brushed up according to review

Added LockingWriteMultipleOnly and LockingWriteOnceOnly interfaces,
so that further extensions are possible (in this package or others).

Moved common SetLocked behavior into promisoid.

Made comments say things that were implied.

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/fairqueuing/promise/interface.go

@@ -16,8 +16,10 @@ limitations under the License.
 
 package promise
 
-// This file defines interfaces for promsies and futures and related
-// things.
+// This file defines interfaces for promises and futures and related
+// things.  These are about coordination among multiple goroutines and
+// so are safe for concurrent calls --- although moderated in some
+// cases by a requirement that the caller hold a certain lock.
 
 // Readable represents a variable that is initially not set and later
 // becomes set.  Some instances may be set to multiple values in
@@ -39,10 +41,16 @@ type Readable interface {
 type LockingReadable interface {
 	Readable
 
-	// GetLocked is like Get but the caller must already hold the lock
+	// GetLocked is like Get but the caller must already hold the
+	// lock.  GetLocked may release, and later re-acquire, the lock
+	// any number of times.  Get may acquire, and later release, the
+	// lock any number of times.
 	GetLocked() interface{}
 
-	// IsSetLocked is like IsSet but the caller must already hold the lock
+	// IsSetLocked is like IsSet but the caller must already hold the
+	// lock.  IsSetLocked may release, and later re-acquire, the lock
+	// any number of times.  IsSet may acquire, and later release, the
+	// lock any number of times.
 	IsSetLocked() bool
 }
 
@@ -52,7 +60,8 @@ type WriteOnceOnly interface {
 	// Set normally writes a value into this variable, unblocks every
 	// goroutine waiting for this variable to have a value, and
 	// returns true.  In the unhappy case that this variable is
-	// already set, this method returns false.
+	// already set, this method returns false without modifying the
+	// variable's value.
 	Set(interface{}) bool
 }
 
@@ -64,14 +73,23 @@ type WriteOnce interface {
 	WriteOnceOnly
 }
 
+// LockingWriteOnceOnly is a WriteOnceOnly whose implementation is
+// protected by a lock.
+type LockingWriteOnceOnly interface {
+	WriteOnceOnly
+
+	// SetLocked is like Set but the caller must already hold the
+	// lock.  SetLocked may release, and later re-acquire, the lock
+	// any number of times.  Set may acquire, and later release, the
+	// lock any number of times
+	SetLocked(interface{}) bool
+}
+
 // LockingWriteOnce is a WriteOnce whose implementation is protected
 // by a lock.
 type LockingWriteOnce interface {
 	LockingReadable
-	WriteOnceOnly
-
-	// SetLocked is like Set but the caller must already hold the lock
-	SetLocked(interface{}) bool
+	LockingWriteOnceOnly
 }
 
 // WriteMultipleOnly represents a variable that is initially not set
@@ -91,12 +109,21 @@ type WriteMultiple interface {
 	WriteMultipleOnly
 }
 
+// LockingWriteMultipleOnly is a WriteMultipleOnly whose
+// implementation is protected by a lock.
+type LockingWriteMultipleOnly interface {
+	WriteMultipleOnly
+
+	// SetLocked is like Set but the caller must already hold the
+	// lock.  SetLocked may release, and later re-acquire, the lock
+	// any number of times.  Set may acquire, and later release, the
+	// lock any number of times
+	SetLocked(interface{})
+}
+
 // LockingWriteMultiple is a WriteMultiple whose implementation is
 // protected by a lock.
 type LockingWriteMultiple interface {
 	LockingReadable
-	WriteMultipleOnly
-
-	// SetLocked is like Set but the caller must already hold the lock
-	SetLocked(interface{})
+	LockingWriteMultipleOnly
 }

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/fairqueuing/promise/lockingpromise/lockingpromise.go

@@ -23,12 +23,12 @@ import (
 	"k8s.io/apiserver/pkg/util/flowcontrol/fairqueuing/promise"
 )
 
-// promisoid is the data and reading behavior common to all the
-// promise-like abstractions implemented here.  This implementation is
-// based on a condition variable.  This implementation tracks active
-// goroutines: the given counter is decremented for a goroutine
-// waiting for this varible to be set and incremented when such a
-// goroutine is unblocked.
+// promisoid is the data and behavior common to all the promise-like
+// abstractions implemented here.  This implementation is based on a
+// condition variable.  This implementation tracks active goroutines:
+// the given counter is decremented for a goroutine waiting for this
+// varible to be set and incremented when such a goroutine is
+// unblocked.
 type promisoid struct {
 	lock          sync.Locker
 	cond          sync.Cond
@@ -63,6 +63,16 @@ func (pr *promisoid) IsSetLocked() bool {
 	return pr.isSet
 }
 
+func (pr *promisoid) SetLocked(value interface{}) {
+	pr.isSet = true
+	pr.value = value
+	if pr.waitingCount > 0 {
+		pr.activeCounter.Add(pr.waitingCount)
+		pr.waitingCount = 0
+		pr.cond.Broadcast()
+	}
+}
+
 type writeOnce struct {
 	promisoid
 }
@@ -88,13 +98,7 @@ func (wr *writeOnce) SetLocked(value interface{}) bool {
 	if wr.isSet {
 		return false
 	}
-	wr.isSet = true
-	wr.value = value
-	if wr.waitingCount > 0 {
-		wr.activeCounter.Add(wr.waitingCount)
-		wr.waitingCount = 0
-		wr.cond.Broadcast()
-	}
+	wr.promisoid.SetLocked(value)
 	return true
 }
 
@@ -118,13 +122,3 @@ func (wr *writeMultiple) Set(value interface{}) {
 	defer wr.lock.Unlock()
 	wr.SetLocked(value)
 }
-
-func (wr *writeMultiple) SetLocked(value interface{}) {
-	wr.isSet = true
-	wr.value = value
-	if wr.waitingCount > 0 {
-		wr.activeCounter.Add(wr.waitingCount)
-		wr.waitingCount = 0
-		wr.cond.Broadcast()
-	}
-}