add: support pre-add hook

Documentation/git-add.adoc

@@ -11,7 +11,7 @@ SYNOPSIS
 git add [--verbose | -v] [--dry-run | -n] [--force | -f] [--interactive | -i] [--patch | -p]
 	[--edit | -e] [--[no-]all | -A | --[no-]ignore-removal | [--update | -u]] [--sparse]
 	[--intent-to-add | -N] [--refresh] [--ignore-errors] [--ignore-missing] [--renormalize]
-	[--chmod=(+|-)x] [--pathspec-from-file=<file> [--pathspec-file-nul]]
+	[--no-verify] [--chmod=(+|-)x] [--pathspec-from-file=<file> [--pathspec-file-nul]]
 	[--] [<pathspec>...]
 
 DESCRIPTION
@@ -42,10 +42,11 @@ use the `--force` option to add ignored files. If you specify the exact
 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 used to reject `git add` (see linkgit:githooks[5]).
+
 Please see linkgit:git-commit[1] for alternative ways to add content to a
 commit.
 
-
 OPTIONS
 -------
 `<pathspec>...`::
@@ -163,6 +164,10 @@ for `git add --no-all <pathspec>...`, i.e. ignored removed files.
 	Don't add the file(s), but only refresh their stat()
 	information in the index.
 
+`--no-verify`::
+	Bypass the `pre-add` hook if it exists. See linkgit:githooks[5] for
+	more information about hooks.
+
 `--ignore-errors`::
 	If some files could not be added because of errors indexing
 	them, do not abort the operation, but continue adding the
@@ -451,6 +456,7 @@ linkgit:git-reset[1]
 linkgit:git-mv[1]
 linkgit:git-commit[1]
 linkgit:git-update-index[1]
+linkgit:githooks[5]
 
 GIT
 ---

Documentation/githooks.adoc

@@ -94,6 +94,36 @@ and is invoked after the patch is applied and a commit is made.
 This hook is meant primarily for notification, and cannot affect
 the outcome of `git am`.
 
+pre-add
+~~~~~~~
+
+This hook is invoked by linkgit:git-add[1], and can be bypassed with the
+`--no-verify` option. It is not invoked for `--interactive`, `--patch`,
+`--edit`, or `--dry-run`.
+
+It takes two arguments: the path to the index file for this invocation
+of `git add`, and the path to the lockfile containing the proposed
+index after staging. If no index exists yet, the first argument names
+a path that does not exist and should be treated as an empty index.
+
+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 index
+path. Exiting with a non-zero status causes `git add` to reject the
+proposed state, roll back the lockfile, and leave the index unchanged.
+Exiting with zero status allows the index update to be committed. The
+hook accepts or rejects the entire proposed update; per-path filtering
+is not supported. Both files should be treated as read-only by the hook.
+
+Hook authors may set `GIT_INDEX_FILE="$1"` to inspect the current index
+state and `GIT_INDEX_FILE="$2"` to inspect the proposed index state.
+
+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

@@ -25,6 +25,8 @@
 #include "strvec.h"
 #include "submodule.h"
 #include "add-interactive.h"
+#include "hook.h"
+#include "abspath.h"
 
 static const char * const builtin_add_usage[] = {
 	N_("git add [<options>] [--] <pathspec>..."),
@@ -36,6 +38,7 @@ static int take_worktree_changes;
 static int add_renormalize;
 static int pathspec_file_nul;
 static int include_sparse;
+static int no_verify;
 static const char *pathspec_from_file;
 
 static int chmod_pathspec(struct repository *repo,
@@ -271,6 +274,7 @@ static struct option builtin_add_options[] = {
 	OPT_BOOL( 0 , "refresh", &refresh_only, N_("don't add, only refresh the index")),
 	OPT_BOOL( 0 , "ignore-errors", &ignore_add_errors, N_("just skip files which cannot be added because of errors")),
 	OPT_BOOL( 0 , "ignore-missing", &ignore_missing, N_("check if - even missing - files are ignored in dry run")),
+	OPT_BOOL( 0 , "no-verify", &no_verify, N_("bypass pre-add hook")),
 	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")),
@@ -391,6 +395,8 @@ 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;
+	char *orig_index_path = NULL;
 
 	repo_config(repo, add_config, NULL);
 
@@ -576,6 +582,11 @@ int cmd_add(int argc,
 		string_list_clear(&only_match_skip_worktree, 0);
 	}
 
+	if (!show_only && !no_verify && hook_exists(repo, "pre-add")) {
+		run_pre_add = 1;
+		orig_index_path = absolute_pathdup(repo_get_index_file(repo));
+	}
+
 	transaction = odb_transaction_begin(repo->objects);
 
 	ps_matched = xcalloc(pathspec.nr, 1);
@@ -598,9 +609,30 @@ 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 && repo->index->cache_changed) {
+		struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;
+
+		if (write_locked_index(repo->index, &lock_file,
+				SKIP_INDEX_CHANGE_HOOK))
+			die(_("unable to write proposed index"));
+
+		strvec_push(&opt.args, 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;
+		} else if (commit_lock_file(&lock_file)) {
+			die(_("unable to write new index file"));
+		} else {
+			emit_post_index_change(repo->index);
+		}
+	} else {
+		if (write_locked_index(repo->index, &lock_file,
+				       COMMIT_LOCK | SKIP_IF_UNCHANGED))
+			die(_("unable to write new index file"));
+	}
+
+	free(orig_index_path);
 
 	free(ps_matched);
 	dir_clear(&dir);

read-cache-ll.h

@@ -284,6 +284,9 @@ int is_index_unborn(struct index_state *);
 /* For use with `write_locked_index()`. */
 #define COMMIT_LOCK		(1 << 0)
 #define SKIP_IF_UNCHANGED	(1 << 1)
+#define SKIP_INDEX_CHANGE_HOOK	(1 << 2)
+
+void emit_post_index_change(struct index_state *istate);
 
 /*
  * Write the index while holding an already-taken lock. Close the lock,

read-cache.c

@@ -3161,13 +3161,18 @@ static int do_write_locked_index(struct index_state *istate,
 	else
 		ret = close_lock_file_gently(lock);
 
+	if (!(flags & SKIP_INDEX_CHANGE_HOOK))
+		emit_post_index_change(istate);
+	return ret;
+}
+
+void emit_post_index_change(struct index_state *istate)
+{
 	run_hooks_l(the_repository, "post-index-change",
 		    istate->updated_workdir ? "1" : "0",
 		    istate->updated_skipworktree ? "1" : "0", NULL);
 	istate->updated_workdir = 0;
 	istate->updated_skipworktree = 0;
-
-	return ret;
 }
 
 static int write_split_index(struct index_state *istate,

t/meson.build

@@ -415,6 +415,7 @@ 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',