[PATCH v2] add: support pre-add hook

From: Chandra Kethi-Reddy <chandrakr@pm.me>

"git add" has no hook that lets users inspect what is about to be staged. Users who want to reject certain paths or content must wrap the command in a shell alias or wait for pre-commit, which fires after staging is already done and objects may already be in the object database.

Introduce a "pre-add" hook that runs after "git add" computes the new index state but before committing it to disk. The hook receives two arguments:

  $1 -- path to a temporary copy of the index before this "git add"
  $2 -- path to the lockfile containing the proposed index

$1 on first add can be a non-existent path representing an empty index.

Hook authors can inspect the computed result with ordinary tools:

  GIT_INDEX_FILE="$2" git diff --cached --name-only HEAD

without needing to interpret pathspec or mode flags like "-u" or "--renormalize" -- the proposed index already reflects their effect.

The implementation creates a temporary copy of the index via the tempfile API when find_hook("pre-add") reports a hook is present, then lets all staging proceed normally. At the finish label, write_locked_index() writes the proposed index to the lockfile without COMMIT_LOCK. If the hook approves, commit_lock_file() atomically replaces the index. If the hook rejects, rollback_lock_file() discards the lockfile and the original index is left unchanged. When no hook is installed, the existing write_locked_index(COMMIT_LOCK | SKIP_IF_UNCHANGED) path is still taken.

The hook is bypassed with "--no-verify" and is not invoked for --interactive, --patch, --edit, or --dry-run, nor by "git commit -a" which stages through its own code path.

Register t3706-pre-add-hook.sh in t/meson.build to synchronize Meson and Makefile lists.

Signed-off-by: Chandra Kethi-Reddy <chandrakr@pm.me>
---
    add: support pre-add hook
    
    
    Summary
    =======
    
     * v2 reworks pre-add to follow Junio's suggested architecture: snapshot
       original index, compute staging normally, write proposed index to
       lockfile, run hook with ($1 original, $2 proposed), then
       commit_lock_file() or rollback_lock_file().
     * Hook authors now inspect computed results directly with
       GIT_INDEX_FILE="$1" / GIT_INDEX_FILE="$2" instead of trying to
       emulate pathspec/mode behavior.
     * Added tests for two-argument contract, original-vs-proposed
       comparison, explicit rollback behavior on hook rejection, and example
       policies (filename/content rejection).
    
    
    Notes
    =====
    
     * This design intentionally trades ODB prevention for correctness of
       hook inputs: blobs may already be written to object storage when the
       hook runs, but hook rejection still leaves the on-disk index
       unchanged.
     * Conflicts with ar/parallel-hooks on seen:
       RUN_HOOKS_OPT_INIT → RUN_HOOKS_OPT_INIT_SERIAL.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2045%2Fshatachandra%2Fpre-add-hooks-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2045/shatachandra/pre-add-hooks-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/2045

Range-diff vs v1:

 1:  964bec5b9ea ! 1:  10244150e24 add: support pre-add hook
     @@ Metadata
       ## Commit message ##
          add: support pre-add hook
      
     -    git has no hook that fires during 'git add'.  Users who want to
     -    validate files before staging must wrap 'git add' in a shell alias
     -    or wait for pre-commit, which fires after staging is already done.
     +    "git add" has no hook that lets users inspect what is about to be
     +    staged. Users who want to reject certain paths or content must
     +    wrap the command in a shell alias or wait for pre-commit, which
     +    fires after staging is already done and objects may already be in
     +    the object database.
      
     -    Add a pre-add hook that runs after pathspec validation and before
     -    any files are staged.  If the hook exits non-zero, 'git add' aborts
     -    without modifying the index.  The hook receives GIT_INDEX_FILE in
     -    its environment, following the same convention as pre-commit.
     +    Introduce a "pre-add" hook that runs after "git add" computes the
     +    new index state but before committing it to disk. The hook
     +    receives two arguments:
      
     -    The hook is bypassed with '--no-verify' (long flag only, since '-n'
     -    is already '--dry-run' in 'git add').  It is not invoked for
     -    --interactive, --patch, --edit, or --dry-run modes, nor by
     -    'git commit -a' which stages files through its own code path in
     -    builtin/commit.c.
     +      $1 -- path to a temporary copy of the index before this "git add"
     +      $2 -- path to the lockfile containing the proposed index
      
     -    The implementation calls run_hooks_opt() directly rather than the
     -    run_commit_hook() wrapper, which sets GIT_EDITOR=: and is not
     -    relevant for 'git add'.  When no hook is installed, there is no
     -    performance impact.
     +    $1 on first add can be a non-existent path representing an empty
     +    index.
      
