pathspec: give better message for submodule related pathspec error

Every once in a while someone complains to the mailing list to have
run into this weird assertion[1]. The usual response from the mailing
list is link to old discussions[2], and acknowledging the problem
stating it is known.

This patch accomplishes two things:

  1. Switch assert() to die("BUG") to give a more readable message.

  2. Take one of the cases where we hit a BUG and turn it into a normal
     "there was something wrong with the input" message.

     This assertion triggered for cases where there wasn't a programming
     bug, but just bogus input. In particular, if the user asks for a
     pathspec that is inside a submodule, we shouldn't assert() or
     die("BUG"); we should tell the user their request is bogus.

     The only reason we did not check for it, is the expensive nature
     of such a check, so callers avoid setting the flag
     PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE. However when we die due
     to bogus input, the expense of CPU cycles spent outweighs the user
     wondering what went wrong, so run that check unconditionally before
     dying with a more generic error message.

Note: There is a case (e.g. "git -C submodule add .") in which we call
strip_submodule_slash_expensive, as git-add requests it via the flag
PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE, but the assert used to
trigger nevertheless, because the flag PATHSPEC_LITERAL was not set,
such that we executed

	if (item->nowildcard_len < prefixlen)
		item->nowildcard_len = prefixlen;

and prefixlen was not adapted (e.g. it was computed from "submodule/")
So in the die_inside_submodule_path function we also need handle paths,
that were stripped before, i.e. are the exact submodule path. This
is why the conditions in die_inside_submodule_path are slightly
different than in strip_submodule_slash_expensive.

[1] https://www.google.com/search?q=item-%3Enowildcard_len
[2] http://git.661346.n2.nabble.com/assert-failed-in-submodule-edge-case-td7628687.html
    https://www.spinics.net/lists/git/msg249473.html

Helped-by: Jeff King <peff@peff.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pathspec.c

@@ -296,6 +296,27 @@ static void strip_submodule_slash_expensive(struct pathspec_item *item)
 	}
 }
 
+static void die_inside_submodule_path(struct pathspec_item *item)
+{
+	int i;
+
+	for (i = 0; i < active_nr; i++) {
+		struct cache_entry *ce = active_cache[i];
+		int ce_len = ce_namelen(ce);
+
+		if (!S_ISGITLINK(ce->ce_mode))
+			continue;
+
+		if (item->len < ce_len ||
+		    !(item->match[ce_len] == '/' || item->match[ce_len] == '\0') ||
+		    memcmp(ce->name, item->match, ce_len))
+			continue;
+
+		die(_("Pathspec '%s' is in submodule '%.*s'"),
+		    item->original, ce_len, ce->name);
+	}
+}
+
 /*
  * Perform the initialization of a pathspec_item based on a pathspec element.
  */
@@ -391,8 +412,18 @@ static void init_pathspec_item(struct pathspec_item *item, unsigned flags,
 	}
 
 	/* sanity checks, pathspec matchers assume these are sane */
-	assert(item->nowildcard_len <= item->len &&
-	       item->prefix         <= item->len);
+	if (item->nowildcard_len > item->len ||
+	    item->prefix         > item->len) {
+		/*
+		 * This case can be triggered by the user pointing us to a
+		 * pathspec inside a submodule, which is an input error.
+		 * Detect that here and complain, but fallback in the
+		 * non-submodule case to a BUG, as we have no idea what
+		 * would trigger that.
+		 */
+		die_inside_submodule_path(item);
+		die ("BUG: item->nowildcard_len > item->len || item->prefix > item->len)");
+	}
 }
 
 static int pathspec_item_cmp(const void *a_, const void *b_)

t/t6134-pathspec-in-submodule.sh

@@ -0,0 +1,36 @@
+#!/bin/sh
+
+test_description='test case exclude pathspec'
+
+. ./test-lib.sh
+
+test_expect_success 'setup a submodule' '
+	test_create_repo pretzel &&
+	: >pretzel/a &&
+	git -C pretzel add a &&
+	git -C pretzel commit -m "add a file" -- a &&
+	git submodule add ./pretzel sub &&
+	git commit -a -m "add submodule" &&
+	git submodule deinit --all
+'
+
+cat <<EOF >expect
+fatal: Pathspec 'sub/a' is in submodule 'sub'
+EOF
+
+test_expect_success 'error message for path inside submodule' '
+	echo a >sub/a &&
+	test_must_fail git add sub/a 2>actual &&
+	test_cmp expect actual
+'
+
+cat <<EOF >expect
+fatal: Pathspec '.' is in submodule 'sub'
+EOF
+
+test_expect_success 'error message for path inside submodule from within submodule' '
+	test_must_fail git -C sub add . 2>actual &&
+	test_cmp expect actual
+'
+
+test_done