[PATCH v2 3/3] builtin/grep: introduce --use-index argument

[Thomas Gummerer <t.gummerer@gmail.com>]

Currently when git grep is used outside of a git repository without the
--no-index option git simply dies.  Introduce a new --use-index command line
argument, which can be set to always, optional, or never.  If it is set
to always, git grep continues to behave as it currently does.  If set to
optional, git grep will work normally if executed inside of a git
repository, and behave like git grep --no-index if used outside of a git
repository.  If set to never, git grep will behave like git grep
--no-index regardless of whether it is used inside or outside of a git
repository.

Helped-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
---
 Documentation/git-grep.txt | 13 ++++++++
 builtin/grep.c             | 47 +++++++++++++++++++++++---
 t/t7810-grep.sh            | 82 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 137 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-grep.txt b/Documentation/git-grep.txt
index 4a44d6d..90ff643 100644
--- a/Documentation/git-grep.txt
+++ b/Documentation/git-grep.txt
@@ -25,6 +25,7 @@ SYNOPSIS
 	   [-W | --function-context]
 	   [-f <file>] [-e] <pattern>
 	   [--and|--or|--not|(|)|-e <pattern>...]
+	   [--use-index=<always|optional|never>]
 	   [ [--[no-]exclude-standard] [--cached | --no-index | --untracked] | <tree>...]
 	   [--] [<pathspec>...]
 
@@ -66,6 +67,18 @@ OPTIONS
 --no-index::
 	Search files in the current directory that is not managed by Git.
 
+--use-index::
+	Set the behavior of git grep when used in- or outside of a git
+	repository.  If set to always, the command can only be used
+	inside of a git repository, and will fail if it is used
+	outside of a git repository.  If set to never, it will work
+	both inside and outside of a git repository, but will not use
+	any information in the index if used inside of a repository.
+	If set to optional, it will work both inside and outside of a
+	git repository, using the information available in the index
+	if used inside a repository.  Always and optional can be
+	overridden by `--no-index`.
+
 --untracked::
 	In addition to searching in the tracked files in the working
 	tree, search also in untracked files.
diff --git a/builtin/grep.c b/builtin/grep.c
index 3a27bd5..bb2add9 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -19,6 +19,9 @@
 #include "dir.h"
 #include "pathspec.h"
 
+#define GREP_NO_INDEX_EXPLICIT 1
+#define GREP_NO_INDEX_IMPLICIT 2
+
 static char const * const grep_usage[] = {
 	N_("git grep [<options>] [-e] <pattern> [<rev>...] [[--] <path>...]"),
 	NULL
@@ -626,13 +629,17 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	int i;
 	int dummy;
 	int no_index = 0;
+	const char *use_index = NULL;
 	int pattern_type_arg = GREP_PATTERN_TYPE_UNSPECIFIED;
 
 	struct option options[] = {
 		OPT_BOOL(0, "cached", &cached,
 			N_("search in index instead of in the work tree")),
 		OPT_BIT(0, "no-index", &no_index,
-			N_("find in contents not managed by git"), 1),
+			N_("find in contents not managed by git"),
+			GREP_NO_INDEX_EXPLICIT),
+		OPT_STRING(0, "use-index", &use_index, N_("always|optional|never"),
+			   N_("options for using the index")),
 		OPT_BOOL(0, "untracked", &untracked,
 			N_("search in both tracked and untracked files")),
 		OPT_SET_INT(0, "exclude-standard", &opt_exclude,
@@ -755,6 +762,25 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			     PARSE_OPT_STOP_AT_NON_OPTION);
 	grep_commit_pattern_type(pattern_type_arg, &opt);
 
+	if (use_index) {
+		if (!strcmp(use_index, "always")) {
+			/* same as git grep without --use-index */
+		} else if (!strcmp(use_index, "optional")) {
+			if (!no_index && !startup_info->have_repository) {
+				int nongit = 0;
+
+				setup_git_directory_gently(&nongit);
+				if (nongit)
+					no_index = GREP_NO_INDEX_IMPLICIT;
+			}
+		} else if (!strcmp(use_index, "never")) {
+			no_index = GREP_NO_INDEX_EXPLICIT;
+		} else {
+			die(_("invalid option \"%s\" for --use-index, expecting"
+			      " always, optional or never"), use_index);
+		}
+	}
+
 	if (!no_index && !startup_info->have_repository)
 		/* die the same way as if we did it at the beginning */
 		setup_git_directory();
@@ -873,13 +899,24 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	if (!show_in_pager && !opt.status_only)
 		setup_pager();
 
