refactor(persistence): migrate credential-manager to canonical atomic-write util (DC-106) [glm-grade=B]

All three writeFileSync sites in the encrypted-credentials manager now
delegate to src/utils/atomic-write.js atomicWriteJSON — rotateEncryptionKey
save, _ensureFileExists bootstrap, and the _lockedUpdate commit under the
proper-lockfile lock. A crash can no longer tear credentials.json
mid-write: power loss through the old path could leave an empty/short
file and silently drop every stored credential (DNS provider tokens etc).
Lock-safety pre-study: proper-lockfile only stats its own sibling .lock
dir, never the target file, so the rename swap cannot trip ECOMPROMISED.

Test mock extended to the fd-level fs API (openSync/writeSync/
fsyncSync/closeSync/renameSync + fdMap/closedTmp state) so the canonical
path is exercised end-to-end under the mock; write assertions moved from
writeFileSync.mock.calls to destination-state reads. New DC-106 pins:
wx+0600+fsync+rename discipline, plaintext-secret canary never on disk,
fd-lifecycle order fsync->close->rename via invocationCallOrder, and
atomic _ensureFileExists create at 0600. Eighth store migrated
(DC-099..DC-105 preceded).

Judge: GLM-5.3 cold read, round-1 B/ship (deleg_b3c038c2).
URN urn:ump:vscequdet7wt5un7jhtl2nlg6cfkabsbazy5m2tjnyyguu5wstxa
(readback verified: grade B, topic codex-judge-verdict).
Sole finding is pre-existing and non-blocking: rotateEncryptionKey
persists the new key before writing rotated creds (crash window) —
queued as DC-107 follow-up.
Full suite: 121 suites / 2783 tests green.

dashcaddy-api/__tests__/credential-manager.test.js

@@ -23,13 +23,65 @@ jest.mock('proper-lockfile', () => ({
   check: jest.fn().mockResolvedValue(false),
 }));
 
+// DC-106: fd-level mock exercising the canonical atomic-write path
+// (openSync('wx') -> writeSync -> fsyncSync -> closeSync -> renameSync).
+const mockFsState = {
+  files: {},          // path -> content (destination state after rename)
+  fdMap: new Map(),   // open fd -> { p, content }
+  closedTmp: new Map(), // closed-but-not-yet-renamed tmp path -> content
+  openedWith: [],     // { p, flags, mode } per openSync call
+  nextFd: 0,
+};
+
 jest.mock('fs', () => ({
-  existsSync: jest.fn().mockReturnValue(true),
-  readFileSync: jest.fn().mockReturnValue('{}'),
-  writeFileSync: jest.fn(),
+  existsSync: jest.fn((p) => mockFsState.files[p] !== undefined),
+  readFileSync: jest.fn((p) => {
+    if (mockFsState.files[p] === undefined) {
+      const e = new Error(`ENOENT: ${p}`);
+      e.code = 'ENOENT';
+      throw e;
+    }
+    return mockFsState.files[p];
+  }),
   mkdirSync: jest.fn(),
+  // DC-105/DC-106 canonical atomic-write path (atomic-write.js).
+  openSync: jest.fn((p, flags, mode) => {
+    mockFsState.openedWith.push({ p, flags, mode });
+    mockFsState.nextFd += 1;
+    mockFsState.fdMap.set(mockFsState.nextFd, { p, content: '' });
+    return mockFsState.nextFd;
+  }),
+  writeSync: jest.fn((fd, content) => {
+    const rec = mockFsState.fdMap.get(fd);
+    if (!rec) throw new Error(`EBADF: fd ${fd}`);
+    rec.content += content;
+  }),
+  fsyncSync: jest.fn(),
+  closeSync: jest.fn((fd) => {
+    const rec = mockFsState.fdMap.get(fd);
+    if (rec) {
+      mockFsState.closedTmp.set(rec.p, rec.content);
+      mockFsState.fdMap.delete(fd);
+    }
+  }),
+  renameSync: jest.fn((src, dst) => {
+    const content = mockFsState.closedTmp.has(src)
+      ? mockFsState.closedTmp.get(src)
+      : mockFsState.files[src];
+    mockFsState.files[dst] = content;
+    mockFsState.closedTmp.delete(src);
+    delete mockFsState.files[src];
+  }),
+  unlinkSync: jest.fn(),
 }));
 
+// DC-106: mirror the production path resolution so assertions read the same
+// destination the manager writes to, regardless of env overrides.
+const path = require('path');
+const platformPaths = require('../platform-paths');
+const CREDENTIALS_FILE = process.env.CREDENTIALS_FILE
+  || path.join(platformPaths.dataDir, 'credentials.json');
+
 describe('CredentialManager', () => {
   let credentialManager;
   let fs, lockfile, keychainManager, cryptoUtils;
@@ -43,10 +95,30 @@ describe('CredentialManager', () => {
     keychainManager = require('../src/security/keychain-manager');
     cryptoUtils = require('../src/security/crypto-utils');
 