     -    Disclosure: developed with guidance from Claude Code (Anthropic)
     -    and Codex CLI (OpenAI) for development, review and standards
     -    compliance. The contributor handtyped and reviewed all tests, code,
     -    and documentation.
     +    Hook authors can inspect the computed result with ordinary tools:
     +
     +      GIT_INDEX_FILE="$2" git diff --cached --name-only HEAD
     +
     +    without needing to interpret pathspec or mode flags like "-u" or
     +    "--renormalize" -- the proposed index already reflects their effect.
     +
     +    The implementation creates a temporary copy of the index via the
     +    tempfile API when find_hook("pre-add") reports a hook is present,
     +    then lets all staging proceed normally. At the finish label,
     +    write_locked_index() writes the proposed index to the lockfile
     +    without COMMIT_LOCK. If the hook approves, commit_lock_file()
     +    atomically replaces the index. If the hook rejects,
     +    rollback_lock_file() discards the lockfile and the original index
     +    is left unchanged. When no hook is installed, the existing
     +    write_locked_index(COMMIT_LOCK | SKIP_IF_UNCHANGED) path is still
     +    taken.
     +
     +    The hook is bypassed with "--no-verify" and is not invoked for
     +    --interactive, --patch, --edit, or --dry-run, nor by "git commit -a"
     +    which stages through its own code path.
     +
     +    Register t3706-pre-add-hook.sh in t/meson.build to synchronize Meson
     +    and Makefile lists.
      
          Signed-off-by: Chandra Kethi-Reddy <chandrakr@pm.me>
      
     @@ Documentation/git-add.adoc: use the `--force` option to add ignored files. If yo
       filename of an ignored file, `git add` will fail with a list of ignored
       files. Otherwise it will silently ignore the file.
       
     -+A pre-add hook can be run to inspect or reject the add operation before
     -+it stages files. See linkgit:githooks[5] for details.
     ++A pre-add hook can be run to inspect or reject the proposed index update
     ++after `git add` computes staging and writes it to the index lockfile,
     ++but before writing it to the final index. See linkgit:githooks[5].
      +
       Please see linkgit:git-commit[1] for alternative ways to add content to a
       commit.
     @@ Documentation/githooks.adoc: and is invoked after the patch is applied and a com
      +~~~~~~~
      +
      +This hook is invoked by linkgit:git-add[1], and can be bypassed with the
     -+`--no-verify` option. This hook is not invoked for `--interactive`, `--patch`,
     -+`--edit`, or `--dry-run`. It takes no parameters, and is invoked after pathspec
     -+validation and before any files are staged. Exiting with a non-zero status
     -+from this script causes the `git add` command to abort without modifying the
     -+index.
     ++`--no-verify` option. It is not invoked for `--interactive`, `--patch`,
     ++`--edit`, or `--dry-run`.
      +
     -+This hook is invoked with the environment variable `GIT_INDEX_FILE`
     -+which points to the index file. This allows the hook to inspect what
     -+files would be staged before the operation proceeds.
     ++It takes two parameters: the path to a copy of the index before this
     ++invocation of `git add`, and the path to the lockfile containing the
     ++proposed index after staging. It does not read from standard input.
     ++If no index exists yet, the first parameter names a path that does not
     ++exist and should be treated as an empty index. No special environment
     ++variables are set. The hook is invoked after the index has been updated
     ++in memory and written to the lockfile, but before it is committed to the
     ++final location.
      +
     -+This hook is not invoked by `git commit -a` or `git commit --include` which
     -+still can run the pre-commit hook, providing a control point at commit time.
     ++Exiting with a non-zero status causes `git add` to abort and leaves the
     ++index unchanged. Exiting with zero status causes the staged changes to
     ++take effect.
     ++
     ++This hook can be used to prevent staging of files based on names, content,
     ++or sizes (e.g., to block `.env` files, secret keys, or large files).
     ++
     ++This hook is not invoked by `git commit -a` or `git commit --include`
     ++which still can run the pre-commit hook, providing a control point at
     ++commit time.
      +
       pre-commit
       ~~~~~~~~~~
     @@ builtin/add.c
       #include "submodule.h"
       #include "add-interactive.h"
      +#include "hook.h"
     ++#include "copy.h"
       