-	if (no_index && (untracked || cached))
-		die(_("--cached or --untracked cannot be used with --no-index."));
+	if (no_index && (untracked || cached)) {
+		if (no_index == GREP_NO_INDEX_IMPLICIT)
+			die(_("--cached or --untracked cannot be used outside "
+			      "of a git repository."));
+		else
+			die(_("--cached or --untracked cannot be used with "
+			      "--no-index or --no-index=never."));
+	}
 
 	if (no_index || untracked) {
 		int use_exclude = (opt_exclude < 0) ? !no_index : !!opt_exclude;
-		if (list.nr)
-			die(_("--no-index or --untracked cannot be used with revs."));
+		if (list.nr) {
+			if (no_index == GREP_NO_INDEX_IMPLICIT)
+				die(_("cannot use revs outside of a git repository."));
+			else
+				die(_("--no-index, --use-index=never or "
+				      "--untracked cannot be used with revs."));
+		}
 		hit = grep_directory(&opt, &pathspec, use_exclude);
 	} else if (0 <= opt_exclude) {
 		die(_("--[no-]exclude-standard cannot be used for tracked contents."));
diff --git a/t/t7810-grep.sh b/t/t7810-grep.sh
index cc4b97d..9776f15 100755
--- a/t/t7810-grep.sh
+++ b/t/t7810-grep.sh
@@ -821,6 +821,88 @@ test_expect_success 'outside of git repository' '
 	)
 '
 
+test_expect_success 'outside of git repository with --use-index=optional' '
+	rm -fr non &&
+	mkdir -p non/git/sub &&
+	echo hello >non/git/file1 &&
+	echo world >non/git/sub/file2 &&
+	cat <<-\EOF >non/expect.full &&
+	file1:hello
+	sub/file2:world
+	EOF
+	echo file2:world >non/expect.sub &&
+	(
+		GIT_CEILING_DIRECTORIES="$(pwd)/non" &&
+		export GIT_CEILING_DIRECTORIES &&
+		cd non/git &&
+		test_must_fail git grep --use-index=always o &&
+		git grep --use-index=optional o >../actual.full &&
+		test_cmp ../expect.full ../actual.full &&
+		cd sub &&
+		test_must_fail git grep --use-index=always o &&
+		git grep --use-index=optional o >../../actual.sub &&
+		test_cmp ../../expect.sub ../../actual.sub
+	) &&
+
+	echo ".*o*" >non/git/.gitignore &&
+	(
+		GIT_CEILING_DIRECTORIES="$(pwd)/non" &&
+		export GIT_CEILING_DIRECTORIES &&
+		cd non/git &&
+		test_must_fail git grep --use-index=always o &&
+		git grep --use-index=optional --exclude-standard o >../actual.full &&
+		test_cmp ../expect.full ../actual.full &&
+
+		{
+			echo ".gitignore:.*o*" &&
+			cat ../expect.full
+		} >../expect.with.ignored &&
+		git grep --use-index=optional --no-exclude o >../actual.full &&
+		test_cmp ../expect.with.ignored ../actual.full
+	)
+'
+
+test_expect_success 'outside of git repository with --use-index=never' '
+	rm -fr non &&
+	mkdir -p non/git/sub &&
+	echo hello >non/git/file1 &&
+	echo world >non/git/sub/file2 &&
+	cat <<-\EOF >non/expect.full &&
+	file1:hello
+	sub/file2:world
+	EOF
+	echo file2:world >non/expect.sub &&
+	(
+		GIT_CEILING_DIRECTORIES="$(pwd)/non" &&
+		export GIT_CEILING_DIRECTORIES &&
+		cd non/git &&
+		test_must_fail git grep --use-index=always o &&
+		git grep --use-index=never o >../actual.full &&
+		test_cmp ../expect.full ../actual.full &&
+		cd sub &&
+		test_must_fail git grep --use-index=always o &&
+		git grep --use-index=never o >../../actual.sub &&
+		test_cmp ../../expect.sub ../../actual.sub
+	) &&
+
+	echo ".*o*" >non/git/.gitignore &&
+	(
+		GIT_CEILING_DIRECTORIES="$(pwd)/non" &&
+		export GIT_CEILING_DIRECTORIES &&
+		cd non/git &&
+		test_must_fail git grep --use-index=always o &&
+		git grep --use-index=never --exclude-standard o >../actual.full &&
+		test_cmp ../expect.full ../actual.full &&
+
+		{
+			echo ".gitignore:.*o*" &&
+			cat ../expect.full
+		} >../expect.with.ignored &&
+		git grep --use-index=never --no-exclude o >../actual.full &&
+		test_cmp ../expect.with.ignored ../actual.full
+	)
+'
+
 test_expect_success 'inside git repository but with --no-index' '
 	rm -fr is &&
 	mkdir -p is/git/sub &&
-- 
2.7.0.3.g214d8d0.dirty