-    // Reset mock implementations
-    fs.existsSync.mockReturnValue(true);
-    fs.readFileSync.mockReturnValue('{}');
-    fs.writeFileSync.mockImplementation(() => {});
+    // Reset mock implementations and fd-level atomic-write state
+    for (const k of Object.keys(mockFsState.files)) delete mockFsState.files[k];
+    mockFsState.fdMap.clear();
+    mockFsState.closedTmp.clear();
+    mockFsState.openedWith.length = 0;
+    mockFsState.nextFd = 0;
+    // Default world: credentials.json exists with empty payload (the previous
+    // mock's existsSync=true / readFileSync='{}' semantics, now truthful).
+    mockFsState.files[CREDENTIALS_FILE] = '{}';
+    fs.existsSync.mockImplementation((p) => mockFsState.files[p] !== undefined);
+    fs.readFileSync.mockImplementation((p) => {
+      if (mockFsState.files[p] === undefined) {
+        const e = new Error(`ENOENT: ${p}`);
+        e.code = 'ENOENT';
+        throw e;
+      }
+      return mockFsState.files[p];
+    });
+    fs.openSync.mockClear();
+    fs.writeSync.mockClear();
+    fs.fsyncSync.mockClear();
+    fs.closeSync.mockClear();
+    fs.renameSync.mockClear();
+    fs.unlinkSync.mockClear();
     lockfile.lock.mockResolvedValue(jest.fn().mockResolvedValue());
     keychainManager.available = false;
 
@@ -59,7 +131,7 @@ describe('CredentialManager', () => {
       const result = await credentialManager.store('test.key', 'secret-value');
       expect(result).toBe(true);
       expect(cryptoUtils.encrypt).toHaveBeenCalledWith('secret-value');
-      expect(fs.writeFileSync).toHaveBeenCalled();
+      expect(fs.renameSync).toHaveBeenCalled(); // DC-106: canonical write landed
     });
 
     it('stores value in keychain when available', async () => {
@@ -67,9 +139,10 @@ describe('CredentialManager', () => {
       // Need to get a fresh instance that sees available=true
       jest.resetModules();
       fs = require('fs');
-      fs.existsSync.mockReturnValue(true);
-      fs.readFileSync.mockReturnValue('{}');
-      fs.writeFileSync.mockImplementation(() => {});
+      for (const k of Object.keys(mockFsState.files)) delete mockFsState.files[k];
+      mockFsState.fdMap.clear();
+      mockFsState.closedTmp.clear();
+      mockFsState.openedWith.length = 0;
       lockfile = require('proper-lockfile');
       lockfile.lock.mockResolvedValue(jest.fn().mockResolvedValue());
       keychainManager = require('../src/security/keychain-manager');
@@ -86,9 +159,10 @@ describe('CredentialManager', () => {
       keychainManager.available = true;
       jest.resetModules();
       fs = require('fs');
-      fs.existsSync.mockReturnValue(true);
-      fs.readFileSync.mockReturnValue('{}');
-      fs.writeFileSync.mockImplementation(() => {});
+      for (const k of Object.keys(mockFsState.files)) delete mockFsState.files[k];
+      mockFsState.fdMap.clear();
+      mockFsState.closedTmp.clear();
+      mockFsState.openedWith.length = 0;
       lockfile = require('proper-lockfile');
       lockfile.lock.mockResolvedValue(jest.fn().mockResolvedValue());
       keychainManager = require('../src/security/keychain-manager');
@@ -226,8 +300,7 @@ describe('CredentialManager', () => {
       });
 
       expect(lockfile.lock).toHaveBeenCalled();
-      expect(fs.writeFileSync).toHaveBeenCalled();
-      const writtenData = JSON.parse(fs.writeFileSync.mock.calls[0][1]);
+      const writtenData = JSON.parse(mockFsState.files[CREDENTIALS_FILE]);
       expect(writtenData).toEqual({ a: 1, b: 2 });
       expect(releaseFn).toHaveBeenCalled();
     });
@@ -264,7 +337,7 @@ describe('CredentialManager', () => {
       const result = await credentialManager.rotateEncryptionKey();
       expect(result).toBe(true);
       expect(cryptoUtils.rotateKey).toHaveBeenCalled();
-      expect(fs.writeFileSync).toHaveBeenCalled();
+      expect(fs.renameSync).toHaveBeenCalled(); // DC-106: canonical write landed
     });
 
     it('clears cache after rotation', async () => {
@@ -326,6 +399,90 @@ describe('CredentialManager', () => {
     });
   });
 