       static const char * const builtin_add_usage[] = {
       	N_("git add [<options>] [--] <pathspec>..."),
     @@ builtin/add.c: static struct option builtin_add_options[] = {
       	OPT_BOOL(0, "sparse", &include_sparse, N_("allow updating entries outside of the sparse-checkout cone")),
       	OPT_STRING(0, "chmod", &chmod_arg, "(+|-)x",
       		   N_("override the executable bit of the listed files")),
     +@@ builtin/add.c: int cmd_add(int argc,
     + 	char *ps_matched = NULL;
     + 	struct lock_file lock_file = LOCK_INIT;
     + 	struct odb_transaction *transaction;
     ++	int run_pre_add = 0;
     ++	struct tempfile *orig_index = NULL;
     ++	char *orig_index_path = NULL;
     + 
     + 	repo_config(repo, add_config, NULL);
     + 
      @@ builtin/add.c: int cmd_add(int argc,
       		string_list_clear(&only_match_skip_worktree, 0);
       	}
       
     -+	if (!show_only && !no_verify) {
     ++	if (!show_only && !no_verify && find_hook(repo, "pre-add")) {
     ++		int fd_in, status;
     ++		const char *index_file = repo_get_index_file(repo);
     ++		char *template;
     ++
     ++		run_pre_add = 1;
     ++		template = xstrfmt("%s.pre-add.XXXXXX", index_file);
     ++		orig_index = xmks_tempfile(template);
     ++		free(template);
     ++
     ++		fd_in = open(index_file, O_RDONLY);
     ++		if (fd_in >= 0) {
     ++			status = copy_fd(fd_in, get_tempfile_fd(orig_index));
     ++			if (close(fd_in))
     ++				die_errno(_("unable to close index for pre-add hook"));
     ++			if (close_tempfile_gently(orig_index))
     ++				die_errno(_("unable to close temporary index copy"));
     ++			if (status < 0)
     ++				die(_("failed to copy index for pre-add hook"));
     ++		} else if (errno == ENOENT) {
     ++			orig_index_path = xstrdup(get_tempfile_path(orig_index));
     ++			if (delete_tempfile(&orig_index))
     ++				die_errno(_("unable to remove temporary index copy"));
     ++		} else {
     ++			die_errno(_("unable to open index for pre-add hook"));
     ++		}
     ++	}
     ++
     + 	transaction = odb_transaction_begin(repo->objects);
     + 
     + 	ps_matched = xcalloc(pathspec.nr, 1);
     +@@ builtin/add.c: int cmd_add(int argc,
     + 						  include_sparse, flags);
     + 
     + 	if (take_worktree_changes && !add_renormalize && !ignore_add_errors &&
     +-	    report_path_error(ps_matched, &pathspec))
     ++	    report_path_error(ps_matched, &pathspec)) {
     ++		if (orig_index)
     ++			delete_tempfile(&orig_index);
     ++		free(orig_index_path);
     + 		exit(128);
     ++	}
     + 
     + 	if (add_new_files)
     + 		exit_status |= add_files(repo, &dir, flags);
     +@@ builtin/add.c: int cmd_add(int argc,
     + 	odb_transaction_commit(transaction);
     + 
     + finish:
     +-	if (write_locked_index(repo->index, &lock_file,
     +-			       COMMIT_LOCK | SKIP_IF_UNCHANGED))
     +-		die(_("unable to write new index file"));
     ++	if (run_pre_add && !exit_status && repo->index->cache_changed) {
      +		struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;
      +
     -+		strvec_pushf(&opt.env, "GIT_INDEX_FILE=%s",
     -+			     repo_get_index_file(repo));
     ++		if (write_locked_index(repo->index, &lock_file, 0))
     ++			die(_("unable to write new index file"));
     ++
     ++		strvec_push(&opt.args, orig_index ? get_tempfile_path(orig_index) :
     ++					     orig_index_path);
     ++		strvec_push(&opt.args, get_lock_file_path(&lock_file));
      +		if (run_hooks_opt(repo, "pre-add", &opt)) {
     ++			rollback_lock_file(&lock_file); /* hook rejected */
      +			exit_status = 1;
     -+			goto finish;
     ++		} else {
     ++			if (commit_lock_file(&lock_file)) /* hook approved */
     ++				die(_("unable to write new index file"));
      +		}
     ++	} else {
     ++		if (write_locked_index(repo->index, &lock_file,
     ++				       COMMIT_LOCK | SKIP_IF_UNCHANGED))
     ++			die(_("unable to write new index file"));
      +	}
      +
     - 	transaction = odb_transaction_begin(repo->objects);
     ++	delete_tempfile(&orig_index);
     ++	free(orig_index_path);
       
