ignore: match git's rule negation rules

A rule can only negate something which was explicitly mentioned in the
rules before it. Change our parsing to ignore a negative rule which does
not negate something mentioned in the rules above it.

While here, fix a wrong allocator usage. The memory for the match string
comes from pool allocator. We must not free it with the general
allocator. We can instead simply forget the string and it will be
cleaned up.

src/ignore.c

@@ -4,11 +4,87 @@
 #include "attrcache.h"
 #include "path.h"
 #include "config.h"
+#include "fnmatch.h"
 
 #define GIT_IGNORE_INTERNAL		"[internal]exclude"
 
 #define GIT_IGNORE_DEFAULT_RULES ".\n..\n.git\n"
 
+/**
+ * A negative ignore can only unignore a file which is given explicitly before, thus
+ *
+ *    foo
+ *    !foo/bar
+ *
+ * does not unignore 'foo/bar' as it's not in the list. However
+ *
+ *    foo/<star>
+ *    !foo/bar
+ *
+ * does unignore 'foo/bar', as it is contained within the 'foo/<star>' rule.
+ */
+static int does_negate_rule(int *out, git_vector *rules, git_attr_fnmatch *match)
+{
+	int error = 0;
+	size_t i;
+	git_attr_fnmatch *rule;
+	char *path;
+	git_buf buf = GIT_BUF_INIT;
+
+	/* path of the file relative to the workdir, so we match the rules in subdirs */
+	if (match->containing_dir) {
+		git_buf_puts(&buf, match->containing_dir);
+	}
+	if (git_buf_puts(&buf, match->pattern) < 0)
+		return -1;
+
+	path = git_buf_detach(&buf);
+
+	git_vector_foreach(rules, i, rule) {
+		/* no chance of matching w/o a wilcard */
+		if (!(rule->flags & GIT_ATTR_FNMATCH_HASWILD))
+			continue;
+
+	/*
+	 * If we're dealing with a directory (which we know via the
+	 * strchr() check) we want to use 'dirname/<star>' as the
+	 * pattern so p_fnmatch() honours FNM_PATHNAME
+	 */
+		git_buf_clear(&buf);
+		if (rule->containing_dir) {
+			git_buf_puts(&buf, rule->containing_dir);
+		}
+		if (!strchr(rule->pattern, '*'))
+			error = git_buf_printf(&buf, "%s/*", rule->pattern);
+		else
+			error = git_buf_puts(&buf, rule->pattern);
+
+		if (error < 0)
+			goto out;
+
+
+		if ((error = p_fnmatch(git_buf_cstr(&buf), path, FNM_PATHNAME)) < 0) {
+			giterr_set(GITERR_INVALID, "error matching pattern");
+			goto out;
+		}
+
+		/* if we found a match, we want to keep this rule */
+		if (error != FNM_NOMATCH) {
+			*out = 1;
+			error = 0;
+			goto out;
+		}
+	}
+
+	*out = 0;
+	error = 0;
+
+out:
+	git__free(path);
+	git_buf_free(&buf);
+	return error;
+}
+
 static int parse_ignore_file(
 	git_repository *repo, git_attr_file *attrs, const char *data)
 {
@@ -32,6 +108,8 @@ static int parse_ignore_file(
 	}
 
 	while (!error && *scan) {
+		int valid_rule = 1;
+
 		if (!match && !(match = git__calloc(1, sizeof(*match)))) {
 			error = -1;
 			break;
@@ -48,11 +126,16 @@ static int parse_ignore_file(
 				match->flags |= GIT_ATTR_FNMATCH_ICASE;
 
 			scan = git__next_line(scan);
-			error = git_vector_insert(&attrs->rules, match);
+
+			/* if a negative match doesn't actually do anything, throw it away */
+			if (match->flags & GIT_ATTR_FNMATCH_NEGATIVE)
+				error = does_negate_rule(&valid_rule, &attrs->rules, match);
+
+			if (!error && valid_rule)
+				error = git_vector_insert(&attrs->rules, match);
 		}
 
-		if (error != 0) {
-			git__free(match->pattern);
+		if (error != 0 || !valid_rule) {
 			match->pattern = NULL;
 
 			if (error == GIT_ENOTFOUND)