fix: populate timezone data when formatting time

The `jv2tm` function was zeroing fields of `struct tm` that were not
specified by the standard. However, depending on the libc this produced
incorrect timezone data when used together with formatting functions.
This change tries to fill the timezone data using either `mktime`,
`timegm`, or manually.

src/builtin.c

@@ -1555,7 +1555,7 @@ static jv f_strptime(jq_state *jq, jv a, jv b) {
   return r;
 }
 
-static int jv2tm(jv a, struct tm *tm) {
+static int jv2tm(jv a, struct tm *tm, int localtime) {
   memset(tm, 0, sizeof(*tm));
   static const size_t offsets[] = {
     offsetof(struct tm, tm_year),
@@ -1585,13 +1585,25 @@ static int jv2tm(jv a, struct tm *tm) {
     jv_free(n);
   }
 
-  // We use UTC everywhere (gettimeofday, gmtime) and UTC does not do DST.
-  // Setting tm_isdst to 0 is done by the memset.
-  // tm->tm_isdst = 0;
+  if (localtime) {
+    tm->tm_isdst = -1;
+    mktime(tm);
+  } else {
+#ifdef HAVE_TIMEGM
+    timegm(tm);
+#elif HAVE_TM_TM_GMT_OFF
+    // tm->tm_gmtoff = 0;
+    tm->tm_zone = "GMT";
+#elif HAVE_TM___TM_GMT_OFF
+    // tm->__tm_gmtoff = 0;
+    tm->__tm_zone = "GMT";
+#endif
+    // tm->tm_isdst = 0;
 
-  // The standard permits the tm structure to contain additional members. We
-  // hope it is okay to initialize them to zero, because the standard does not
-  // provide an alternative.
+    // The standard permits the tm structure to contain additional members. We
+    // hope it is okay to initialize them to zero, because the standard does not
+    // provide an alternative.
+  }
 
   jv_free(a);
   return 1;
@@ -1603,7 +1615,7 @@ static jv f_mktime(jq_state *jq, jv a) {
   if (jv_get_kind(a) != JV_KIND_ARRAY)
     return ret_error(a, jv_string("mktime requires array inputs"));
   struct tm tm;
-  if (!jv2tm(a, &tm))
+  if (!jv2tm(a, &tm, 0))
     return jv_invalid_with_msg(jv_string("mktime requires parsed datetime inputs"));
   time_t t = my_mktime(&tm);
   if (t == (time_t)-1)
@@ -1701,7 +1713,7 @@ static jv f_strftime(jq_state *jq, jv a, jv b) {
   if (jv_get_kind(b) != JV_KIND_STRING)
     return ret_error2(a, b, jv_string("strftime/1 requires a string format"));
   struct tm tm;
-  if (!jv2tm(a, &tm))
+  if (!jv2tm(a, &tm, 0))
     return ret_error(b, jv_string("strftime/1 requires parsed datetime inputs"));
 
   const char *fmt = jv_string_value(b);
@@ -1732,7 +1744,7 @@ static jv f_strflocaltime(jq_state *jq, jv a, jv b) {
   if (jv_get_kind(b) != JV_KIND_STRING)
     return ret_error2(a, b, jv_string("strflocaltime/1 requires a string format"));
   struct tm tm;
-  if (!jv2tm(a, &tm))
+  if (!jv2tm(a, &tm, 1))
     return ret_error(b, jv_string("strflocaltime/1 requires parsed datetime inputs"));
   const char *fmt = jv_string_value(b);
   size_t alloced = strlen(fmt) + 100;