[RFC PATCH v4 1/3] Add support for nested aliases

[Tim Schumacher <timschumi@gmx.de>]

Aliases can only contain non-alias git commands and their
arguments, not other user-defined aliases. Resolving further
(nested) aliases is prevented by breaking the loop after the
first alias was processed. Git then fails with a command-not-found
error.

Allow resolving nested aliases by not breaking the loop in
run_argv() after the first alias was processed. Instead, continue
the loop until `handle_alias()` fails, which means that there are
no further aliases that can be processed. Prevent looping aliases
by storing substituted commands in `cmd_list` and checking if
a command has been substituted previously.

While we're at it, fix a styling issue just below the added code.

Signed-off-by: Tim Schumacher <timschumi@gmx.de>
---
Changes since v3:
 - Print the command that the user entered instead of the command
   which caused the loop (and a nicer, more explanatory error message)
 - Use unsorted_string_list_has_string() instead of the sorted version
 - Fix a code style issue just below the modified code
 - done_alias is a simple boolean again (instead of a counter)

 git.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/git.c b/git.c
index c27c38738..15727c17f 100644
--- a/git.c
+++ b/git.c
@@ -674,6 +674,7 @@ static void execv_dashed_external(const char **argv)
 static int run_argv(int *argcp, const char ***argv)
 {
 	int done_alias = 0;
+	struct string_list cmd_list = STRING_LIST_INIT_NODUP;
 
 	while (1) {
 		/*
@@ -691,17 +692,25 @@ static int run_argv(int *argcp, const char ***argv)
 		/* .. then try the external ones */
 		execv_dashed_external(*argv);
 
-		/* It could be an alias -- this works around the insanity
+		if (unsorted_string_list_has_string(&cmd_list, *argv[0])) {
+			die(_("alias loop detected: expansion of '%s' does"
+			      " not terminate"), cmd_list.items[0].string);
+		}
+
+		string_list_append(&cmd_list, *argv[0]);
+
+		/*
+		 * It could be an alias -- this works around the insanity
 		 * of overriding "git log" with "git show" by having
 		 * alias.log = show
 		 */
-		if (done_alias)
-			break;
 		if (!handle_alias(argcp, argv))
 			break;
 		done_alias = 1;
 	}
 
+	string_list_clear(&cmd_list, 0);
+
 	return done_alias;
 }
 
-- 
2.19.0.rc2.1.g4c98b8d69.dirty