efi-string: Add startswith8()

startswith() from string-util-fundamental.h is defined for sd_char
which is char16_t, so let's add an implementation for char as well.

src/boot/efi/efi-string.c

@@ -212,6 +212,21 @@ char16_t *xstrn8_to_16(const char *str8, size_t n) {
         return str16;
 }
 
+char *startswith8(const char *s, const char *prefix) {
+        size_t l;
+
+        assert(prefix);
+
+        if (!s)
+                return NULL;
+
+        l = strlen8(prefix);
+        if (!strneq8(s, prefix, l))
+                return NULL;
+
+        return (char*) s + l;
+}
+
 static bool efi_fnmatch_prefix(const char16_t *p, const char16_t *h, const char16_t **ret_p, const char16_t **ret_h) {
         assert(p);
         assert(h);

src/boot/efi/efi-string.h

@@ -101,6 +101,8 @@ static inline char16_t *xstr8_to_16(const char *str8) {
         return xstrn8_to_16(str8, strlen8(str8));
 }
 
+char *startswith8(const char *s, const char *prefix);
+
 bool efi_fnmatch(const char16_t *pattern, const char16_t *haystack);
 
 bool parse_number8(const char *s, uint64_t *ret_u, const char **ret_tail);

src/boot/efi/test-efi-string.c

@@ -355,6 +355,18 @@ TEST(xstrn8_to_16) {
         free(s);
 }
 
+TEST(startswith8) {
+        assert_se(streq8(startswith8("", ""), ""));
+        assert_se(streq8(startswith8("x", ""), "x"));
+        assert_se(!startswith8("", "x"));
+        assert_se(!startswith8("", "xxxxxxxx"));
+        assert_se(streq8(startswith8("xxx", "x"), "xx"));
+        assert_se(streq8(startswith8("xxx", "xx"), "x"));
+        assert_se(streq8(startswith8("xxx", "xxx"), ""));
+        assert_se(!startswith8("xxx", "xxxx"));
+        assert_se(!startswith8(NULL, ""));
+}
+
 #define TEST_FNMATCH_ONE(pattern, haystack, expect)                                     \
         ({                                                                              \
                 assert_se(fnmatch(pattern, haystack, 0) == (expect ? 0 : FNM_NOMATCH)); \