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PiperOrigin-RevId: 194583126
Change-Id: Ica1d8821a90f74e7e745962d71801c598c652463

1 files changed, 158 insertions, 0 deletions

diff --git a/pkg/sync/seqcount.go b/pkg/sync/seqcount.go
new file mode 100644
index 000000000..8e3304d69
--- /dev/null
+++ b/pkg/sync/seqcount.go
@@ -0,0 +1,158 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sync
+
+import (
+	"fmt"
+	"reflect"
+	"runtime"
+	"sync/atomic"
+)
+
+// SeqCount is a synchronization primitive for optimistic reader/writer
+// synchronization in cases where readers can work with stale data and
+// therefore do not need to block writers.
+//
+// Compared to sync/atomic.Value:
+//
+// - Mutation of SeqCount-protected data does not require memory allocation,
+// whereas atomic.Value generally does. This is a significant advantage when
+// writes are common.
+//
+// - Atomic reads of SeqCount-protected data require copying. This is a
+// disadvantage when atomic reads are common.
+//
+// - SeqCount may be more flexible: correct use of SeqCount.ReadOk allows other
+// operations to be made atomic with reads of SeqCount-protected data.
+//
+// - SeqCount may be less flexible: as of this writing, SeqCount-protected data
+// cannot include pointers.
+//
+// - SeqCount is more cumbersome to use; atomic reads of SeqCount-protected
+// data require instantiating function templates using go_generics (see
+// seqatomic.go).
+type SeqCount struct {
+	// epoch is incremented by BeginWrite and EndWrite, such that epoch is odd
+	// if a writer critical section is active, and a read from data protected
+	// by this SeqCount is atomic iff epoch is the same even value before and
+	// after the read.
+	epoch uint32
+}
+
+// SeqCountEpoch tracks writer critical sections in a SeqCount.
+type SeqCountEpoch struct {
+	val uint32
+}
+
+// We assume that:
+//
+// - All functions in sync/atomic that perform a memory read are at least a
+// read fence: memory reads before calls to such functions cannot be reordered
+// after the call, and memory reads after calls to such functions cannot be
+// reordered before the call, even if those reads do not use sync/atomic.
+//
+// - All functions in sync/atomic that perform a memory write are at least a
+// write fence: memory writes before calls to such functions cannot be
+// reordered after the call, and memory writes after calls to such functions
+// cannot be reordered before the call, even if those writes do not use
+// sync/atomic.
+//
+// As of this writing, the Go memory model completely fails to describe
+// sync/atomic, but these properties are implied by
+// https://groups.google.com/forum/#!topic/golang-nuts/7EnEhM3U7B8.
+
+// BeginRead indicates the beginning of a reader critical section. Reader
+// critical sections DO NOT BLOCK writer critical sections, so operations in a
+// reader critical section MAY RACE with writer critical sections. Races are
+// detected by ReadOk at the end of the reader critical section. Thus, the
+// low-level structure of readers is generally:
+//
+//     for {
+//         epoch := seq.BeginRead()
+//         // do something idempotent with seq-protected data
+//         if seq.ReadOk(epoch) {
+//             break
+//         }
+//     }
+//
+// However, since reader critical sections may race with writer critical
+// sections, the Go race detector will (accurately) flag data races in readers
+// using this pattern. Most users of SeqCount will need to use the
+// SeqAtomicLoad function template in seqatomic.go.
+func (s *SeqCount) BeginRead() SeqCountEpoch {
+	epoch := atomic.LoadUint32(&s.epoch)
+	for epoch&1 != 0 {
+		runtime.Gosched()
+		epoch = atomic.LoadUint32(&s.epoch)
+	}
+	return SeqCountEpoch{epoch}
+}
+
+// ReadOk returns true if the reader critical section initiated by a previous
+// call to BeginRead() that returned epoch did not race with any writer critical
+// sections.
+//
+// ReadOk may be called any number of times during a reader critical section.
+// Reader critical sections do not need to be explicitly terminated; the last
+// call to ReadOk is implicitly the end of the reader critical section.
+func (s *SeqCount) ReadOk(epoch SeqCountEpoch) bool {
+	return atomic.LoadUint32(&s.epoch) == epoch.val
+}
+
+// BeginWrite indicates the beginning of a writer critical section.
+//
+// SeqCount does not support concurrent writer critical sections; clients with
+// concurrent writers must synchronize them using e.g. sync.Mutex.
+func (s *SeqCount) BeginWrite() {
+	if epoch := atomic.AddUint32(&s.epoch, 1); epoch&1 == 0 {
+		panic("SeqCount.BeginWrite during writer critical section")
+	}
+}
+
+// EndWrite ends the effect of a preceding BeginWrite.
+func (s *SeqCount) EndWrite() {
+	if epoch := atomic.AddUint32(&s.epoch, 1); epoch&1 != 0 {
+		panic("SeqCount.EndWrite outside writer critical section")
+	}
+}
+
+// PointersInType returns a list of pointers reachable from values named
+// valName of the given type.
+//
+// PointersInType is not exhaustive, but it is guaranteed that if typ contains
+// at least one pointer, then PointersInTypeOf returns a non-empty list.
+func PointersInType(typ reflect.Type, valName string) []string {
+	switch kind := typ.Kind(); kind {
+	case reflect.Bool, reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64, reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr, reflect.Float32, reflect.Float64, reflect.Complex64, reflect.Complex128:
+		return nil
+
+	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice, reflect.String, reflect.UnsafePointer:
+		return []string{valName}
+
+	case reflect.Array:
+		return PointersInType(typ.Elem(), valName+"[]")
+
+	case reflect.Struct:
+		var ptrs []string
+		for i, n := 0, typ.NumField(); i < n; i++ {
+			field := typ.Field(i)
+			ptrs = append(ptrs, PointersInType(field.Type, fmt.Sprintf("%s.%s", valName, field.Name))...)
+		}
+		return ptrs
+
+	default:
+		return []string{fmt.Sprintf("%s (of type %s with unknown kind %s)", valName, typ, kind)}
+	}
+}