fix(fleet): preserve managed link provenance

AMD1213-B3: record Mosaic-owned links and refuse foreign or retargeted symlink mutations. Out-of-scope review follow-up: settings output/snapshot apply-time TOCTOU remains reported, not patched.
2026-08-13 14:38:25 -05:00
parent 4fde3f622d
commit fe2cf19461
2 changed files with 350 additions and 35 deletions
@@ -1,13 +1,17 @@
 import {
+  closeSync,
  lstatSync,
  mkdirSync,
+  openSync,
  readFileSync,
  readlinkSync,
  readdirSync,
  realpathSync,
+  renameSync,
  rmSync,
  symlinkSync,
  writeFileSync,
+  writeSync,
  type Stats,
 } from 'node:fs';
 import { basename, dirname, isAbsolute, join, relative, resolve, sep } from 'node:path';
@@ -91,6 +95,16 @@ interface PlannedLink {
  readonly target: string;
 }

+interface ManagedLinkManifest {
+  readonly links: Record<string, string>;
+}
+
+interface ManagedLinkState {
+  readonly path: string;
+  readonly links: Map<string, string>;
+  readonly existed: boolean;
+}
+
 export interface FleetLaunchComposition {
  readonly name: string;
  readonly profilePath: string;
@@ -113,6 +127,7 @@ export interface FleetLaunchComposition {
    readonly link: string;
    readonly target: string;
  };
+  readonly managedLinks: ManagedLinkState;
  readonly installs: readonly PlannedLink[];
  readonly prune: readonly string[];
  readonly env: Readonly<Record<string, string>>;
@@ -374,6 +389,7 @@ function resolveCredential(
  profile: FleetAgentLaunchProfile,
  userHome: string,
  seatHome: string,
+  managedLinks: ManagedLinkState,
 ): Pick<FleetLaunchComposition, 'bundle' | 'credential'> {
  assertRealDirectory(userHome, 'user Mosaic root');
  const realUserHome = realpathSync(userHome);
@@ -425,6 +441,16 @@ function resolveCredential(

  const credentialLink = join(seatHome, CREDENTIAL_FILES[profile.harness]);
  const seatInfo = lstatIfPresent(credentialLink);
+  if (seatInfo?.isSymbolicLink() && !managedLinks.existed) {
+    const existingTarget = currentLinkTarget(credentialLink);
+    try {
+      assertContained(resolvedAuthRoot, realpathSync(existingTarget), 'credential migration');
+      // A pre-manifest seat may adopt only a credential link inside the central auth root.
+      managedLinks.links.set(credentialLink, existingTarget);
+    } catch {
+      // Foreign links remain unrecorded and are refused by apply before any writes.
+    }
+  }
  if (seatInfo && !seatInfo.isSymbolicLink()) {
    throw new FleetLaunchError(
      'FIRST_AUTH_REFUSAL',
@@ -449,11 +475,16 @@ function resolveCredential(
  };
 }

+function currentLinkTarget(link: string): string {
+  return resolve(dirname(link), readlinkSync(link));
+}
+
 function resolveManagedLinks(
  kind: PlannedLink['kind'],
  names: readonly string[],
  userHome: string,
  seatHome: string,
+  managedLinks: ManagedLinkState,
 ): { installs: PlannedLink[]; prune: string[] } {
  const plural = kind === 'plugin' ? 'plugins' : 'skills';
  const storeRoot = join(userHome, plural);
@@ -494,7 +525,6 @@ function resolveManagedLinks(
  if (installRootInfo?.isDirectory()) {
    for (const entry of readdirSync(installRoot, { withFileTypes: true })) {
      const path = join(installRoot, entry.name);
-      if (desired.has(entry.name)) continue;
      if (!entry.isSymbolicLink()) {
        // The harness writes its own metadata files (e.g. installed_plugins.json)
        // beside the managed links; only a real directory is an unmanaged entry
@@ -507,12 +537,107 @@ function resolveManagedLinks(
        }
        continue;
      }
-      prune.push(path);
+      const currentTarget = currentLinkTarget(path);
+      const recordedTarget = managedLinks.links.get(path);
+      if (recordedTarget === undefined) {
+        if (
+          !managedLinks.existed &&
+          (() => {
+            try {
+              assertContained(
+                realpathSync(storeRoot),
+                realpathSync(currentTarget),
+                `${kind} migration`,
+              );
+              return true;
+            } catch {
+              return false;
+            }
+          })()
+        ) {
+          // A pre-manifest seat may adopt only links to the central Mosaic store; foreign links refuse.
+          managedLinks.links.set(path, currentTarget);
+        }
+      } else if (recordedTarget !== currentTarget) {
+        throw new FleetLaunchError(
+          'COMPOSITION_FAILED',
+          `managed ${kind} symlink target changed since composition: ${path}`,
+        );
+      }
+      if (!desired.has(entry.name)) prune.push(path);
    }
  }
  return { installs, prune };
 }

