leapdb: support leap-seconds.list as second source

The existing implementation of getting leap second information from a
timezone in get_tz_leap() relies on non-portable C library behaviour.

Specifically, mktime is not required to return '60' in the tm_sec field
when a leap second is inserted leading to "Timezone right/UTC failed
leap second check, ignoring" errors on musl based systems.

This patch adds support for getting leap second information from the
leap-seconds.list file included with tzdata and adds a new configuration
directive leapseclist to switch on the feature.

leapdb.c

@@ -3,6 +3,7 @@
 
  **********************************************************************
  * Copyright (C) Miroslav Lichvar  2009-2018, 2020, 2022
+ * Copyright (C) Patrick Oppenlander 2023, 2024
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of version 2 of the GNU General Public License as
@@ -30,11 +31,20 @@
 #include "conf.h"
 #include "leapdb.h"
 #include "logging.h"
+#include "util.h"
 
 /* ================================================== */
 
-/* Name of a system timezone containing leap seconds occuring at midnight */
-static char *leap_tzname;
+/* Source of leap second data */
+enum {
+  SRC_NONE,
+  SRC_TIMEZONE,
+  SRC_LIST,
+} leap_src;
+
+/* Offset between leap-seconds.list timestamp epoch and Unix epoch.
+   leap-seconds.list epoch is 1 Jan 1900, 00:00:00 */
+#define LEAP_SEC_LIST_OFFSET 2208988800
 
 /* ================================================== */
 
@@ -59,7 +69,7 @@ get_tz_leap(time_t when, int *tai_offset)
       return tz_leap;
     strcpy(tz_orig, tz_env);
   }
-  setenv("TZ", leap_tzname, 1);
+  setenv("TZ", CNF_GetLeapSecTimezone(), 1);
   tzset();
 
   /* Get the TAI-UTC offset, which started at the epoch at 10 seconds */
@@ -93,6 +103,91 @@ get_tz_leap(time_t when, int *tai_offset)
 
 /* ================================================== */
 
+static NTP_Leap
+get_list_leap(time_t when, int *tai_offset)
+{
+  FILE *f;
+  char line[1024];
+  NTP_Leap ret_leap = LEAP_Normal;
+  int ret_tai_offset = 0, prev_lsl_tai_offset = 10;
+  int64_t lsl_updated = 0, lsl_expiry = 0;
+  const char *leap_sec_list = CNF_GetLeapSecList();
+
+  if (!(f = UTI_OpenFile(NULL, leap_sec_list, NULL, 'r', 0))) {
+    LOG(LOGS_ERR, "Failed to open leap seconds list %s", leap_sec_list);
+    goto out;
+  }
+
+  /* Leap second happens at midnight */
+  when = (when / (24 * 3600) + 1) * (24 * 3600);
+
+  /* leap-seconds.list timestamps are relative to 1 Jan 1900, 00:00:00 */
+  when += LEAP_SEC_LIST_OFFSET;
+
+  while (fgets(line, sizeof line, f) > 0) {
+    int64_t lsl_when;
+    int lsl_tai_offset;
+    char *p;
+
+    /* Ignore blank lines */
+    for (p = line; *p && isspace(*p); ++p)
+      ;
+    if (!*p)
+      continue;
+
+    if (*line == '#') {
+      /* Update time line starts with #$ */
+      if (line[1] == '$' && sscanf(line + 2, "%"SCNd64, &lsl_updated) != 1)
+        goto error;
+      /* Expiration time line starts with #@ */
+      if (line[1] == '@' && sscanf(line + 2, "%"SCNd64, &lsl_expiry) != 1)
+        goto error;
+      /* Comment or a special comment we don't care about */
+      continue;
+    }
+
+    /* Leap entry */
+    if (sscanf(line, "%"SCNd64" %d", &lsl_when, &lsl_tai_offset) != 2)
+      goto error;
+
+    if (when == lsl_when) {
+      if (lsl_tai_offset > prev_lsl_tai_offset)
+        ret_leap = LEAP_InsertSecond;
+      else if (lsl_tai_offset < prev_lsl_tai_offset)
+        ret_leap = LEAP_DeleteSecond;
+      /* When is rounded to the end of the day, so offset hasn't changed yet! */
+      ret_tai_offset = prev_lsl_tai_offset;
+    } else if (when > lsl_when) {
+      ret_tai_offset = lsl_tai_offset;
+    }
+
+    prev_lsl_tai_offset = lsl_tai_offset;
+  }
+
+  /* Make sure the file looks sensible */
+  if (!feof(f) || !lsl_updated || !lsl_expiry)
+    goto error;
+
+  if (when >= lsl_expiry)
+    LOG(LOGS_WARN, "Leap second list %s needs update", leap_sec_list);
+
+  goto out;
+
+error:
+  if (f)
+    fclose(f);
+  LOG(LOGS_ERR, "Failed to parse leap seconds list %s", leap_sec_list);
+  return LEAP_Normal;
+
+out:
+  if (f)
+    fclose(f);
+  *tai_offset = ret_tai_offset;
+  return ret_leap;
+}
+
+/* ================================================== */
+
 static int
 check_leap_source(NTP_Leap (*src)(time_t when, int *tai_offset))
 {
@@ -111,14 +206,27 @@ check_leap_source(NTP_Leap (*src)(time_t when, int *tai_offset))
 void
 LDB_Initialise(void)
 {
+  const char *leap_tzname, *leap_sec_list;
+
   leap_tzname = CNF_GetLeapSecTimezone();
   if (leap_tzname && !check_leap_source(get_tz_leap)) {
     LOG(LOGS_WARN, "Timezone %s failed leap second check, ignoring", leap_tzname);
     leap_tzname = NULL;
   }
 
-  if (leap_tzname)
+  leap_sec_list = CNF_GetLeapSecList();
+  if (leap_sec_list && !check_leap_source(get_list_leap)) {
+    LOG(LOGS_WARN, "Leap second list %s failed check, ignoring", leap_sec_list);
+    leap_sec_list = NULL;
+  }
+
+  if (leap_sec_list) {
+    LOG(LOGS_INFO, "Using leap second list %s", leap_sec_list);
+    leap_src = SRC_LIST;
+  } else if (leap_tzname) {
     LOG(LOGS_INFO, "Using %s timezone to obtain leap second data", leap_tzname);
+    leap_src = SRC_TIMEZONE;
+  }
 }
 
 /* ================================================== */
@@ -139,8 +247,16 @@ LDB_GetLeap(time_t when, int *tai_offset)
   ldb_leap = LEAP_Normal;
   ldb_tai_offset = 0;
 
-  if (leap_tzname)
+  switch (leap_src) {
+  case SRC_NONE:
+    break;
+  case SRC_TIMEZONE:
     ldb_leap = get_tz_leap(when, &ldb_tai_offset);
+    break;
+  case SRC_LIST:
+    ldb_leap = get_list_leap(when, &ldb_tai_offset);
+    break;
+  }
 
 out:
   *tai_offset = ldb_tai_offset;