Minimize blocking in Filesystem.getCachedDiskUsage (#53)

server/filesystem.go

@@ -41,10 +41,11 @@ func IsPathResolutionError(err error) bool {
 }
 
 type Filesystem struct {
-	mu sync.RWMutex
+	mu sync.Mutex
 
-	lastLookupTime time.Time
-	diskUsage      int64
+	lastLookupTime   time.Time
+	lookupInProgress int32
+	diskUsage        int64
 
 	Server *Server
 }
@@ -211,8 +212,10 @@ func (fs *Filesystem) ParallelSafePath(paths []string) ([]string, error) {
 //
 // Because determining the amount of space being used by a server is a taxing operation we
 // will load it all up into a cache and pull from that as long as the key is not expired.
-func (fs *Filesystem) HasSpaceAvailable() bool {
-	size, err := fs.getCachedDiskUsage()
+//
+// This operation will potentially block unless nonBlocking is true
+func (fs *Filesystem) HasSpaceAvailable(nonBlocking bool) bool {
+	size, err := fs.getCachedDiskUsage(nonBlocking)
 	if err != nil {
 		fs.Server.Log().WithField("error", err).Warn("failed to determine root server directory size")
 	}
@@ -238,20 +241,40 @@ func (fs *Filesystem) HasSpaceAvailable() bool {
 // as needed without overly taxing the system. This will prioritize the value from the cache to avoid
 // excessive IO usage. We will only walk the filesystem and determine the size of the directory if there
 // is no longer a cached value.
-func (fs *Filesystem) getCachedDiskUsage() (int64, error) {
+// This will potentially block unless nonBlocking is true.
+func (fs *Filesystem) getCachedDiskUsage(nonBlocking bool) (int64, error) {
+
+	// Check if cache is expired...
+	if !fs.lastLookupTime.After(time.Now().Add(time.Second * -150)) {
+		// We're OK with blocking, so go ahead and block
+		if !nonBlocking {
+			return fs.updateCachedDiskUsage()
+		}
+
+		// Otherwise, we're fine with not blocking, but still need to update the cache. (If it isn't being done already)
+		if atomic.LoadInt32(&fs.lookupInProgress) != 1 {
+			go fs.updateCachedDiskUsage()
+		}
+
+	}
+
+	// Go ahead and return the cached value
+	return atomic.LoadInt64(&fs.diskUsage), nil
+}
+
+func (fs *Filesystem) updateCachedDiskUsage() (int64, error) {
+
 	// Obtain an exclusive lock on this process so that we don't unintentionally run it at the same
 	// time as another running process. Once the lock is available it'll read from the cache for the
 	// second call rather than hitting the disk in parallel.
-	//
-	// This effectively the same speed as running this call in parallel since this cache will return
-	// instantly on the second call.
 	fs.mu.Lock()
 	defer fs.mu.Unlock()
 
-	// Expire the cache after 2.5 minutes.
-	if fs.lastLookupTime.After(time.Now().Add(time.Second * -150)) {
-		return fs.diskUsage, nil
-	}
+	// Always clear the in progress flag
+	defer atomic.StoreInt32(&fs.lookupInProgress, 0)
+
+	// Signal that we're currently updating the disk size, to prevent other routines to block on this.
+	atomic.StoreInt32(&fs.lookupInProgress, 1)
 
 	// If there is no size its either because there is no data (in which case running this function
 	// will have effectively no impact), or there is nothing in the cache, in which case we need to