feat: cache precision per device

See #103

Author: julien-f

lib/mtime-precision.js

@@ -3,40 +3,48 @@
 const cacheSymbol = Symbol();
 
 function probe(file, fs, callback) {
-    const cachedPrecision = fs[cacheSymbol];
+    let cachedPrecisions = fs[cacheSymbol];
+    if (cachedPrecisions === undefined) {
+        cachedPrecisions = new Map();
 
-    if (cachedPrecision) {
-        return fs.stat(file, (err, stat) => {
-            /* istanbul ignore if */
-            if (err) {
-                return callback(err);
-            }
-
-            callback(null, stat.mtime, cachedPrecision);
-        });
+        // Cache the precisions in a non-enumerable way
+        Object.defineProperty(fs, cacheSymbol, { value: cachedPrecisions });
     }
 
-    // Set mtime by ceiling Date.now() to seconds + 5ms so that it's "not on the second"
-    const mtime = new Date((Math.ceil(Date.now() / 1000) * 1000) + 5);
-
-    fs.utimes(file, mtime, mtime, (err) => {
+    return fs.stat(file, (err, stat) => {
         /* istanbul ignore if */
         if (err) {
             return callback(err);
         }
 
-        fs.stat(file, (err, stat) => {
+        const dev = stat.dev;
+
+        // Precisions are cached by device, see #103
+        const precision = cachedPrecisions[dev];
+        if (precision !== undefined) {
+            return callback(null, stat.mtime, precision);
+        }
+
+        // Set mtime by ceiling Date.now() to seconds + 5ms so that it's "not on the second"
+        const mtime = new Date(Math.ceil(Date.now() / 1000) * 1000 + 5);
+
+        fs.utimes(file, mtime, mtime, (err) => {
             /* istanbul ignore if */
             if (err) {
                 return callback(err);
             }
 
-            const precision = stat.mtime.getTime() % 1000 === 0 ? 's' : 'ms';
+            fs.stat(file, (err, stat) => {
+                /* istanbul ignore if */
+                if (err) {
+                    return callback(err);
+                }
 
-            // Cache the precision in a non-enumerable way
-            Object.defineProperty(fs, cacheSymbol, { value: precision });
+                const precision = stat.mtime.getTime() % 1000 === 0 ? 's' : 'ms';
+                cachedPrecisions.set(dev, precision);
 
-            callback(null, stat.mtime, precision);
+                callback(null, stat.mtime, precision);
+            });
         });
     });
 }