
+
   async write(filename, data) {
     try {
       await this.ensureDirExists();
+      await this.writeDepMtimes(data);
       await fs.writeFile(this.getCacheFile(filename), JSON.stringify(data));
       this.invalidated.delete(filename);
     } catch (err) {
       console.error('Error writing to cache', err);
     }
   }
 
+  async checkDepMtimes(data) {
+    // Check mtimes for files that are already compiled into this asset
+    // If any of them changed, invalidate.
+    for (let dep of data.dependencies) {
+      if (dep.includedInParent) {
+        let stats = await fs.stat(dep.name);
+        if (stats.mtime > dep.mtime) {
+          return false;
+        }
+      }
+    }
+
+    return true;
+  }
+
   async read(filename) {
     if (this.invalidated.has(filename)) {
       return null;
@@ -55,8 +80,13 @@ class FSCache {
         return null;
       }
 
-      let data = await fs.readFile(cacheFile);
-      return json5.parse(data);
+      let json = await fs.readFile(cacheFile);
+      let data = JSON.parse(json);
+      if (!await this.checkDepMtimes(data)) {
+        return null;
+      }
+
+      return data;
     } catch (err) {
       return null;
     }