Remove GIT_INDEX_FILE from env before running hooks

crates/prek/src/git.rs

@@ -45,7 +45,7 @@ pub(crate) static GIT_ROOT: LazyLock<Result<PathBuf, Error>> = LazyLock::new(||
 /// and set `GIT_INDEX_FILE` to point to it.
 /// We need to keep the `GIT_INDEX_FILE` env var to make sure `git write-tree` works correctly.
 /// <https://stackoverflow.com/questions/65639403/git-pre-commit-hook-how-can-i-get-added-modified-files-when-commit-with-a-flag/65647202#65647202>
-pub(crate) static GIT_ENV_TO_REMOVE: LazyLock<Vec<(String, String)>> = LazyLock::new(|| {
+pub(crate) static GIT_ENVS_TO_REMOVE: LazyLock<Vec<(String, String)>> = LazyLock::new(|| {
     let keep = &[
         "GIT_EXEC_PATH",
         "GIT_SSH",

crates/prek/src/process.rs

@@ -408,7 +408,7 @@ impl Cmd {
 
     /// Remove some git-specific environment variables to make git commands isolated.
     pub fn remove_git_envs(&mut self) -> &mut Self {
-        for (key, _) in crate::git::GIT_ENV_TO_REMOVE.iter() {
+        for (key, _) in crate::git::GIT_ENVS_TO_REMOVE.iter() {
             self.inner.env_remove(key);
         }
         self

crates/prek/tests/run.rs

@@ -2118,6 +2118,106 @@ fn git_commit_a() -> Result<()> {
     Ok(())
 }
 
+#[cfg(unix)]
+#[test]
+fn git_commit_a_currently_fails_when_hook_writes_to_temp_git_index() -> Result<()> {
+    let context = TestContext::new();
+    context.init_project();
+
+    // Repro for #1786 documenting the current behavior.
+    // `git commit -a` exports `GIT_INDEX_FILE=.git/index.lock` to the pre-commit hook
+    // process. If the hook inherits that env var and then runs a git command that writes
+    // to an index in a different repository, Git will write those entries into the parent
+    // repo's temporary index instead.
+    //
+    // The important detail is that the temp repo stages `file.txt`, matching a tracked
+    // path in the parent repo. That makes prek's post-hook `git diff` read the corrupted
+    // parent index entry and fail with `fatal: unable to read <hash>`.
+    context
+        .work_dir()
+        .child("hook.sh")
+        .write_str(indoc::indoc! {r#"
+        set -eu
+        tmpdir="$(mktemp -d)"
+        trap 'rm -rf "$tmpdir"' EXIT
+        cd "$tmpdir"
+        git init >/dev/null 2>&1
+        printf 'hook version\n' > file.txt
+        git add file.txt
+    "#})?;
+
+    context.write_pre_commit_config(indoc::indoc! {r"
+        repos:
+          - repo: local
+            hooks:
+              - id: write-temp-index
+                name: write-temp-index
+                language: system
+                entry: sh hook.sh
+                pass_filenames: false
+                always_run: true
+                verbose: true
+    "});
+
+    let cwd = context.work_dir();
+    let file = cwd.child("file.txt");
+    file.write_str("Hello, world!\n")?;
+
+    cmd_snapshot!(context.filters(), context.install(), @r#"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    prek installed at `.git/hooks/pre-commit`
+
+    ----- stderr -----
+    "#);
+
+    context.git_add(".");
+    context.git_commit("Initial commit");
+
+    // `git commit` does not set `GIT_INDEX_FILE`; `git commit -a` does.
+    // The repro only triggers on the `-a` path.
+    file.write_str("Hello again!\n")?;
+
+    let mut commit = git_cmd(cwd);
+    commit
+        .arg("commit")
+        .arg("-a")
+        .arg("-m")
+        .arg("Update file")
+        .env(EnvVars::PREK_HOME, &**context.home_dir());
+
+    let filters = context
+        .filters()
+        .into_iter()
+        .chain([
+            (r"\[master \w{7}\]", r"[master COMMIT]"),
+            (
+                r"fatal: unable to read [0-9a-f]{40}",
+                "fatal: unable to read [HASH]",
+            ),
+        ])
+        .collect::<Vec<_>>();
+
+    cmd_snapshot!(filters, commit, @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Command `git diff` exited with an error:
+
+    [status]
+    exit status: 128
+
+    [stderr]
+    fatal: unable to read [HASH]
+    "
+    );
+
+    Ok(())
+}
+
 fn write_pre_commit_config(path: &Path, hooks: &[(&str, &str)]) -> Result<()> {
     let mut yaml = String::from(indoc::indoc! {"
         repos: