perf(#315): memoize subrepo detection within loadConfig (3 scans → 1) (#398)

loadConfig called detectSubRepos(cwd) at up to 3 sites per invocation (root-config
requiresFilesystem migration, workstream-config requiresFilesystem migration, and the
planning.sub_repos filesystem re-sync) — all with the same cwd, yielding identical
results. Introduce a per-call lazy memo (getDetectedSubRepos) so the directory scan
runs at most once per loadConfig call while preserving all conditional logic. Fixes #315.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/perf-315-loadconfig-subrepo-scan.test.cjs

@@ -0,0 +1,118 @@
+'use strict';
+
+/**
+ * Regression test for #315 — perf: repeated subrepo detection in loadConfig.
+ *
+ * Before the fix, loadConfig called detectSubRepos(cwd) up to 3 times per
+ * invocation (all with the same cwd):
+ *   Site 1 (~line 305): root-config requiresFilesystem migration (workstream path)
+ *   Site 2 (~line 353): workstream-config requiresFilesystem migration
+ *   Site 3 (~line 367): planning.sub_repos filesystem re-sync
+ *
+ * After the fix, a per-call lazy memo ensures detectSubRepos(cwd) is called
+ * EXACTLY once regardless of how many sites trigger.
+ *
+ * Fixture triggers:
+ *   - options.workstream set → loads root config (site 1 candidate)
+ *   - root config has `multiRepo: true` → requiresFilesystem → site 1 fires
+ *   - workstream config has `planning.sub_repos: [...]` → site 3 fires
+ *   - workstream config also has `multiRepo: true` → requiresFilesystem → site 2 fires
+ *   Combined: 3 distinct call sites attempted; memo should collapse to 1 scan.
+ */
+
+const { describe, test, afterEach } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const os = require('node:os');
+
+// Import loadConfig directly (sync, no CLI subprocess needed)
+const { loadConfig } = require('../get-shit-done/bin/lib/core.cjs');
+
+// ─── helpers ──────────────────────────────────────────────────────────────────
+
+function createProjectWithSubRepo(prefix = 'gsd-315-test-') {
+  const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), prefix));
+
+  // Create a child dir that looks like a subrepo
+  const subRepoDir = path.join(tmpDir, 'sub-service');
+  fs.mkdirSync(path.join(subRepoDir, '.git'), { recursive: true });
+
+  // Create .planning root layout
+  fs.mkdirSync(path.join(tmpDir, '.planning', 'phases'), { recursive: true });
+
+  return tmpDir;
+}
+
+function writeRootConfig(tmpDir, obj) {
+  const configPath = path.join(tmpDir, '.planning', 'config.json');
+  fs.writeFileSync(configPath, JSON.stringify(obj, null, 2), 'utf-8');
+}
+
+function writeWorkstreamConfig(tmpDir, wsName, obj) {
+  const wsDir = path.join(tmpDir, '.planning', 'workstreams', wsName, 'phases');
+  fs.mkdirSync(wsDir, { recursive: true });
+  const configPath = path.join(tmpDir, '.planning', 'workstreams', wsName, 'config.json');
+  fs.writeFileSync(configPath, JSON.stringify(obj, null, 2), 'utf-8');
+}
+
+// ─── test ─────────────────────────────────────────────────────────────────────
+
+describe('perf-315 — loadConfig calls detectSubRepos at most once per invocation', () => {
+  let tmpDir;
+
+  afterEach(() => {
+    if (tmpDir) {
+      fs.rmSync(tmpDir, { recursive: true, force: true });
+      tmpDir = null;
+    }
+  });
+
+  test('detectSubRepos scan count is exactly 1 regardless of how many sites trigger', () => {
+    tmpDir = createProjectWithSubRepo();
+
+    const wsName = 'test-ws';
+
+    // Root config: multiRepo: true triggers requiresFilesystem at site 1
+    writeRootConfig(tmpDir, { multiRepo: true, model_profile: 'balanced' });
+
+    // Workstream config:
+    //   - planning.sub_repos set  → triggers site 3 (fs-resync)
+    //   - multiRepo: true         → triggers site 2 (requiresFilesystem normalization)
+    writeWorkstreamConfig(tmpDir, wsName, {
+      multiRepo: true,
+      planning: { sub_repos: ['sub-service'] },
+      model_profile: 'balanced',
+    });
+
+    // Spy on fs.readdirSync — count only calls that match detectSubRepos' signature
+    // (first arg === tmpDir, options object containing withFileTypes: true)
+    let scanCount = 0;
+    const originalReaddirSync = fs.readdirSync;
+    fs.readdirSync = function spyReaddirSync(dirPath, opts) {
+      if (dirPath === tmpDir && opts && opts.withFileTypes === true) {
+        scanCount += 1;
+      }
+      return originalReaddirSync.call(this, dirPath, opts);
+    };
+
+    try {
+      // Load config with workstream option so all three sites can be reached
+      const config = loadConfig(tmpDir, { workstream: wsName });
+
+      // Behavior lock: sub_repos is correctly resolved
+      assert.ok(
+        Array.isArray(config.sub_repos) || Array.isArray(config.planning?.sub_repos),
+        'sub_repos should be an array in the returned config'
+      );
+
+      // Performance assertion: only one scan regardless of how many sites triggered
+      assert.strictEqual(
+        scanCount, 1,
+        `Expected detectSubRepos to scan cwd exactly once, but fs.readdirSync was called ${scanCount} time(s) with the cwd + {withFileTypes:true} signature`
+      );
+    } finally {
+      fs.readdirSync = originalReaddirSync;
+    }
+  });
+});