     - 	ps_matched = xcalloc(pathspec.nr, 1);
     + 	free(ps_matched);
     + 	dir_clear(&dir);
     +
     + ## t/meson.build ##
     +@@ t/meson.build: integration_tests = [
     +   't3703-add-magic-pathspec.sh',
     +   't3704-add-pathspec-file.sh',
     +   't3705-add-sparse-checkout.sh',
     ++  't3706-pre-add-hook.sh',
     +   't3800-mktag.sh',
     +   't3900-i18n-commit.sh',
     +   't3901-i18n-patch.sh',
      
       ## t/t3706-pre-add-hook.sh (new) ##
      @@
     @@ t/t3706-pre-add-hook.sh (new)
      +	test_path_is_missing actual
      +'
      +
     -+test_expect_success 'hook receives GIT_INDEX_FILE environment variable' '
     -+	test_when_finished "rm -f actual expected" &&
     -+	echo "hook-saw-env" >expected &&
     ++test_expect_success 'hook receives original and proposed index as arguments' '
     ++	test_when_finished "rm -f tracked expected hook-ran" &&
     ++	echo "initial" >tracked &&
     ++	git add tracked &&
     ++	git commit -m "initial" &&
      +	test_hook pre-add <<-\EOF &&
     -+	if test -z "$GIT_INDEX_FILE"
     -+	then
     -+		echo hook-missing-env >>actual
     -+	else
     -+		echo hook-saw-env >>actual
     -+	fi
     ++	test $# -eq 2 &&
     ++	test -f "$1" &&
     ++	test -f "$2" &&
     ++	echo pass >hook-ran
      +	EOF
      +
     -+	echo content >file &&
     -+	git add file &&
     -+	test_cmp expected actual
     ++	echo "modified" >tracked &&
     ++	git add tracked &&
     ++	echo pass >expected &&
     ++	test_cmp expected hook-ran
     ++'
     ++
     ++test_expect_success 'hook handles first add with no existing index' '
     ++	test_when_finished "rm -rf no-index" &&
     ++	test_create_repo no-index &&
     ++	echo ok >no-index/expected &&
     ++	test_hook -C no-index pre-add <<-\EOF &&
     ++	test $# -eq 2 &&
     ++	test ! -e "$1" &&
     ++	test -f "$2" &&
     ++	echo ok >hook-ran
     ++	EOF
     ++
     ++	echo first >no-index/file &&
     ++	git -C no-index add file &&
     ++	test_cmp no-index/expected no-index/hook-ran
      +'
      +
     -+test_expect_success 'with --dry-run (show-only) the hook is not invoked' '
     ++test_expect_success 'hook is not invoked with --dry-run (show-only)' '
      +	test_when_finished "rm -f actual" &&
      +	test_hook pre-add <<-\EOF &&
      +	echo should-not-run >>actual
     @@ t/t3706-pre-add-hook.sh (new)
      +	test_cmp expected actual
      +'
      +
     ++test_expect_success 'hook can compare original and proposed index' '
     ++	test_when_finished "rm -f old-raw new-raw old-list new-list \
     ++			    expected-old expected-new" &&
     ++	echo "initial" >file1 &&
     ++	echo "initial" >file2 &&
     ++	git add file1 file2 &&
     ++	git commit -m "initial" &&
     ++	echo "staged-before" >file1 &&
     ++	git add file1 &&
     ++	test_hook pre-add <<-\EOF &&
     ++	GIT_INDEX_FILE="$1" git diff --cached --name-only HEAD >old-raw &&
     ++	GIT_INDEX_FILE="$2" git diff --cached --name-only HEAD >new-raw &&
     ++	sort old-raw >old-list &&
