From 764cdac01aff837c84cc0cb7490e5424906bcef5 Mon Sep 17 00:00:00 2001
From: Mahmood Ali <mahmood@hashicorp.com>
Date: Mon, 1 Jun 2020 10:18:59 -0400
Subject: [PATCH 1/2] test for paused workers upon leadership revocation

---
 nomad/leader_test.go | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/nomad/leader_test.go b/nomad/leader_test.go
index 03333f639ff..d88c8aac4af 100644
--- a/nomad/leader_test.go
+++ b/nomad/leader_test.go
@@ -1325,6 +1325,37 @@ func TestLeader_TransitionsUpdateConsistencyRead(t *testing.T) {
 	require.True(t, s1.isReadyForConsistentReads())
 }
 
+// TestLeader_PausingWorkers asserts that scheduling workers are paused
+// (and unpaused) upon leader elections (and step downs).
+func TestLeader_PausingWorkers(t *testing.T) {
+	s1, cleanupS1 := TestServer(t, func(c *Config) {
+		c.NumSchedulers = 12
+	})
+	defer cleanupS1()
+	testutil.WaitForLeader(t, s1.RPC)
+	require.Len(t, s1.workers, 12)
+
+	pausedWorkers := func() int {
+		c := 0
+		for _, w := range s1.workers {
+			w.pauseLock.Lock()
+			if w.paused {
+				c++
+			}
+			w.pauseLock.Unlock()
+		}
+		return c
+	}
+
+	// pause 3/4 of the workers
+	require.Equal(t, 9, pausedWorkers())
+
+	err := s1.revokeLeadership()
+	require.NoError(t, err)
+
+	require.Zero(t, pausedWorkers())
+}
+
 // Test doing an inplace upgrade on a server from raft protocol 2 to 3
 // This verifies that removing the server and adding it back with a uuid works
 // even if the server's address stays the same.

From 01fcd2ba9bc1ca8cd69dad96bb228c9d439f86ca Mon Sep 17 00:00:00 2001
From: Mahmood Ali <mahmood@hashicorp.com>
Date: Mon, 1 Jun 2020 10:57:53 -0400
Subject: [PATCH 2/2] leadership: pause and unpause workers consistently

This fixes a bug where leadership establishment pauses 3/4 of workers
but stepping down unpause only 1/2!
---
 nomad/leader.go | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/nomad/leader.go b/nomad/leader.go
index a2281fbc88c..5d5c417b0ee 100644
--- a/nomad/leader.go
+++ b/nomad/leader.go
@@ -206,12 +206,8 @@ func (s *Server) establishLeadership(stopCh chan struct{}) error {
 
 	// Disable workers to free half the cores for use in the plan queue and
 	// evaluation broker
-	if numWorkers := len(s.workers); numWorkers > 1 {
-		// Disabling 3/4 of the workers frees CPU for raft and the
-		// plan applier which uses 1/2 the cores.
-		for i := 0; i < (3 * numWorkers / 4); i++ {
-			s.workers[i].SetPause(true)
-		}
+	for _, w := range s.pausableWorkers() {
+		w.SetPause(true)
 	}
 
 	// Initialize and start the autopilot routine
@@ -913,14 +909,29 @@ func (s *Server) revokeLeadership() error {
 	}
 
 	// Unpause our worker if we paused previously
-	if len(s.workers) > 1 {
-		for i := 0; i < len(s.workers)/2; i++ {
-			s.workers[i].SetPause(false)
-		}
+	for _, w := range s.pausableWorkers() {
+		w.SetPause(false)
 	}
+
 	return nil
 }
 
+// pausableWorkers returns a slice of the workers
+// to pause on leader transitions.
+//
+// Upon leadership establishment, pause workers to free half
+// the cores for use in the plan queue and evaluation broker
+func (s *Server) pausableWorkers() []*Worker {
+	n := len(s.workers)
+	if n <= 1 {
+		return []*Worker{}
+	}
+
+	// Disabling 3/4 of the workers frees CPU for raft and the
+	// plan applier which uses 1/2 the cores.
+	return s.workers[:3*n/4]
+}
+
 // reconcile is used to reconcile the differences between Serf
 // membership and what is reflected in our strongly consistent store.
 func (s *Server) reconcile() error {