kvserver: don't GC if protected timestamp information isn't available

We only want to run GC on a replica that some PTS information (even if
it's stale). We don't want to run GC on a replica if no PTS information
is available however. This can happen if a Replica is being considered
for GC before the initial scan of the KVSubscriber has completed.

This wasn't an issue before this patch for implicit reasons -- this
patch just makes the check explicit and adds a test. Previously, we
wouldn't run GC if no PTS information was available because our lease
was guaranteed to be newer than the empty timestamp.

Release note: None

pkg/kv/kvserver/replica_protected_timestamp.go

@@ -132,6 +132,17 @@ func (r *Replica) checkProtectedTimestampsForGC(
 	if err != nil {
 		return false, hlc.Timestamp{}, hlc.Timestamp{}, hlc.Timestamp{}, hlc.Timestamp{}, err
 	}
+
+	if read.readAt.IsEmpty() {
+		// We don't want to allow GC to proceed if no protected timestamp
+		// information is available. This can happen if the initial scan of the
+		// rangefeed established by the spanconfig.KVSubscriber hasn't completed
+		// yet.
+		log.VEventf(ctx, 1,
+			"not gc'ing replica %v because protected timestamp information is unavailable", r)
+		return false, hlc.Timestamp{}, hlc.Timestamp{}, hlc.Timestamp{}, hlc.Timestamp{}, nil
+	}
+
 	gcTimestamp = read.readAt
 	if !read.earliestProtectionTimestamp.IsEmpty() {
 		// NB: we want to allow GC up to the timestamp preceding the earliest valid

pkg/kv/kvserver/replica_protected_timestamp_test.go

@@ -53,6 +53,16 @@ func TestCheckProtectedTimestampsForGC(t *testing.T) {
 				require.Zero(t, gcTimestamp)
 			},
 		},
+		{
+			name: "no PTS information is available",
+			test: func(t *testing.T, r *Replica, mp *manualPTSReader) {
+				mp.asOf = hlc.Timestamp{}
+				canGC, _, gcTimestamp, _, _, err := r.checkProtectedTimestampsForGC(ctx, makeTTLDuration(10))
+				require.NoError(t, err)
+				require.False(t, canGC)
+				require.Zero(t, gcTimestamp)
+			},
+		},
 		{
 			name: "have overlapping but new enough that it's okay",
 			test: func(t *testing.T, r *Replica, mp *manualPTSReader) {