Publish only a commit on its module's trunk; post a pull request's change plan (hq ADR 0238)

One commit, one plan: a commit off the trunk — a pull request's head, a branch built by
hand, a rebuild or replay of one — is for checking. The build seat reads from its clone
which branches hold the commit, and the controller records and never registers a build
whose commit is not on the branch the module follows (the repository's default for a
new one), so nothing off the trunk can be sent.

A pull request's check now carries its change plan, computed by the planner: what a
merge would build in which order, what each machine would receive, and what is not an
ordinary send — the bus step, a module waiting for a person, a provider's consumers.

cmd/mesh-controller/checks_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"strings"
 	"testing"
 
@@ -169,3 +170,43 @@ func TestAPullRequestReachesWhatAMergeOfItWouldPlan(t *testing.T) {
 		t.Fatalf("a change to the source gitea packages reaches %v (manifests %v)", s.Modules, s.Manifests)
 	}
 }
+
+// **Only a commit on the trunk is published** (novox/hq ADR 0238): a build of a commit off its repository's
+// default branch — a pull request's head, a branch built by hand, a rebuild or replay of one — is recorded
+// and never registered, so nothing can send it; a check or a dry run never is either.
+func TestABuildOffTheTrunkIsNeverPublished(t *testing.T) {
+	on := link.BuildResult{ID: "b", Commit: "abc", Trunk: "main", OnTrunk: true}
+	if err := publishable(on, ""); err != nil {
+		t.Errorf("a build on the trunk was refused: %v", err)
+	}
+	off := link.BuildResult{ID: "b", Commit: "abc", Trunk: "main"}
+	if err := publishable(off, ""); !errors.Is(err, errOffTheTrunk) || !strings.Contains(err.Error(), "main") {
+		t.Errorf("a build off the trunk was let through: %v", err)
+	}
+	for _, r := range []link.BuildResult{
+		{ID: "c", Trunk: "main", OnTrunk: true, Check: &link.CheckOutcome{Verdict: "pass"}},
+		{ID: "d", Trunk: "main", OnTrunk: true, Checked: &link.CheckRequest{}},
+		{ID: "e", Trunk: "main", OnTrunk: true, DryRun: true},
+	} {
+		if publishable(r, "") == nil {
+			t.Errorf("%s, a check or a dry run, was publishable", r.ID)
+		}
+	}
+	// A module that follows a branch other than the default: that branch is its trunk, and the default
+	// is not.
+	follows := link.BuildResult{ID: "g", Commit: "abc", Trunk: "master", Branches: []string{"nox-mesh"}}
+	if err := publishable(follows, "nox-mesh"); err != nil {
+		t.Errorf("a commit on the branch a module follows was refused: %v", err)
+	}
+	if err := publishable(link.BuildResult{ID: "h", Commit: "abc", Trunk: "master", OnTrunk: true,
+		Branches: []string{"master"}}, "nox-mesh"); err == nil {
+		t.Error("a commit on the default but not on the branch the module follows was published")
+	}
+	if err := publishable(link.BuildResult{ID: "i", Commit: "abc", Trunk: "main", Branches: []string{"feat/x"}}, ""); err == nil {
+		t.Error("a feature branch's commit was published")
+	}
+	// A build seat older than the rule says nothing: let through and said, so the rule can reach the mesh.
+	if err := publishable(link.BuildResult{ID: "f", Commit: "abc"}, ""); err != nil {
+		t.Errorf("a build seat that said nothing was refused: %v", err)
+	}
+}