From bab077199cc12e2d3c37eb1a42db8f947165cf7b Mon Sep 17 00:00:00 2001
From: Seebs <seebs@seebs.net>
Date: Thu, 14 Nov 2019 17:00:34 -0600
Subject: [PATCH 1/2] use the right lock for Enqueue

The request for a non-read lock blocks until all existing read
locks exit, meaning that if an Immediate operation is already
going for a fragment, an Enqueue operation will hang forever
holding the fragment's lock, while the Immediate operation has
probably relinquished the fragment's lock to wait for the
queue worker to process it. But the queue worker can't process
it, because the incoming Enqueue still holds the fragment's
lock. Solution: Don't block the Enqueue operation like that.
It shouldn't coexist with things that actually change the sq
channels, like Stop(), but it is fine for it to coexist with
other queue operations.
---
 snapshotqueue.go | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/snapshotqueue.go b/snapshotqueue.go
index 210c2b772..c7eb8cabb 100644
--- a/snapshotqueue.go
+++ b/snapshotqueue.go
@@ -200,8 +200,8 @@ func (sq *prioritySnapshotQueue) Enqueue(f *fragment) {
 	if f.snapshotPending {
 		return
 	}
-	sq.mu.Lock()
-	defer sq.mu.Unlock()
+	sq.mu.RLock()
+	defer sq.mu.RUnlock()
 	if sq.normal == nil {
 		sq.logger.Printf("requested snapshot after snapshot queue was closed")
 		return

From f204b37760b79b9eaf39405bae7193dd60448ae6 Mon Sep 17 00:00:00 2001
From: Seebs <seebs@seebs.net>
Date: Thu, 14 Nov 2019 17:39:25 -0600
Subject: [PATCH 2/2] use atomics instead of locking for stats

---
 snapshotqueue.go | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/snapshotqueue.go b/snapshotqueue.go
index c7eb8cabb..d355824e3 100644
--- a/snapshotqueue.go
+++ b/snapshotqueue.go
@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"os"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/pilosa/pilosa/v2/logger"
@@ -108,8 +109,8 @@ type prioritySnapshotQueue struct {
 	mu               sync.RWMutex
 	scanWG, workerWG sync.WaitGroup
 	stats            struct {
-		enqueued int64
-		skipped  int64
+		enqueued uint64
+		skipped  uint64
 	}
 }
 
@@ -214,10 +215,10 @@ func (sq *prioritySnapshotQueue) Enqueue(f *fragment) {
 	// try to enqueue snapshot
 	select {
 	case sq.normal <- snapshotRequest{frag: f, when: time.Now()}:
-		sq.stats.enqueued++
+		atomic.AddUint64(&sq.stats.enqueued, 1)
 		return
 	default:
-		sq.stats.skipped++
+		atomic.AddUint64(&sq.stats.skipped, 1)
 		f.snapshotPending = false
 		return
 	}