feat(firestore): add WithCommitResponseTo TransactionOption

.github/workflows/vet.sh

@@ -61,6 +61,7 @@ golint ./... 2>&1 | (
     grep -v "firestore.arrayUnion" |
     grep -v "firestore.arrayRemove" |
     grep -v "maxAttempts" |
+    grep -v "firestore.commitResponse" |
     grep -v "UptimeCheckIpIterator" |
     grep -vE "apiv[0-9]+" |
     grep -v "ALL_CAPS" |

firestore/examples_test.go

@@ -564,6 +564,9 @@ func ExampleClient_RunTransaction() {
 	}
 	defer client.Close()
 
+	// write the CommitResponse here, via firestore.WithCommitResponse (below)
+	var cr firestore.CommitResponse
+
 	nm := client.Doc("States/NewMexico")
 	err = client.RunTransaction(ctx, func(ctx context.Context, tx *firestore.Transaction) error {
 		doc, err := tx.Get(nm) // tx.Get, NOT nm.Get!
@@ -575,10 +578,11 @@ func ExampleClient_RunTransaction() {
 			return err
 		}
 		return tx.Update(nm, []firestore.Update{{Path: "pop", Value: pop.(float64) + 0.2}})
-	})
+	}, firestore.WithCommitResponseTo(&cr))
 	if err != nil {
 		// TODO: Handle error.
 	}
+	// CommitResponse can be accessed here
 }
 
 func ExampleArrayUnion_create() {

firestore/transaction.go

@@ -17,6 +17,7 @@ package firestore
 import (
 	"context"
 	"errors"
+	"time"
 
 	pb "cloud.google.com/go/firestore/apiv1/firestorepb"
 	"cloud.google.com/go/internal/trace"
@@ -41,6 +42,7 @@ type Transaction struct {
 // A TransactionOption is an option passed to Client.Transaction.
 type TransactionOption interface {
 	config(t *Transaction)
+	handleCommitResponse(r *pb.CommitResponse)
 }
 
 // MaxAttempts is a TransactionOption that configures the maximum number of times to
@@ -49,7 +51,8 @@ func MaxAttempts(n int) maxAttempts { return maxAttempts(n) }
 
 type maxAttempts int
 
-func (m maxAttempts) config(t *Transaction) { t.maxAttempts = int(m) }
+func (m maxAttempts) config(t *Transaction)                     { t.maxAttempts = int(m) }
+func (m maxAttempts) handleCommitResponse(r *pb.CommitResponse) {}
 
 // DefaultTransactionMaxAttempts is the default number of times to attempt a transaction.
 const DefaultTransactionMaxAttempts = 5
@@ -60,7 +63,35 @@ var ReadOnly = ro{}
 
 type ro struct{}
 
-func (ro) config(t *Transaction) { t.readOnly = true }
+func (ro) config(t *Transaction)                     { t.readOnly = true }
+func (ro) handleCommitResponse(r *pb.CommitResponse) {}
+
+// CommitResponse exposes information about a committed transaction.
+type CommitResponse struct {
+	response *pb.CommitResponse
+}
+
+// CommitTime returns the commit time from the commit response.
+func (r *CommitResponse) CommitTime() time.Time {
+	return r.response.CommitTime.AsTime()
+}
+
+// commitResponse is the TransactionOption to record a commit response.
+type commitResponse struct {
+	responseTo *CommitResponse
+}
+
+func (c commitResponse) config(t *Transaction) {}
+func (c commitResponse) handleCommitResponse(r *pb.CommitResponse) {
+	c.responseTo.response = r
+}
+
+// WithCommitResponseTo returns a TransactionOption that specifies where the
+// CommitResponse should be written on successful commit. Nothing is written
+// on a failed commit.
+func WithCommitResponseTo(r *CommitResponse) commitResponse {
+	return commitResponse{responseTo: r}
+}
 
 var (
 	// Defined here for testing.
@@ -115,6 +146,7 @@ func (c *Client) RunTransaction(ctx context.Context, f func(context.Context, *Tr
 		}
 	}
 	var backoff gax.Backoff
+	var commitResponse *pb.CommitResponse
 	// TODO(jba): use other than the standard backoff parameters?
 	// TODO(jba): get backoff time from gRPC trailer metadata? See
 	// extractRetryDelay in https://code.googlesource.com/gocloud/+/master/spanner/retry.go.
@@ -142,13 +174,20 @@ func (c *Client) RunTransaction(ctx context.Context, f func(context.Context, *Tr
 			return err
 		}
 		t.ctx = trace.StartSpan(t.ctx, "cloud.google.com/go/firestore.Client.Commit")
-		_, err = t.c.c.Commit(t.ctx, &pb.CommitRequest{
+		commitResponse, err = t.c.c.Commit(t.ctx, &pb.CommitRequest{
 			Database:    t.c.path(),
 			Writes:      t.writes,
 			Transaction: t.id,
 		})
 		trace.EndSpan(t.ctx, err)
 
+		// on success, handle the commit response
+		if err == nil {
+			for _, opt := range opts {
+				opt.handleCommitResponse(commitResponse)
+			}
+		}
+
 		// If a read-write transaction returns Aborted, retry.
 		// On success or other failures, return here.
 		if t.readOnly || status.Code(err) != codes.Aborted {

firestore/transaction_test.go

@@ -84,6 +84,7 @@ func TestRunTransaction(t *testing.T) {
 		},
 		&pb.CommitResponse{CommitTime: aTimestamp3},
 	)
+	var commitResponse CommitResponse
 	err = c.RunTransaction(ctx, func(_ context.Context, tx *Transaction) error {
 		docref := c.Collection("C").Doc("a")
 		doc, err := tx.Get(docref)
@@ -95,11 +96,17 @@ func TestRunTransaction(t *testing.T) {
 			return err
 		}
 		return tx.Update(docref, []Update{{Path: "count", Value: count.(int64) + 1}})
-	})
+	}, WithCommitResponseTo(&commitResponse))
 	if err != nil {
 		t.Fatal(err)
 	}
 
+	// validate commit time
+	commitTime := commitResponse.CommitTime()
+	if commitTime != aTimestamp3.AsTime() {
+		t.Fatalf("commit time %v should equal %v", commitTime, aTimestamp3)
+	}
+
 	// Query
 	srv.reset()
 	srv.addRPC(beginReq, beginRes)