Catch up on merges the bus announced and never handed over (hq issue 266)

The controller acted only on what its events consumer handed it, so a merge
the bus skipped left modules behind with nothing said. The stream is now read
back every five minutes on a single-filter consumer, and any merge that would
still move a module after ten minutes is said and acted on.

internal/link/merges.go

@@ -0,0 +1,79 @@
+package link
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"sort"
+	"time"
+
+	"github.com/nats-io/nats.go"
+
+	"github.com/novox/mesh-controller/internal/broker"
+)
+
+// AnnouncedMerge is one merge the forge announced, as the events stream holds it: what it said, and
+// when the bus took it.
+type AnnouncedMerge struct {
+	SourceMoved
+	// At is when the bus took the announcement — the stream's own time, not the forge's.
+	At time.Time
+	// Seq is its place in the events stream.
+	Seq uint64
+}
+
+// MergedSubject is where the forge's merges land: the controller's own follow of them.
+var MergedSubject = broker.ControllerFollows[3]
+
+// readQuiet is how long a read of the stream waits for one more message before it takes the stream
+// as read to its end. The stream answers at once when it holds something; this is only the wait at
+// the end.
+const readQuiet = 2 * time.Second
+
+// AnnouncedMerges is every merge the forge announced since a moment, oldest first, read back from
+// the events stream (novox/hq issue 266).
+//
+// **Read on a consumer of its own, filtered on the one subject.** The controller's durable consumer
+// carries several filters, and the bus server the mesh ran when this was written (2.10) skips a
+// message now and then on a consumer with more than one filter: it moves its delivered pointer past
+// the message without ever handing it over, so the controller never hears of it and nothing says so.
+// A consumer filtered on a single subject reads the stream another way and was not seen to skip. This
+// one is ordered, ephemeral and acknowledges nothing, so reading it changes nothing on the bus.
+func (s *Server) AnnouncedMerges(ctx context.Context, since time.Time) ([]AnnouncedMerge, error) {
+	if s.js == nil {
+		return nil, errors.New("this control plane is not on the bus, so it cannot read what the forge announced")
+	}
+	sub, err := s.js.Context().SubscribeSync(MergedSubject, nats.OrderedConsumer(), nats.StartTime(since))
+	if err != nil {
+		return nil, fmt.Errorf("reading the forge's merges from the events stream: %w", err)
+	}
+	defer func() { _ = sub.Unsubscribe() }()
+
+	var out []AnnouncedMerge
+	for {
+		wait, cancel := context.WithTimeout(ctx, readQuiet)
+		msg, err := sub.NextMsgWithContext(wait)
+		cancel()
+		if err != nil {
+			if ctx.Err() != nil {
+				return nil, ctx.Err()
+			}
+			// Nothing more within the quiet wait: the stream has been read to its end.
+			break
+		}
+		meta, err := msg.Metadata()
+		if err != nil {
+			continue
+		}
+		var moved SourceMoved
+		if json.Unmarshal(msg.Data, &moved) == nil && moved.Commit != "" && moved.Repo != "" {
+			out = append(out, AnnouncedMerge{SourceMoved: moved, At: meta.Timestamp, Seq: meta.Sequence.Stream})
+		}
+		if meta.NumPending == 0 {
+			break
+		}
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].Seq < out[j].Seq })
+	return out, nil
+}