+function readManagedLinkState(seatHome: string): ManagedLinkState {
+  const path = join(seatHome, '.mosaic-managed-links.json');
+  const info = lstatIfPresent(path);
+  if (!info) return { path, links: new Map(), existed: false };
+  if (!info.isFile() || info.isSymbolicLink()) {
+    throw new FleetLaunchError(
+      'COMPOSITION_FAILED',
+      `managed link manifest must be a real, non-symlink JSON file: ${path}`,
+    );
+  }
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(readFileSync(path, 'utf8')) as unknown;
+  } catch (error: unknown) {
+    const detail = error instanceof Error ? error.message : String(error);
+    throw new FleetLaunchError(
+      'COMPOSITION_FAILED',
+      `managed link manifest is invalid JSON: ${detail}`,
+    );
+  }
+  if (!isPlainObject(parsed) || !isPlainObject(parsed['links'])) {
+    throw new FleetLaunchError(
+      'COMPOSITION_FAILED',
+      `managed link manifest has invalid shape: ${path}`,
+    );
+  }
+  const links = new Map<string, string>();
+  for (const [link, target] of Object.entries(parsed['links'])) {
+    if (typeof target !== 'string' || !isAbsolute(link) || !isAbsolute(target)) {
+      throw new FleetLaunchError(
+        'COMPOSITION_FAILED',
+        `managed link manifest has invalid entry: ${path}`,
+      );
+    }
+    links.set(link, target);
+  }
+  return { path, links, existed: true };
+}
+
+interface PreparedManagedLinkManifest {
+  readonly path: string;
+  readonly descriptor: number;
+}
+
+function prepareManagedLinkManifest(managedLinks: ManagedLinkState): PreparedManagedLinkManifest {
+  const path = `${managedLinks.path}.tmp`;
+  try {
+    return { path, descriptor: openSync(path, 'wx', 0o600) };
+  } catch (error: unknown) {
+    const detail = error instanceof Error ? error.message : String(error);
+    throw new FleetLaunchError(
+      'COMPOSITION_FAILED',
+      `managed link manifest temporary file cannot be created exclusively: ${detail}`,
+    );
+  }
+}
+
+function writeManagedLinkState(
+  managedLinks: ManagedLinkState,
+  prepared: PreparedManagedLinkManifest,
+): void {
+  const entries = [...managedLinks.links.entries()].sort(([left], [right]) =>
+    left.localeCompare(right),
+  );
+  const manifest: ManagedLinkManifest = { links: Object.fromEntries(entries) };
+  writeSync(prepared.descriptor, `${JSON.stringify(manifest, null, 2)}\n`);
+}
+
 function buildArgv(
  profile: FleetAgentLaunchProfile,
  seatHome: string,
@@ -594,9 +719,22 @@ export function resolveFleetLaunchComposition(
      : readSettingsLayer('agent', overlayPath, false),
  ];
  const merged = deepMergeSettings(...layers.map((layer) => layer.value));
-  const credential = resolveCredential(profile, roots.userHome, seatHome);
-  const plugins = resolveManagedLinks('plugin', profile.plugins, roots.userHome, seatHome);
-  const skills = resolveManagedLinks('skill', profile.skills, roots.userHome, seatHome);
+  const managedLinks = readManagedLinkState(seatHome);
+  const credential = resolveCredential(profile, roots.userHome, seatHome, managedLinks);
+  const plugins = resolveManagedLinks(
+    'plugin',
+    profile.plugins,
+    roots.userHome,
+    seatHome,
+    managedLinks,
+  );
+  const skills = resolveManagedLinks(
+    'skill',
+    profile.skills,
+    roots.userHome,
+    seatHome,
+    managedLinks,
+  );
  const homeEnvName: Record<RuntimeName, string> = {
    claude: 'CLAUDE_CONFIG_DIR',
    pi: 'PI_CODING_AGENT_DIR',
@@ -621,6 +759,7 @@ export function resolveFleetLaunchComposition(
      snapshot: settingsSnapshot,
    },
    ...credential,
+    managedLinks,
    installs: [...plugins.installs, ...skills.installs],
    prune: [...plugins.prune, ...skills.prune],
    env,
@@ -628,11 +767,20 @@ export function resolveFleetLaunchComposition(
  };
 }

