perf(ci): reuse tree-equivalent merge candidates

.gitea/actions/quickstack-oci/deploy.test.mjs

@@ -1,4 +1,8 @@
 import assert from "node:assert/strict";
+import { execFileSync } from "node:child_process";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
 import test from "node:test";
 
 import {
@@ -17,6 +21,7 @@ import {
   orderApplications,
   resolveDeploymentBranch,
   resolvePullRequestHeadBranch,
+  resolveTreeEquivalentCandidateSha,
   selectDeployment,
   selectPipeline,
   toSavePayload,
@@ -130,6 +135,45 @@ test("same-repository pull requests publish reusable candidates while forks stay
   );
 });
 
+test("tree-equivalent merge commits resolve to their tested candidate parent", () => {
+  const workspace = fs.mkdtempSync(path.join(os.tmpdir(), "quickstack-merge-candidate-"));
+  const git = (args) => execFileSync("git", args, {
+    cwd: workspace,
+    encoding: "utf8",
+    stdio: ["ignore", "pipe", "pipe"],
+  }).trim();
+  try {
+    git(["init"]);
+    git(["checkout", "-b", "main"]);
+    git(["config", "user.email", "ci@example.test"]);
+    git(["config", "user.name", "CI"]);
+    fs.writeFileSync(path.join(workspace, "app.txt"), "base\n");
+    git(["add", "app.txt"]);
+    git(["commit", "-m", "base"]);
+    const baseSha = git(["rev-parse", "HEAD"]);
+    git(["checkout", "-b", "feature"]);
+    fs.writeFileSync(path.join(workspace, "app.txt"), "feature\n");
+    git(["commit", "-am", "feature"]);
+    const featureSha = git(["rev-parse", "HEAD"]);
+    git(["checkout", "main"]);
+    git(["merge", "--no-ff", "feature", "-m", "merge feature"]);
+    const mergeSha = git(["rev-parse", "HEAD"]);
+    git(["checkout", "-b", "prod", baseSha]);
+    git(["merge", "--no-ff", "main", "-m", "merge staging"]);
+    const prodMergeSha = git(["rev-parse", "HEAD"]);
+
+    assert.equal(resolveTreeEquivalentCandidateSha(workspace, mergeSha), featureSha);
+    assert.equal(resolveTreeEquivalentCandidateSha(workspace, prodMergeSha), featureSha);
+    assert.equal(resolveTreeEquivalentCandidateSha(workspace, featureSha), featureSha);
+    assert.equal(
+      resolveTreeEquivalentCandidateSha(workspace, mergeSha, { requireTreeMatch: false }),
+      mergeSha,
+    );
+  } finally {
+    fs.rmSync(workspace, { recursive: true, force: true });
+  }
+});
+
 test("validates split action modes and artifact build slots", () => {
   assert.equal(resolveActionMode(undefined), "all");
   assert.equal(resolveActionMode("BUILD"), "build");