Judge the SDK conformance fixtures against the SDK the controller pins, never a desktop's checkout (issue 449)

A check clones mesh-sdk beside the controller at the commit go.mod pins; elsewhere
the test reads a copy captured at that commit, held to go.mod. A missing clone fails
instead of skipping.

internal/beside/beside.go

@@ -1,5 +1,5 @@
 // Package beside is where a test finds another repository of the mesh it reads: the catalogue's manifests,
-// the node-engine's genesis template (novox/hq issue 432).
+// the node-engine's genesis template (novox/hq issue 432), the SDK's conformance fixtures (issue 449).
 //
 // A test used to read the checkout beside this one, `../../../mesh-catalog`, so its verdict depended on
 // whatever sat on the machine running it: a stale or dirty checkout failed it on a desktop, and where none
@@ -10,7 +10,8 @@
 //     test judges against those clones, so agreement with the other repository is checked where
 //     `mesh/repo-check` runs; a repository missing there fails the test, never skips it. Which clones a
 //     check gets is chosen from the inventory: mesh-catalog by the source of the `nats` module, mesh-host
-//     by the source of `mesh-host`. In a mesh where either module has no source repository nothing is
+//     by the source of `mesh-host`, and mesh-sdk as the controller's sibling at the commit the controller's
+//     go.mod pins. In a mesh where either module has no source repository nothing is
 //     cloned, and these tests fail loudly with "not beside this check": a cause in the setup, not in the
 //     change. And in a delivery group that holds a mesh-catalog pull request, these tests read the
 //     catalogue's main, not the group's head.

internal/link/conformance_test.go

@@ -5,15 +5,21 @@ import (
 	"os"
 	"path/filepath"
 	"regexp"
+	"strings"
 	"testing"
 	"time"
+
+	"github.com/novox/mesh-controller/internal/beside"
 )
 
 // The Go implementation, held to the shared fixtures (novox/hq ADR 0074, design 19).
 //
-// **Read from the sdk's conformance directory by sibling path**, the way the lab finds its
-// siblings — deliberately not copied here. A fixture copied into each implementation is two
-// fixtures, and two fixtures drift, which is the exact failure the suite exists to prevent.
+// **Read from the SDK's own conformance directory, never copied by hand**: a fixture written into each
+// implementation is two fixtures, and two fixtures drift, which is the failure the suite exists to prevent.
+// Which SDK is read is named, never the one a desktop happens to hold beside this checkout (novox/hq issue
+// 449): in a merge check the SDK the build seat clones beside it, at the commit this controller's go.mod
+// pins, and a missing clone fails the test; elsewhere the copy captured in testdata/beside at that same
+// commit (internal/beside, testdata/beside/CAPTURED), held to go.mod by TestTheCapturedSDKIsTheOneGoModPins.
 type fixture struct {
 	Name  string `json:"name"`
 	Given struct {
@@ -32,10 +38,11 @@ type fixture struct {
 
 func loadFixture(t *testing.T, name string) fixture {
 	t.Helper()
-	path := filepath.Join("..", "..", "..", "mesh-sdk", "conformance", name)
+	path := filepath.Join(beside.Dir(t, "mesh-sdk"), "conformance", name)
 	raw, err := os.ReadFile(path)
 	if err != nil {
-		t.Skipf("the sdk's conformance fixtures are not beside this checkout: %v", err)
+		// Failed, never skipped: a skip here passed the suite with nothing judged.
+		t.Fatalf("the SDK's conformance fixture %s: %v", name, err)
 	}
 	var f fixture
 	if err := json.Unmarshal(raw, &f); err != nil {
@@ -44,6 +51,33 @@ func loadFixture(t *testing.T, name string) fixture {
 	return f
 }
 
+// The captured SDK is the one this controller is built against: when go.mod moves the SDK, the copy moves
+// with it, or the tests away from a merge check judge an SDK the controller no longer uses (novox/hq issue
+// 449).
+func TestTheCapturedSDKIsTheOneGoModPins(t *testing.T) {
+	mod, err := os.ReadFile(filepath.Join("..", "..", "go.mod"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	pinned := regexp.MustCompile(`(?m)^\s*git\.novox\.be/novox/mesh-sdk/go v\S+-([0-9a-f]{12})$`).FindSubmatch(mod)
+	if pinned == nil {
+		t.Fatal("go.mod pins no commit of git.novox.be/novox/mesh-sdk/go as a pseudo-version; say here how a " +
+			"release's tag is matched against testdata/beside/CAPTURED")
+	}
+	captured, err := os.ReadFile(filepath.Join(beside.Captured(), "CAPTURED"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	at := regexp.MustCompile(`(?m)^mesh-sdk\s+([0-9a-f]{40})\s`).FindSubmatch(captured)
+	if at == nil {
+		t.Fatal("testdata/beside/CAPTURED names no commit for mesh-sdk")
+	}
+	if !strings.HasPrefix(string(at[1]), string(pinned[1])) {
+		t.Errorf("the SDK is captured at %s but go.mod pins %s: capture it again at the pinned commit, as "+
+			"testdata/beside/CAPTURED says", at[1], pinned[1])
+	}
+}
+
 // Every header the fixture requires is one this implementation actually sets.
 func TestTheGoEmitterSetsEveryRequiredHeader(t *testing.T) {
 	f := loadFixture(t, "events/module-event.json")