-function ensureSymlink(link: string, target: string): void {
+function ensureSymlink(link: string, target: string, managedLinks: ManagedLinkState): void {
  const info = lstatIfPresent(link);
  if (info?.isSymbolicLink()) {
-    const current = resolve(dirname(link), readlinkSync(link));
-    if (current === target) return;
+    const current = currentLinkTarget(link);
+    if (current === target) {
+      managedLinks.links.set(link, target);
+      return;
+    }
+    if (managedLinks.links.get(link) !== current) {
+      throw new FleetLaunchError(
+        'COMPOSITION_FAILED',
+        `unrecorded or retargeted symlink occupies managed path: ${link}`,
+      );
+    }
    rmSync(link);
  } else if (info) {
    throw new FleetLaunchError(
@@ -642,6 +790,30 @@ function ensureSymlink(link: string, target: string): void {
  }
  mkdirSync(dirname(link), { recursive: true });
  symlinkSync(target, link, 'file');
+  managedLinks.links.set(link, target);
+}
+
+function assertManagedLinkMutationAllowed(
+  link: string,
+  target: string | undefined,
+  managedLinks: ManagedLinkState,
+): void {
+  const info = lstatIfPresent(link);
+  if (!info) return;
+  if (!info.isSymbolicLink()) {
+    throw new FleetLaunchError(
+      'COMPOSITION_FAILED',
+      `real object occupies managed symlink path ${link}; refusing to delete it.`,
+    );
+  }
+  const current = currentLinkTarget(link);
+  if (target !== undefined && current === target) return;
+  if (managedLinks.links.get(link) !== current) {
+    throw new FleetLaunchError(
+      'COMPOSITION_FAILED',
+      `unrecorded or retargeted symlink occupies managed path: ${link}`,
+    );
+  }
 }

 function canonicalJson(value: unknown): unknown {
@@ -657,21 +829,57 @@ function canonicalJson(value: unknown): unknown {
 /** Apply a previously resolved plan. No caller should apply a dry-run plan. */
 export function applyFleetLaunchComposition(plan: FleetLaunchComposition): void {
  mkdirSync(plan.seatHome, { recursive: true });
-  const settings = `${JSON.stringify(canonicalJson(plan.settings.merged), null, 2)}\n`;
-  writeFileSync(plan.settings.output, settings, { mode: 0o600 });
-  writeFileSync(plan.settings.snapshot, settings, { mode: 0o600 });
-  ensureSymlink(plan.credential.link, plan.credential.target);
-  for (const path of plan.prune) {
-    const info = lstatIfPresent(path);
-    if (info?.isSymbolicLink()) rmSync(path);
-    else if (info) {
-      throw new FleetLaunchError(
-        'COMPOSITION_FAILED',
-        `real object replaced managed symlink before prune: ${path}`,
-      );
+  const preparedManifest = prepareManagedLinkManifest(plan.managedLinks);
+  let committedManifest = false;
+  try {
+    assertManagedLinkMutationAllowed(
+      plan.credential.link,
+      plan.credential.target,
+      plan.managedLinks,
+    );
+    for (const path of plan.prune) {
+      assertManagedLinkMutationAllowed(path, undefined, plan.managedLinks);
+    }
+    for (const install of plan.installs) {
+      assertManagedLinkMutationAllowed(install.link, install.target, plan.managedLinks);
+    }
+
+    const settings = `${JSON.stringify(canonicalJson(plan.settings.merged), null, 2)}\n`;
+    writeFileSync(plan.settings.output, settings, { mode: 0o600 });
+    writeFileSync(plan.settings.snapshot, settings, { mode: 0o600 });
+    ensureSymlink(plan.credential.link, plan.credential.target, plan.managedLinks);
+    for (const path of plan.prune) {
+      const info = lstatIfPresent(path);
+      if (info?.isSymbolicLink()) {
+        const current = currentLinkTarget(path);
+        if (plan.managedLinks.links.get(path) !== current) {
+          throw new FleetLaunchError(
+            'COMPOSITION_FAILED',
+            `unrecorded or retargeted symlink cannot be pruned: ${path}`,
+          );
+        }
+        rmSync(path);
+        plan.managedLinks.links.delete(path);
+      } else if (info) {
+        throw new FleetLaunchError(
+          'COMPOSITION_FAILED',
+          `real object replaced managed symlink before prune: ${path}`,
+        );
+      }
+    }
+    for (const install of plan.installs) {
+      ensureSymlink(install.link, install.target, plan.managedLinks);
+    }
+    writeManagedLinkState(plan.managedLinks, preparedManifest);
+    closeSync(preparedManifest.descriptor);
+    renameSync(preparedManifest.path, plan.managedLinks.path);
+    committedManifest = true;
+  } finally {
+    if (!committedManifest) {
+      closeSync(preparedManifest.descriptor);
+      rmSync(preparedManifest.path, { force: true });
    }
  }
-  for (const install of plan.installs) ensureSymlink(install.link, install.target);
 }

 /** Stable, auditable text representation used by --dry-run and snapshot tests. */