dashboard: always keep the original message in discussions

It lets us always determine the discussion start date and whether the
discussion was started by a bot or a human.

2 files changed, 44 insertions, 1 deletions

diff --git a/dashboard/app/discussion.go b/dashboard/app/discussion.go
index 04f0370b8..12a0392c5 100644
--- a/dashboard/app/discussion.go
+++ b/dashboard/app/discussion.go
@@ -174,11 +174,17 @@ func (d *Discussion) addMessages(messages []dashapi.DiscussionMessage) Discussio
 			Time:     m.Time,
 		})
 	}
+	if len(d.Messages) == 0 {
+		return diff
+	}
 	sort.Slice(d.Messages, func(i, j int) bool {
 		return d.Messages[i].Time.Before(d.Messages[j].Time)
 	})
+	// Always keep the oldest message.
+	first := d.Messages[0]
 	if len(d.Messages) > maxMessagesInDiscussion {
-		d.Messages = d.Messages[len(d.Messages)-maxMessagesInDiscussion:]
+		d.Messages = append([]DiscussionMessage{first},
+			d.Messages[len(d.Messages)-maxMessagesInDiscussion+1:]...)
 	}
 	return diff
 }
diff --git a/dashboard/app/discussion_test.go b/dashboard/app/discussion_test.go
index a06ee216e..bbe87341b 100644
--- a/dashboard/app/discussion_test.go
+++ b/dashboard/app/discussion_test.go
@@ -5,6 +5,7 @@ package main
 
 import (
 	"fmt"
+	"reflect"
 	"testing"
 	"time"
 
@@ -360,3 +361,39 @@ Hello!
 	c.expectOK(err)
 	client.expectEQ(len(got), 0)
 }
+
+func TestMessageOverflow(t *testing.T) {
+	date := time.Date(2000, time.January, 1, 1, 0, 0, 0, time.UTC)
+	d := &Discussion{}
+	first, last := dashapi.DiscussionMessage{
+		ID:   date.String(),
+		Time: date,
+	}, dashapi.DiscussionMessage{}
+	d.addMessages([]dashapi.DiscussionMessage{first})
+
+	const blockSize = 100
+	for i := 0; i < 2*maxMessagesInDiscussion; i += blockSize {
+		block := []dashapi.DiscussionMessage{}
+		for j := 0; j < blockSize; j++ {
+			date = date.Add(time.Minute)
+			last = dashapi.DiscussionMessage{
+				ID:   date.String(),
+				Time: date,
+			}
+			block = append(block, last)
+		}
+		// Make sure that the first message always remains in place and the last one
+		// is the latest one.
+		d.addMessages(block)
+		if !reflect.DeepEqual(first.ID, d.Messages[0].ID) {
+			t.Fatalf("unexpected first messages")
+		}
+		if !reflect.DeepEqual(last.ID, d.Messages[len(d.Messages)-1].ID) {
+			t.Fatalf("unexpected last messages")
+		}
+	}
+	if len(d.Messages) != maxMessagesInDiscussion {
+		t.Fatalf("expected len to be equal to %d, got %d",
+			maxMessagesInDiscussion, len(d.Messages))
+	}
+}