sd-json: add SD_JSON_PARSE_SEEK0 flag

src/libsystemd/sd-json/sd-json.c

@@ -3502,9 +3502,12 @@ _public_ int sd_json_parse_file_at(
         _cleanup_free_ char *text = NULL;
         int r;
 
-        if (f)
+        if (f) {
+                if (FLAGS_SET(flags, SD_JSON_PARSE_SEEK0) && fseek(f, /* offset= */ 0, SEEK_SET) < 0)
+                        return -errno;
+
                 r = read_full_stream(f, &text, NULL);
-        else
+        } else
                 r = read_full_file_full(dir_fd, path, UINT64_MAX, SIZE_MAX, 0, NULL, &text, NULL);
         if (r < 0)
                 return r;

src/systemd/sd-json.h

@@ -184,6 +184,7 @@ __extension__ typedef enum _SD_ENUM_TYPE_S64(sd_json_parse_flags_t) {
         SD_JSON_PARSE_SENSITIVE      = 1 << 0, /* mark variant as "sensitive", i.e. something containing secret key material or such */
         SD_JSON_PARSE_MUST_BE_OBJECT = 1 << 1, /* refuse parsing if top-level is not an object */
         SD_JSON_PARSE_MUST_BE_ARRAY  = 1 << 2, /* refuse parsing if top-level is not an array */
+        SD_JSON_PARSE_SEEK0          = 1 << 3, /* seek to offset 0 before reading */
         _SD_ENUM_FORCE_S64(JSON_PARSE_FLAGS)
 } sd_json_parse_flags_t;