parser: Support system timezone in Time_Offset

Add the 'System' option to Time_Offset that uses the system timezone
to compute the UTC offset during startup.

As proposed in #593

Signed-off-by: Sijmen Huizenga <sijmenhuizenga@gmail.com>

src/flb_parser.c

@@ -43,6 +43,7 @@
 #include <sys/stat.h>
 #include <limits.h>
 #include <string.h>
+#include <time.h>
 
 static inline uint32_t digits10(uint64_t v) {
     if (v < 10) return 1;
@@ -1009,6 +1010,15 @@ int flb_parser_tzone_offset(const char *str, int len, int *tmdiff)
         return 0;
     }
 
+    /* Check system timezones */
+    if (*p == 'S') {
+        time_t currentTime = time(NULL);
+        struct tm localTime = {0};
+        localtime_r(&currentTime, &localTime);
+        *tmdiff = localTime.tm_gmtoff;
+        return 0;
+    }
+
     /* Unexpected timezone string */
     if (*p != '+' && *p != '-') {
         *tmdiff = 0;

tests/internal/parser.c

@@ -37,6 +37,8 @@ struct tz_check tz_entries_ok[] = {
     {"-0600",  -21600},
     {"-06:00", -21600},
     {"Z",           0},
+    // System is mocked to timezone "HST" is -10 hours
+    {"System", -36000},
 };
 
 struct tz_check tz_entries_error[] = {
@@ -126,6 +128,35 @@ int flb_parser_regex_do(struct flb_parser *parser,
                         void **out_buf, size_t *out_size,
                         struct flb_time *out_time);
 
+static char* mock_timezone(char *tz)
+{
+    char *original_tz = getenv("TZ");
+    if (original_tz) {
+        original_tz = strdup(original_tz);
+        if (!original_tz) {
+            perror("Failed to allocate memory for original_tz");
+            exit(EXIT_FAILURE);
+        }
+    }
+
+    setenv("TZ", tz, 1);
+    tzset();
+
+    return original_tz;
+}
+
+static void undo_mock_timezone(char *original_tz)
+{
+    if (original_tz) {
+        setenv("TZ", original_tz, 1);
+    } else {
+        unsetenv("TZ");
+    }
+    tzset();
+    free(original_tz);
+}
+
+
 /* Parse timezone string and get the offset */
 void test_parser_tzone_offset()
 {
@@ -135,6 +166,9 @@ void test_parser_tzone_offset()
     int diff;
     struct tz_check *t;
 
+    /* Hawaii Standard Time chosen for not having daylight saving time */
+    char *original_timezone = mock_timezone("HST");
+
     /* Valid offsets */
     for (i = 0; i < sizeof(tz_entries_ok) / sizeof(struct tz_check); i++) {
         t = &tz_entries_ok[i];
@@ -154,6 +188,8 @@ void test_parser_tzone_offset()
         ret = flb_parser_tzone_offset(t->val, len, &diff);
         TEST_CHECK(ret != 0);
     }
+
+    undo_mock_timezone(original_timezone);
 }
 
 static void load_json_parsers(struct flb_config *config)