From ee59e61fddc6667538d529eec3b0b19c85d46f10 Mon Sep 17 00:00:00 2001
From: Ibrahim Jarif <ibrahim@dgraph.io>
Date: Wed, 8 Jul 2020 18:49:57 +0530
Subject: [PATCH 1/2] Avoid panic on multiple closer.Signal calls

The closer.Signal function closes a channel and multiple calls to the
closing the same channel would cause a "channel already closed". This PR
avoids the panic by using a sync.Once while closing the channel.

Multiple calls to closer.Signal would be no-op with this commit.
---
 y/y.go | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/y/y.go b/y/y.go
index 14122234f..ea857f1bd 100644
--- a/y/y.go
+++ b/y/y.go
@@ -191,8 +191,9 @@ func FixedDuration(d time.Duration) string {
 // to tell the goroutine to shut down, and a WaitGroup with which to wait for it to finish shutting
 // down.
 type Closer struct {
-	closed  chan struct{}
-	waiting sync.WaitGroup
+	closed    chan struct{}
+	waiting   sync.WaitGroup
+	closeOnce sync.Once
 }
 
 // NewCloser constructs a new Closer, with an initial count on the WaitGroup.
@@ -209,7 +210,9 @@ func (lc *Closer) AddRunning(delta int) {
 
 // Signal signals the HasBeenClosed signal.
 func (lc *Closer) Signal() {
-	close(lc.closed)
+	lc.closeOnce.Do(func() {
+		close(lc.closed)
+	})
 }
 
 // HasBeenClosed gets signaled when Signal() is called.

From 6eb1455d776a125b7d770887722eed67fca89b0f Mon Sep 17 00:00:00 2001
From: Ibrahim Jarif <ibrahim@dgraph.io>
Date: Thu, 9 Jul 2020 14:46:31 +0530
Subject: [PATCH 2/2] Add todo and a unit test

---
 y/y.go      |  1 +
 y/y_test.go | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/y/y.go b/y/y.go
index ea857f1bd..5e6dd5b25 100644
--- a/y/y.go
+++ b/y/y.go
@@ -210,6 +210,7 @@ func (lc *Closer) AddRunning(delta int) {
 
 // Signal signals the HasBeenClosed signal.
 func (lc *Closer) Signal() {
+	// Todo(ibrahim): Change Signal to return error on next badger breaking change.
 	lc.closeOnce.Do(func() {
 		close(lc.closed)
 	})
diff --git a/y/y_test.go b/y/y_test.go
index d1b963184..fe14cf4c8 100644
--- a/y/y_test.go
+++ b/y/y_test.go
@@ -254,3 +254,20 @@ func TestPagebufferReader4(t *testing.T) {
 	require.Equal(t, err, io.EOF, "should return EOF")
 	require.Equal(t, n, 0)
 }
+
+func TestMulipleSignals(t *testing.T) {
+	closer := NewCloser(0)
+	require.NotPanics(t, func() { closer.Signal() })
+	// Should not panic.
+	require.NotPanics(t, func() { closer.Signal() })
+	require.NotPanics(t, func() { closer.SignalAndWait() })
+
+	// Attempt 2.
+	closer = NewCloser(1)
+	require.NotPanics(t, func() { closer.Done() })
+
+	require.NotPanics(t, func() { closer.SignalAndWait() })
+	// Should not panic.
+	require.NotPanics(t, func() { closer.SignalAndWait() })
+	require.NotPanics(t, func() { closer.Signal() })
+}