[libbeat] Enable WriteAheadLimit in the disk queue (elastic#21391)

(cherry picked from commit ef6274d)

libbeat/publisher/queue/diskqueue/config.go

@@ -106,8 +106,8 @@ func DefaultSettings() Settings {
 		MaxSegmentSize: 100 * (1 << 20), // 100MiB
 		MaxBufferSize:  (1 << 30),       // 1GiB
 
-		ReadAheadLimit:  256,
-		WriteAheadLimit: 1024,
+		ReadAheadLimit:  512,
+		WriteAheadLimit: 2048,
 	}
 }
 
@@ -129,6 +129,14 @@ func SettingsForUserConfig(config *common.Config) (Settings, error) {
 		// divided by 10.
 		settings.MaxSegmentSize = uint64(userConfig.MaxSize) / 10
 	}
+
+	if userConfig.ReadAheadLimit != nil {
+		settings.ReadAheadLimit = *userConfig.ReadAheadLimit
+	}
+	if userConfig.WriteAheadLimit != nil {
+		settings.WriteAheadLimit = *userConfig.WriteAheadLimit
+	}
+
 	return settings, nil
 }
 

libbeat/publisher/queue/diskqueue/core_loop.go

@@ -58,10 +58,15 @@ func (dq *diskQueue) run() {
 			// The writer loop completed a request, so check if there is more
 			// data to be sent.
 			dq.maybeWritePending()
-			// We also check whether the reader loop is waiting for the data
-			// that was just written.
+
+			// The data that was just written is now available for reading, so check
+			// if we should start a new read request.
 			dq.maybeReadPending()
 
+			// pendingFrames should now be empty. If any producers were blocked
+			// because pendingFrames hit settings.WriteAheadLimit, wake them up.
+			dq.maybeUnblockProducers()
+
 		// Reader loop handling
 		case readerLoopResponse := <-dq.readerLoop.responseChan:
 			dq.handleReaderLoopResponse(readerLoopResponse)
@@ -417,22 +422,25 @@ func (dq *diskQueue) enqueueWriteFrame(frame *writeFrame) {
 	})
 }
 
-// canAcceptFrameOfSize checks whether there is enough free space in the
-// queue (subject to settings.MaxBufferSize) to accept a new frame with
-// the given size. Size includes both the serialized data and the frame
-// header / footer; the easy way to do this for a writeFrame is to pass
+// canAcceptFrameOfSize checks whether there is enough free space in the queue
+// (subject to settings.{MaxBufferSize, WriteAheadLimit}) to accept a new
+// frame with the given size. Size includes both the serialized data and the
+// frame header / footer; the easy way to do this for a writeFrame is to pass
 // in frame.sizeOnDisk().
 // Capacity calculations do not include requests in the blockedProducers
 // list (that data is owned by its callers and we can't touch it until
 // we are ready to respond). That allows this helper to be used both while
 // handling producer requests and while deciding whether to unblock
 // producers after free capacity increases.
-// If we decide to add limits on how many events / bytes can be stored
-// in pendingFrames (to avoid unbounded memory use if the input is faster
-// than the disk), this is the function to modify.
 func (dq *diskQueue) canAcceptFrameOfSize(frameSize uint64) bool {
+	// If pendingFrames is already at the WriteAheadLimit, we can't accept
+	// any new frames right now.
+	if len(dq.pendingFrames) >= dq.settings.WriteAheadLimit {
+		return false
+	}
+
+	// If the queue size is unbounded (max == 0), we accept.
 	if dq.settings.MaxBufferSize == 0 {
-		// Currently we impose no limitations if the queue size is unbounded.
 		return true
 	}