+
+  describe('DC-106 canonical atomic-write migration', () => {
+    it('writes credentials.json via wx tmp + fsync + rename, mode 0600', async () => {
+      await credentialManager.store('dc106.key', 'dc106-secret');
+
+      // fsyncDir also openSync()s the parent dir (flags 'r') — filter to the
+      // payload tmp opens to assert on the canonical write itself.
+      const wxOpens = mockFsState.openedWith.filter((o) => o.flags === 'wx');
+      expect(wxOpens.length).toBe(1); // file pre-existed -> no ensure-create
+      expect(wxOpens[0].mode).toBe(0o600); // sensitive file mode preserved
+      expect(fs.fsyncSync).toHaveBeenCalled(); // bytes pinned before rename
+      expect(fs.renameSync).toHaveBeenCalled();
+
+      const [tmpSrc, dst] = fs.renameSync.mock.calls
+        .find((c) => c[1] === CREDENTIALS_FILE);
+      expect(tmpSrc).not.toBe(dst);
+      expect(tmpSrc).toMatch(/\.credentials\.json\.tmp-/); // canonical tmp prefix
+      expect(dst).toBe(CREDENTIALS_FILE);
+      expect(mockFsState.files[CREDENTIALS_FILE]).toBeDefined();
+
+      // No leftover tmp files: every payload tmp was renamed away
+      const renamedSrcs = fs.renameSync.mock.calls.map((c) => c[0]);
+      for (const o of wxOpens) {
+        expect(renamedSrcs).toContain(o.p);
+      }
+    });
+
+    it('never writes plaintext secret to disk', async () => {
+      await credentialManager.store('dc106b.key', 'plaintext-canary-9f1a');
+      const raw = mockFsState.files[CREDENTIALS_FILE];
+      expect(raw).toBeDefined();
+      expect(raw).not.toContain('plaintext-canary-9f1a');
+      expect(raw).toContain('enc:'); // crypto-utils mock prefix
+    });
+
+    it('_lockedUpdate closes fd before rename (torn-write window eliminated)', async () => {
+      const releaseFn = jest.fn().mockResolvedValue();
+      lockfile.lock.mockResolvedValue(releaseFn);
+      mockFsState.files[CREDENTIALS_FILE] = '{}';
+
+      await credentialManager._lockedUpdate((creds) => {
+        creds.k = { value: 'enc:x' };
+        return creds;
+      });
+
+      // fd lifecycle: open -> write -> fsync -> close -> rename. The dir
+      // fsync adds a second openSync/closeSync pair — so assert on counts of
+      // payload operations and the GLOBAL invocation order, which jest tracks
+      // across mocks (invocationCallOrder).
+      const wxOpens = mockFsState.openedWith.filter((o) => o.flags === 'wx');
+      expect(wxOpens.length).toBe(1); // exactly one payload write
+      expect(fs.writeSync).toHaveBeenCalledTimes(1); // dir fsync writes nothing
+      expect(fs.renameSync).toHaveBeenCalledTimes(1);
+      const fsyncFirst = fs.fsyncSync.mock.invocationCallOrder[0];
+      const closeFirst = fs.closeSync.mock.invocationCallOrder[0];
+      const renameFirst = fs.renameSync.mock.invocationCallOrder[0];
+      expect(fsyncFirst).toBeDefined();
+      expect(closeFirst).toBeGreaterThan(fsyncFirst); // fsync before close
+      expect(renameFirst).toBeGreaterThan(closeFirst); // close before rename
+      expect(mockFsState.files[CREDENTIALS_FILE]).toContain('enc:x');
+    });
+
+    it('_ensureFileExists creates initial {} atomically at 0600 when absent', async () => {
+      const releaseFn = jest.fn().mockResolvedValue();
+      lockfile.lock.mockResolvedValue(releaseFn);
+      delete mockFsState.files[CREDENTIALS_FILE]; // absent on disk
+
+      await credentialManager._lockedUpdate((c) => {
+        c.k = { value: 'enc:x' };
+        return c;
+      });
+
+      const wxOpens = mockFsState.openedWith.filter((o) => o.flags === 'wx');
+      expect(wxOpens.length).toBe(2); // ensure-created '{}' + the locked update
+      expect(wxOpens[0].mode).toBe(0o600);
+      // The ensure write staged its tmp FIRST and renamed it into place before
+      // the locked update renamed over it — creation itself was atomic.
+      expect(fs.renameSync.mock.calls[0][0]).toBe(wxOpens[0].p);
+      const final = JSON.parse(mockFsState.files[CREDENTIALS_FILE]);
+      expect(final.k.value).toBe('enc:x');
+    });
+
+  });
+
   describe('cache TTL', () => {
     it('cache entries expire after TTL', async () => {
       credentialManager.cache.set('ttl.key', {