misc/dashboard/codereview: record Message-ID of code review thread mails.

This will allow us to properly thread "R=..." mails at a later time.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6135053

misc/dashboard/codereview/dashboard/cl.go

@@ -47,6 +47,7 @@ type CL struct {
 	// Mail information.
 	Subject       string   `datastore:",noindex"`
 	Recipients    []string `datastore:",noindex"`
+	LastMessageID string   `datastore:",noindex"`
 
 	// These are person IDs (e.g. "rsc"); they may be empty
 	Author   string
@@ -193,6 +194,8 @@ func handleAssign(w http.ResponseWriter, r *http.Request) {
 					Subject: cl.Subject + " (issue " + n + ")",
 					Body:    "R=" + rev + "\n\n(sent by gocodereview)",
 				}
+				// TODO(dsymonds): Use cl.LastMessageID as the In-Reply-To header
+				// when the appengine/mail package supports that.
 				sendMailLater.Call(c, msg)
 			}
 		}
@@ -339,7 +342,8 @@ func updateCL(c appengine.Context, n string) error {
 		if err != nil && err != datastore.ErrNoSuchEntity {
 			return err
 		} else if err == nil {
-			// Reviewer is the only field that needs preserving.
+			// LastMessageID and Reviewer need preserving.
+			cl.LastMessageID = ocl.LastMessageID
 			cl.Reviewer = ocl.Reviewer
 		}
 		_, err = datastore.Put(c, key, cl)

misc/dashboard/codereview/dashboard/mail.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"appengine"
+	"appengine/datastore"
 )
 
 func init() {
@@ -35,6 +36,23 @@ func handleMail(w http.ResponseWriter, r *http.Request) {
 	}
 
 	c.Infof("Found issue %q", m[1])
+
+	// Track the MessageID.
+	key := datastore.NewKey(c, "CL", m[1], 0, nil)
+	err = datastore.RunInTransaction(c, func(c appengine.Context) error {
+		cl := new(CL)
+		err := datastore.Get(c, key, cl)
+		if err != nil && err != datastore.ErrNoSuchEntity {
+			return err
+		}
+		cl.LastMessageID = msg.Header.Get("Message-ID")
+		_, err = datastore.Put(c, key, cl)
+		return err
+	}, nil)
+	if err != nil {
+		c.Errorf("datastore transaction failed: %v", err)
+	}
+
 	// Update the CL after a delay to give Rietveld a chance to catch up.
 	UpdateCLLater(c, m[1], 10*time.Second)
 }