Carry a config value held for a pull request's new key into its composed check (issue 1826)

The controller tells mesh-config what a pull request declares, asks the approved held values with each check, and the build agent and merge gate lay them over the change's composition only: a key the trunk does not declare yet can then be approved before the merge (ADR 0502).

internal/builder/check.go

@@ -123,8 +123,41 @@ type CheckSpec struct {
 	// controller could not learn them, and the verdict says so.
 	Siblings        []Sibling
 	SiblingsUnknown string
+	// Held are the approved held values of the check's pull requests (novox/hq ADR 0502 clause 4): written to a file and
+	// handed to the gate, which lays them over the change's composition only. None: no flag, no file.
+	Held []HeldValue
 }
 
+// HeldValue is one approved config value held for a key an open pull request introduces: the value of Key for Module on
+// Node (a machine's real name; the gate finds it in the snapshot by its pseudonym). The gate reads this document.
+type HeldValue struct {
+	Pull     string          `json:"pull"`
+	Head     string          `json:"head,omitempty"`
+	Node     string          `json:"node"`
+	Module   string          `json:"module"`
+	Key      string          `json:"key"`
+	Value    json.RawMessage `json:"value"`
+	Approved string          `json:"approved,omitempty"`
+	By       string          `json:"by,omitempty"`
+	At       string          `json:"at,omitempty"`
+}
+
+// writeHeld writes the held values to held.json under root and answers its path; empty, and nothing written, for none.
+func writeHeld(root string, held []HeldValue) (string, error) {
+	if len(held) == 0 {
+		return "", nil
+	}
+	raw, err := json.Marshal(held)
+	if err != nil {
+		return "", err
+	}
+	file := filepath.Join(root, "held.json")
+	return file, os.WriteFile(file, raw, 0o644)
+}
+
+// heldFlag is how the gate is given the held values: only when there are some, so a judge from before --held still runs.
+const heldFlag = `${MESH_CHECK_HELD:+--held "$MESH_CHECK_HELD"}`
+
 // Sibling is one other open pull request of a delivery group (novox/hq ADR 0382): the repository it is in as
 // owner/repo, its number, its whole head, the branch it merges into, and Dir, the directory the check finds that
 // repository under beside it (an entry of Beside).
@@ -272,6 +305,8 @@ const (
 	// EnvGroup is a delivery group's other heads, as JSON, for the gate (novox/hq ADR 0239); empty for a
 	// pull request checked alone.
 	EnvGroup = "MESH_CHECK_GROUP"
+	// EnvHeld is the file of the approved held values of the check's pull requests (novox/hq ADR 0502); empty for none.
+	EnvHeld = "MESH_CHECK_HELD"
 	// EnvTests is a `go test -run` pattern and EnvStoreOnly is 1 when a branch tried before its pull request
 	// asks for only those tests, or only the store tests (novox/hq issue 686). Never set for a pull request's
 	// check. A script that reads neither runs whole.
@@ -572,6 +607,15 @@ func Check(ctx context.Context, run Runner, spec CheckSpec, workspace, registry
 		}
 	}
 
+	// The approved held values of the check's pull requests, for the gate (novox/hq ADR 0502 clause 4).
+	heldFile, err := writeHeld(root, spec.Held)
+	if err != nil {
+		return CheckVerdict{}, err
+	}
+	if heldFile != "" {
+		say("check", "with %d config value(s) held for its pull request(s)", len(spec.Held))
+	}
+
 	// Every clone is made: the credential is gone before anything of the check runs (novox/hq issue 462).
 	forget()
 
@@ -733,7 +777,7 @@ func Check(ctx context.Context, run Runner, spec CheckSpec, workspace, registry
 		EnvTestStore + "=" + storeURL, EnvTestBus + "=nats://" + bus, EnvRepository + "=" + spec.Owner + "/" + spec.Repo,
 		EnvBeside + "=" + root,
 		EnvModules + "=" + strings.Join(append(append([]string{}, spec.Modules...), spec.New...), ","),
-		EnvGroup + "=" + groupFile,
+		EnvGroup + "=" + groupFile, EnvHeld + "=" + heldFile,
 		"GOCACHE=" + filepath.Join(workspace, "go-cache"), "GOMODCACHE=" + filepath.Join(workspace, "go-modules")}
 	env = append(env, changedEnv...)
 	env = append(env, selection(spec)...)
@@ -1100,7 +1144,7 @@ func gateLayer(ctx context.Context, spec CheckSpec, tree, root, gate, verdictFil
 	if err := running(inToolchain(tree, append(env, EnvVerdict+"="+verdictFile), "sh", "-c",
 		`"$MESH_GATE" merge-gate --facts "$MESH_FACTS" --store "$MESH_GATE_POSTGRES" `+
 			`--repository "$MESH_CHECK_REPOSITORY" --tree . `+gateChangedFlag(env)+` `+
-			`${MESH_CHECK_GROUP:+--group "$MESH_CHECK_GROUP"} --json > "$MESH_CHECK_VERDICT"`)); err != nil {
+			`${MESH_CHECK_GROUP:+--group "$MESH_CHECK_GROUP"} `+heldFlag+` --json > "$MESH_CHECK_VERDICT"`)); err != nil {
 		if ctx.Err() != nil {
 			return "error", "the check was ended during the merge gate"
 		}

internal/builder/check_held_test.go

@@ -0,0 +1,33 @@
+package builder
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// 1826/AC2: the held values are written whole to held.json, and the gate is given the flag only when there are some.
+func TestHeldValuesAreWrittenForTheGateAndNoneWriteNothing(t *testing.T) {
+	root := t.TempDir()
+	if file, err := writeHeld(root, nil); err != nil || file != "" {
+		t.Fatalf("none wrote %q %v", file, err)
+	}
+	if _, err := os.Stat(filepath.Join(root, "held.json")); err == nil {
+		t.Fatal("none wrote a file")
+	}
+	file, err := writeHeld(root, []HeldValue{{Pull: "novox/mesh-catalog#434", Node: "ace", Module: "n8n", Key: "media-gid",
+		Value: []byte("2000"), Approved: "the operator"}})
+	if err != nil || file != filepath.Join(root, "held.json") {
+		t.Fatalf("wrote %q %v", file, err)
+	}
+	raw, _ := os.ReadFile(file)
+	for _, want := range []string{`"node":"ace"`, `"module":"n8n"`, `"key":"media-gid"`, `"value":2000`} {
+		if !strings.Contains(string(raw), want) {
+			t.Fatalf("%s lacks %s", raw, want)
+		}
+	}
+	if !strings.Contains(heldFlag, `--held "$MESH_CHECK_HELD"`) || !strings.HasPrefix(heldFlag, "${MESH_CHECK_HELD:+") {
+		t.Fatalf("the flag is %s", heldFlag)
+	}
+}