     ++	sort new-raw >new-list
     ++	EOF
     ++
     ++	echo "modified" >file2 &&
     ++	git add file2 &&
     ++	echo file1 >expected-old &&
     ++	printf "%s\n" file1 file2 >expected-new &&
     ++	test_cmp expected-old old-list &&
     ++	test_cmp expected-new new-list
     ++'
     ++
     ++test_expect_success 'hook rejection rolls back index unchanged' '
     ++	test_when_finished "rm -f file before after old-raw new-raw \
     ++			    old-list new-list expected-old expected-new" &&
     ++	echo "initial" >file &&
     ++	git add file &&
     ++	git commit -m "initial" &&
     ++	git diff --cached --name-only HEAD >before &&
     ++	test_hook pre-add <<-\EOF &&
     ++	GIT_INDEX_FILE="$1" git diff --cached --name-only HEAD >old-raw &&
     ++	GIT_INDEX_FILE="$2" git diff --cached --name-only HEAD >new-raw &&
     ++	sort old-raw >old-list &&
     ++	sort new-raw >new-list &&
     ++	exit 1
     ++	EOF
     ++
     ++	echo "modified" >file &&
     ++	test_must_fail git add file &&
     ++	git diff --cached --name-only HEAD >after &&
     ++	test_cmp before after &&
     ++	: >expected-old &&
     ++	echo file >expected-new &&
     ++	test_cmp expected-old old-list &&
     ++	test_cmp expected-new new-list
     ++'
     ++
     ++test_expect_success 'hook example: block .env files' '
     ++	test_when_finished "rm -f .env safe.txt new-paths" &&
     ++	echo "initial" >base &&
     ++	git add base &&
     ++	git commit -m "initial" &&
     ++	test_hook pre-add <<-\EOF &&
     ++	GIT_INDEX_FILE="$2" git diff --cached --name-only HEAD >new-paths &&
     ++	while read path
     ++	do
     ++		case "$path" in
     ++		*.env|.env)
     ++			echo "error: $path must not be staged" >&2
     ++			exit 1
     ++			;;
     ++		esac
     ++	done <new-paths
     ++	EOF
     ++
     ++	echo "DB_PASS=secret" >.env &&
     ++	test_must_fail git add .env &&
     ++	echo "safe content" >safe.txt &&
     ++	git add safe.txt
     ++'
     ++
     ++test_expect_success 'hook example: block secrets in content' '
     ++	test_when_finished "rm -f config.txt secret" &&
     ++	echo "initial" >config.txt &&
     ++	git add config.txt &&
     ++	git commit -m "initial" &&
     ++	test_hook pre-add <<-\EOF &&
     ++	GIT_INDEX_FILE="$2" git diff --cached HEAD >secret &&
     ++	if grep -qE "(API_KEY|SECRET_KEY|PRIVATE_KEY)=" secret
     ++	then
     ++		echo "error: staged content contains secrets" >&2
     ++		exit 1
     ++	fi
     ++	EOF
     ++
     ++	echo "API_KEY=sksksk-live-12345" >config.txt &&
     ++	test_must_fail git add config.txt &&
     ++	echo "LOG_LEVEL=debug" >config.txt &&
     ++	git add config.txt
     ++'
     ++
      +test_done

 Documentation/git-add.adoc  |  11 +-
 Documentation/githooks.adoc |  27 +++++
 builtin/add.c               |  68 ++++++++++-
 t/meson.build               |   1 +
 t/t3706-pre-add-hook.sh     | 227 ++++++++++++++++++++++++++++++++++++
 5 files changed, 329 insertions(+), 5 deletions(-)
 create mode 100755 t/t3706-pre-add-hook.sh