Isolate Git object reads from inherited environment

Author: mchwang

docs/implementation/build-step-1.md

@@ -37,6 +37,10 @@ Copilot reported two findings. Fixed scope after rename: scope now travels with
 
 Declined the claim that repository aliases can override `rev-parse`, `rev-list`, `diff`, or `cat-file`: these are built-in commands, and Git ignores aliases that shadow them. A scratch probe and a permanent adapter regression confirmed the shell alias never ran. See [Git's alias documentation](https://git-scm.com/docs/git-config/2.54.0).
 
+## Review round 2
+
+Fixed inherited Git environment redirection. A regression with `GIT_DIR` pointing at a second repository initially returned the foreign repository's content. The adapter now drops inherited `GIT_*` variables, ignores global/system Git configuration, and disables lazy fetch and transport access. The same regression now reads only the requested repository.
+
 ## Remaining gates
 
 This is a working foundation, not a completed application or a claim that all implementation tasks are done. T18's pure validation/edit core is present; its agent adapters, import UI, and persistence are pending. Ledger storage, rebase mappings, and the read-only review screen remain next. The already-fixed GitHub check belongs to the later GitHub/runner integration.

git/history.ts

@@ -3,9 +3,12 @@ import type { FileDelta, FileVersion, History } from '../core/linking.ts';
 
 /** Read-only Git adapter. Never follows working-tree symlinks or runs diff helpers. */
 export function readHistory(repo: string, baseRef: string, headRef = 'HEAD'): History {
-  const run = (...args: string[]) => execFileSync('git', ['--no-pager', '--no-replace-objects', '-c', 'core.hooksPath=/dev/null', ...args], {
+  // Inherited Git variables can redirect repository, index, config, and object lookup.
+  const environment = Object.fromEntries(Object.entries(process.env).filter(([key]) => !/^GIT_/i.test(key)));
+  const run = (...args: string[]) => execFileSync('git', ['--no-pager', '--no-replace-objects', '-c', 'core.hooksPath=/dev/null', '-c', 'protocol.allow=never', ...args], {
     cwd: repo, maxBuffer: 32 * 1024 * 1024, timeout: 30_000,
-    env: { ...process.env, GIT_OPTIONAL_LOCKS: '0', GIT_TERMINAL_PROMPT: '0' },
+    env: { ...environment, GIT_OPTIONAL_LOCKS: '0', GIT_TERMINAL_PROMPT: '0',
+      GIT_NO_LAZY_FETCH: '1', GIT_CONFIG_NOSYSTEM: '1', GIT_CONFIG_GLOBAL: '/dev/null' },
     stdio: ['ignore', 'pipe', 'pipe'],
   });
   const resolve = (ref: string) => run('rev-parse', '--verify', '--end-of-options', `${ref}^{commit}`).toString().trim();

test/history.test.ts

@@ -161,3 +161,15 @@ it('Git built-ins cannot be overridden by repository shell aliases', () => {
   expect(f.segments().length).toBeGreaterThan(0);
   expect(existsSync(join(f.dir, 'alias-executed'))).toBe(false);
 });
+it('isolates repository selection from inherited Git environment variables', () => {
+  const expected = fixture(); expected.write('a.txt', 'expected repo\n'); expected.commit('P1');
+  const foreign = fixture(); foreign.write('a.txt', 'foreign repo\n'); foreign.commit();
+  const previous = process.env.GIT_DIR;
+  try {
+    process.env.GIT_DIR = join(foreign.dir, '.git');
+    const history = readHistory(expected.dir, 'HEAD~1');
+    expect(history.final[0]!.after!.text).toBe('expected repo\n');
+  } finally {
+    if (previous === undefined) delete process.env.GIT_DIR; else process.env.GIT_DIR = previous;
+  }
+});