git add: do not add files from a submodule

It comes quite as a surprise to an unsuspecting Git user that calling
"git add submodule/file" (which is a mistake, alright) _removes_
the submodule in the index, and adds the file.  Instead, complain loudly.

While at it, be nice when the user said "git add submodule/" which is
most likely the consequence of tab-completion, and stage the submodule,
instead of trying to add the contents of that directory.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-add.c

@@ -68,6 +68,33 @@ static void prune_directory(struct dir_struct *dir, const char **pathspec, int p
         free(seen);
 }
 
+static void treat_gitlinks(const char **pathspec)
+{
+	int i;
+
+	if (!pathspec || !*pathspec)
+		return;
+
+	for (i = 0; i < active_nr; i++) {
+		struct cache_entry *ce = active_cache[i];
+		if (S_ISGITLINK(ce->ce_mode)) {
+			int len = ce_namelen(ce), j;
+			for (j = 0; pathspec[j]; j++) {
+				int len2 = strlen(pathspec[j]);
+				if (len2 <= len || pathspec[j][len] != '/' ||
+				    memcmp(ce->name, pathspec[j], len))
+					continue;
+				if (len2 == len + 1)
+					/* strip trailing slash */
+					pathspec[j] = xstrndup(ce->name, len);
+				else
+					die ("Path '%s' is in submodule '%.*s'",
+						pathspec[j], len, ce->name);
+			}
+		}
+	}
+}
+
 static void fill_directory(struct dir_struct *dir, const char **pathspec,
 		int ignored_too)
 {
@@ -261,6 +288,7 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 
 	if (read_cache() < 0)
 		die("index file corrupt");
+	treat_gitlinks(pathspec);
 
 	if (add_new_files)
 		/* This picks up the paths that are not tracked */

t/t7400-submodule-basic.sh

@@ -209,4 +209,29 @@ test_expect_success 'update --init' '
 
 '
 
+test_expect_success 'do not add files from a submodule' '
+
+	git reset --hard &&
+	test_must_fail git add init/a
+
+'
+
+test_expect_success 'gracefully add submodule with a trailing slash' '
+
+	git reset --hard &&
+	git commit -m "commit subproject" init &&
+	(cd init &&
+	 echo b > a) &&
+	git add init/ &&
+	git diff --exit-code --cached init &&
+	commit=$(cd init &&
+	 git commit -m update a >/dev/null &&
+	 git rev-parse HEAD) &&
+	git add init/ &&
+	test_must_fail git diff --exit-code --cached init &&
+	test $commit = $(git ls-files --stage |
+		sed -n "s/^160000 \([^ ]*\).*/\1/p")
+
+'
+
 test_done