util/path: Add path_isancestor() function

Despite its simplicity, I keep finding it hard to get such string
comparison functions right. This function makes it significantly harder
to make any mistakes in path matching, and also keeps the intent of the
code clear.

2 files changed, 27 insertions, 1 deletions

diff --git a/src/util/path.h b/src/util/path.h
index 024e5ad..998864e 100644
--- a/src/util/path.h
+++ b/src/util/path.h
@@ -41,5 +41,14 @@
  */
 char *path_normalize(char *);
 
+
+/* Returns true if a is an ancestor of b. e.g. "/" and "/some" are ancestors of
+ * "/some/path". Both paths are assumed to be absolute and normalized according
+ * to path_normalize(). */
+static inline bool path_isancestor(const char *a, const char *b) {
+	size_t alen = strlen(a);
+	return strncmp(a, b, alen) == 0 && (b[alen] == '/' || (alen == 1 && b[alen]));
+}
+
 #endif
 /* vim: set noet sw=4 ts=4: */
diff --git a/test/path.c b/test/path.c
index 717cd84..b46e83b 100644
--- a/test/path.c
+++ b/test/path.c
@@ -34,7 +34,7 @@
 		free(buf);\
 	} while(0)
 
-int main(int argc, char **argv) {
+static void t_normalize() {
 	T("/", "/");
 	T("//", "/");
 	T("/abc", "/abc");
@@ -73,6 +73,23 @@ int main(int argc, char **argv) {
 	T("/..a..", "/..a..");
 	T("/..a.", "/..a.");
 	T("/.a..", "/.a..");
+}
+
+#undef T
+
+
+int main(int argc, char **argv) {
+	t_normalize();
+
+	assert(path_isancestor("/", "/some/path"));
+	assert(path_isancestor("/some", "/some/path"));
+	assert(path_isancestor("/", "/some"));
+	assert(!path_isancestor("/", "/"));
+	assert(!path_isancestor("/some/path", "/some/path"));
+	assert(!path_isancestor("/some/path/a", "/some/path"));
+	assert(!path_isancestor("/some/patha", "/some/path"));
+	assert(!path_isancestor("/some/pa", "/some/path"));
+	assert(!path_isancestor("/som", "/some/path"));
 	return 0;
 }