netchan: added drain method to importer.

Fixes #1868.

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/4550093

src/pkg/netchan/import.go

@@ -11,6 +11,7 @@ import (
 	"os"
 	"reflect"
 	"sync"
+	"time"
 )
 
 // Import
@@ -31,6 +32,9 @@ type Importer struct {
 	chans    map[int]*netChan
 	errors   chan os.Error
 	maxId    int
+	mu       sync.Mutex // protects remaining fields
+	unacked  int64      // number of unacknowledged sends.
+	seqLock  sync.Mutex // guarantees messages are in sequence, only locked under mu
 }
 
 // NewImporter creates a new Importer object to import a set of channels
@@ -42,6 +46,7 @@ func NewImporter(conn io.ReadWriter) *Importer {
 	imp.chans = make(map[int]*netChan)
 	imp.names = make(map[string]*netChan)
 	imp.errors = make(chan os.Error, 10)
+	imp.unacked = 0
 	go imp.run()
 	return imp
 }
@@ -80,8 +85,10 @@ func (imp *Importer) run() {
 	for {
 		*hdr = header{}
 		if e := imp.decode(hdrValue); e != nil {
-			impLog("header:", e)
-			imp.shutdown()
+			if e != os.EOF {
+				impLog("header:", e)
+				imp.shutdown()
+			}
 			return
 		}
 		switch hdr.PayloadType {
@@ -114,6 +121,9 @@ func (imp *Importer) run() {
 			nch := imp.getChan(hdr.Id, true)
 			if nch != nil {
 				nch.acked()
+				imp.mu.Lock()
+				imp.unacked--
+				imp.mu.Unlock()
 			}
 			continue
 		default:
@@ -220,10 +230,17 @@ func (imp *Importer) ImportNValues(name string, chT interface{}, dir Dir, size,
 					}
 					return
 				}
+				// We hold the lock during transmission to guarantee messages are
+				// sent in order.
+				imp.mu.Lock()
+				imp.unacked++
+				imp.seqLock.Lock()
+				imp.mu.Unlock()
 				if err = imp.encode(hdr, payData, val.Interface()); err != nil {
 					impLog("error encoding client send:", err)
 					return
 				}
+				imp.seqLock.Unlock()
 			}
 		}()
 	}
@@ -244,3 +261,27 @@ func (imp *Importer) Hangup(name string) os.Error {
 	nc.close()
 	return nil
 }
+
+func (imp *Importer) unackedCount() int64 {
+	imp.mu.Lock()
+	n := imp.unacked
+	imp.mu.Unlock()
+	return n
+}
+
+// Drain waits until all messages sent from this exporter/importer, including
+// those not yet sent to any server and possibly including those sent while
+// Drain was executing, have been received by the exporter.  In short, it
+// waits until all the importer's messages have been received.
+// If the timeout (measured in nanoseconds) is positive and Drain takes
+// longer than that to complete, an error is returned.
+func (imp *Importer) Drain(timeout int64) os.Error {
+	startTime := time.Nanoseconds()
+	for imp.unackedCount() > 0 {
+		if timeout > 0 && time.Nanoseconds()-startTime >= timeout {
+			return os.ErrorString("timeout")
+		}
+		time.Sleep(100 * 1e6)
+	}
+	return nil
+}

src/pkg/netchan/netchan_test.go

@@ -178,6 +178,16 @@ func TestExportDrain(t *testing.T) {
 	<-done
 }
 
+// Not a great test but it does at least invoke Drain.
+func TestImportDrain(t *testing.T) {
+	exp, imp := pair(t)
+	expDone := make(chan bool)
+	go exportReceive(exp, t, expDone)
+	<-expDone
+	importSend(imp, closeCount, t, nil)
+	imp.Drain(0)
+}
+
 // Not a great test but it does at least invoke Sync.
 func TestExportSync(t *testing.T) {
 	exp, imp := pair(t)