Build a change once: the check builds what its merge moves, the walk takes it by key

A merge built every moved module a second time although its check had judged
the same tree (issue 649). The check now builds into the scratch namespace
under a reuse key (source fingerprint plus the builder's own digest), and the
walk of the trunk commit copies a build of the same key by digest instead of
building, saying per module why when it cannot (novox/hq ADR 0350).

internal/link/build.go

@@ -92,6 +92,24 @@ type BuildRequest struct {
 	// facts snapshot, raises the throwaway stores the check needs, runs the repository's own
 	// merge-check.sh and answers its verdict. Nothing is built, published or registered.
 	Check *CheckRequest `json:"check,omitempty"`
+	// Reuse makes this ask a walk's build of a trunk commit (novox/hq ADR 0350 §2): the builder takes the reuse key
+	// of the commit's tree and, where a check build holds the same key, copies it by digest into the module's
+	// namespace instead of building. Pulls are the pull requests the walk answers, whose checks name the part of a
+	// differing key. Nil for every other ask — a check, a dry run, `build --ref`, `rebuild`, `replay` — which never
+	// takes a check's build. A builder from before it ignores it and builds.
+	Reuse *ReuseAsk `json:"reuse,omitempty"`
+}
+
+// ReuseAsk is a walk's leave to take a check's build (novox/hq ADR 0350 §2).
+type ReuseAsk struct {
+	Pulls []PullRef `json:"pulls,omitempty"`
+}
+
+// PullRef names a pull request.
+type PullRef struct {
+	Owner  string `json:"owner"`
+	Repo   string `json:"repo"`
+	Number int    `json:"number"`
 }
 
 // CheckRequest is what a merge check needs beyond the repository and its head.
@@ -128,6 +146,34 @@ type CheckRequest struct {
 	// repository's own check is not run for a group: each member's pull request runs its own.
 	Group   string        `json:"group,omitempty"`
 	Members []GroupMember `json:"members,omitempty"`
+	// Builds are the modules the delivery plan moves, in the walk's order (novox/hq ADR 0350 §1): the check builds
+	// each, from the merge it judged, into the scratch namespace once it has judged and not failed, for the walk to
+	// take. A build seat from before it builds nothing, and the walk builds as before.
+	Builds []CheckBuild `json:"builds,omitempty"`
+}
+
+// CheckBuild is one module a check builds for its merge: from the merge it made when Merged — a module built from
+// the repository checked, at the branch merged into — else from Repository at Ref, as the walk would.
+type CheckBuild struct {
+	Module     string `json:"module"`
+	Repository string `json:"repository,omitempty"`
+	Path       string `json:"path,omitempty"`
+	Ref        string `json:"ref,omitempty"`
+	Merged     bool   `json:"merged,omitempty"`
+	Tier       int    `json:"tier"`
+	// Skip says why the check cannot build it as the walk would — a module that reads the repository checked
+	// through a context, which a check cannot hand the merge — and nothing after it is built either.
+	Skip string `json:"skip,omitempty"`
+}
+
+// CheckBuilt is one build a check made for its merge, as its verdict lists it (novox/hq ADR 0350 §1): the reuse key
+// and the artifacts, in the scratch namespace — or why it was not built.
+type CheckBuilt struct {
+	Module   string         `json:"module"`
+	Key      string         `json:"key,omitempty"`
+	Made     []MadeArtifact `json:"made,omitempty"`
+	NotBuilt string         `json:"not-built,omitempty"`
+	Took     string         `json:"took,omitempty"`
 }
 
 // GroupMember is one other head of a delivery group's composed check.
@@ -175,6 +221,8 @@ type CheckOutcome struct {
 	Trunk   string            `json:"trunk,omitempty"`
 	Beside  map[string]string `json:"beside,omitempty"`
 	Members []CheckedMember   `json:"members,omitempty"`
+	// Built are the builds it made for its merge (novox/hq ADR 0350 §1).
+	Built []CheckBuilt `json:"built,omitempty"`
 }
 
 // SourceOnSeat names a repository by the seat whose holder serves it and its path there.
@@ -264,6 +312,14 @@ type BuildResult struct {
 	// Failed is why, when it did.
 	Failed string `json:"failed,omitempty"`
 
+	// ReuseKey is the build's reuse key (novox/hq ADR 0350 §1). Reused, on a walk's build that took a check's build
+	// instead of building, is "reused from <check build>" with its digests, and ReusedFrom which; BuiltWhy, on one
+	// that built, why it took none (§3, §4). Both empty from a builder that predates it.
+	ReuseKey   string      `json:"reuse-key,omitempty"`
+	Reused     string      `json:"reused,omitempty"`
+	ReusedFrom *ReusedFrom `json:"reused-from,omitempty"`
+	BuiltWhy   string      `json:"built-why,omitempty"`
+
 	// Source is the request's, echoed: the seat form of the repository, for whoever registers.
 	Source *SourceOnSeat `json:"source,omitempty"`
 
@@ -300,3 +356,25 @@ type MadeArtifact struct {
 	Kind      string `json:"kind"`
 	Reference string `json:"reference"`
 }
+
+// ReusedFrom is the check build a walk's build took (novox/hq ADR 0350 §4): its pull request, the head and the trunk
+// commit the check merged it into, the check's ask, and its key.
+type ReusedFrom struct {
+	Pull  PullRef `json:"pull"`
+	Head  string  `json:"head,omitempty"`
+	Trunk string  `json:"trunk,omitempty"`
+	Check string  `json:"check,omitempty"`
+	Key   string  `json:"key"`
+}
+
+// MadeWords is what a walk says a module's build did (novox/hq ADR 0350 §4): "reused from <check build>", or "built:
+// <why>". A build seat from before it says neither, and that is said.
+func (r BuildResult) MadeWords() string {
+	switch {
+	case r.Reused != "":
+		return r.Reused
+	case r.BuiltWhy != "":
+		return "built: " + r.BuiltWhy
+	}
+	return "built: the build seat did not say whether a check's build could be taken (it predates ADR 0350)"
+}

internal/link/events.go

@@ -290,6 +290,9 @@ type Checked struct {
 	Trunk  string            `json:"trunk,omitempty"`
 	Beside map[string]string `json:"beside,omitempty"`
 	Took   string            `json:"took,omitempty"`
+	// Built are the builds the check made for its merge, each with its reuse key and digests, or why not
+	// (novox/hq ADR 0350 §1): what the walk of the merge takes instead of building.
+	Built []CheckBuilt `json:"built,omitempty"`
 }
 
 // CheckedMember is one head a group's composed check judged, and the trunk commit it was merged into (novox/hq