Gate: take a retargeted pull request's new base, so a retarget to main is seeded and asked and one away from main is not (issue 1422)

A re-announcement updated the base head but never the base: a pull request retargeted from nox-mesh to main stayed 'into
nox-mesh' and was never asked or seeded, and one retargeted away from main kept its open ask.

modules/mesh-delivery-gate/cmd/mesh-delivery-gate/holder.go

@@ -352,6 +352,8 @@ func (h *Holder) pullUpdated(p PullEvent) (isNew bool) {
 			// A delivery made from its verdict before its head was heard (issue 1726) holds none of what an announcement
 			// carries: filled from this one, so its base is known and the operator is asked.
 			h.fillFromAnnouncement(d, p)
+			// A pull request retargeted to another base (review of issue 1422): what it merges into is the forge's word now.
+			h.retarget(d, p)
 			// The review record, corrected in the description without a push, is read again (ADR 0421 §5).
 			if by, at := ReviewIn(p.Body); p.Body != "" && (by != d.ReviewedBy || at != d.ReviewedAt) {
 				d.ReviewedBy, d.ReviewedAt = by, at
@@ -453,6 +455,27 @@ func (h *Holder) fillFromAnnouncement(d *Delivery, p PullEvent) {
 	h.Poke()
 }
 
+// retarget takes the base a live, unmerged delivery's pull request was announced again with when it differs from the one
+// kept: into main it is asked and merged as any other, and source.main may be seeded from it (seedTrunks); out of main an
+// ask nobody answered yet is taken back, and the next look says it is not asked (ready.go). A delivery whose base is not yet
+// known is fillFromAnnouncement's. Said in the log. Under the lock.
+func (h *Holder) retarget(d *Delivery, p PullEvent) {
+	if p.Base == "" || d.Base == "" || p.Base == d.Base || d.State.Final() || d.MergedAs != "" ||
+		(d.Ready != nil && d.Ready.Merged != nil) {
+		return
+	}
+	from := d.Base
+	d.Base = p.Base
+	if r := d.Ready; r != nil {
+		r.OtherBase = ""
+		if r.AskID != "" && r.Approved == nil {
+			h.cancels = append(h.cancels, h.withdrawAsk(r.AskID, fmt.Sprintf("%s was retargeted from %s to %s", d.ID, from, p.Base)))
+		}
+	}
+	h.Logf("[mesh-delivery] %s: its pull request was retargeted from %s to %s", d.ID, from, p.Base)
+	h.keep(d)
+}
+
 // PullEdited takes a pull request's edit (novox/hq issue 1079): its description is read again for the review record, the
 // `after:` lines and the issues it names, exactly as an announcement reads them. An Approve ask still unanswered whose words
 // the new record changes is withdrawn and asked again with it read, without a word on the pull request: nothing was

modules/mesh-delivery-gate/cmd/mesh-delivery-gate/issue1726_trunk_test.go

@@ -98,6 +98,37 @@ func TestStartingWritesTheNewestMergeOfEachRepositoryNotYetInSourceMain(t *testi
 		t.Fatalf("seeded with %s; wanted %s, the base head of the latest announcement", got, trunkF)
 	}
 
+	// A green pull request retargeted from nox-mesh to main is seeded and asked; one retargeted from main to nox-mesh has
+	// its ask taken back and is not merged (review round 2).
+	retarget := func(from, to string) (*world, int) {
+		w := newWorld(t)
+		w.op.auto = false
+		p := pr(never, 7, readyHead, "feat/x", "src/a.go")
+		p.Base, p.BaseSHA = from, trunkN
+		w.h.PullUpdated(p)
+		w.h.Checked(judgedAt(never, 7, readyHead, "pass", "t1t1t1t1t1t1"))
+		open := openPull(never, 7, readyHead)
+		open.Base = from
+		w.forge.open = []OpenPull{open}
+		w.h.SeeOpenPulls()
+		w.settleAll()
+		before := len(w.op.open())
+		p.Base, open.Base = to, to
+		w.forge.open = []OpenPull{open}
+		w.h.PullUpdated(p)
+		w.h.SeeOpenPulls()
+		w.settleAll()
+		w.later(2 * time.Minute)
+		w.settleAll()
+		return w, before
+	}
+	if r, before := retarget("nox-mesh", "main"); before != 0 || r.source.main == nil || r.source.main.Heads[never] != trunkN || len(r.op.open()) != 1 {
+		t.Fatalf("retargeted to main: %d asks before, source.main %+v, open asks %v; wanted none before, then seeded at %s and asked", before, r.source.main, r.op.open(), trunkN)
+	}
+	if r, before := retarget("main", "nox-mesh"); before != 1 || len(r.op.open()) != 0 || len(r.forge.merges) != 0 || !strings.Contains(r.logs(), "retargeted") {
+		t.Fatalf("retargeted to nox-mesh: %d asks before, open asks %v, merges %v; wanted one before, then none and the retarget said", before, r.op.open(), r.forge.merges)
+	}
+
 	o := newWorld(t)
 	o.source.main = &sdk.Source{V: sdk.Version, Group: "newer", Heads: map[string]string{i1726Repo: i1726Merge2}}
 	o.h.PullUpdated(pr(i1726Repo, 471, head2, "fix/b", "modules/app/b.go"))