util-lib: make timestamp generation and parsing reversible (#3869)

This patch improves parsing and generation of timestamps and calendar
specifications in two ways:

- The week day is now always printed in the abbreviated English form, instead
  of the locale's setting. This makes sure we can always parse the week day
  again, even if the locale is changed. Given that we don't follow locale
  settings for printing timestamps in any other way either (for example, we
  always use 24h syntax in order to make uniform parsing possible), it only
  makes sense to also stick to a generic, non-localized form for the timestamp,
  too.

- When parsing a timestamp, the local timezone (in its DST or non-DST name)
  may be specified, in addition to "UTC". Other timezones are still not
  supported however (not because we wouldn't want to, but mostly because libc
  offers no nice API for that). In itself this brings no new features, however
  it ensures that any locally formatted timestamp's timezone is also parsable
  again.

These two changes ensure that the output of format_timestamp() may always be
passed to parse_timestamp() and results in the original input. The related
flavours for usec/UTC also work accordingly. Calendar specifications are
extended in a similar way.

The man page is updated accordingly, in particular this removes the claim that
timestamps systemd prints wouldn't be parsable by systemd. They are now.

The man page previously showed invalid timestamps as examples. This has been
removed, as the man page shouldn't be a unit test, where such negative examples
would be useful. The man page also no longer mentions the names of internal
functions, such as format_timestamp_us() or UNIX error codes such as EINVAL.

src/test/test-calendarspec.c

@@ -88,6 +88,27 @@ static void test_next(const char *input, const char *new_tz, usec_t after, usec_
         tzset();
 }
 
+static void test_timestamp(void) {
+        char buf[FORMAT_TIMESTAMP_MAX];
+        _cleanup_free_ char *t = NULL;
+        CalendarSpec *c;
+        usec_t x, y;
+
+        /* Ensure that a timestamp is also a valid calendar specification. Convert forth and back */
+
+        x = now(CLOCK_REALTIME);
+
+        assert_se(format_timestamp_us(buf, sizeof(buf), x));
+        printf("%s\n", buf);
+        assert_se(calendar_spec_from_string(buf, &c) >= 0);
+        assert_se(calendar_spec_to_string(c, &t) >= 0);
+        calendar_spec_free(c);
+        printf("%s\n", t);
+
+        assert_se(parse_timestamp(t, &y) >= 0);
+        assert_se(y == x);
+}
+
 int main(int argc, char* argv[]) {
         CalendarSpec *c;
 
@@ -155,5 +176,7 @@ int main(int argc, char* argv[]) {
         assert_se(calendar_spec_from_string("00:00:00/0.00000001", &c) < 0);
         assert_se(calendar_spec_from_string("00:00:00.0..00.9", &c) < 0);
 
+        test_timestamp();
+
         return 0;
 }

src/test/test-time.c

@@ -19,6 +19,7 @@
 
 #include "strv.h"
 #include "time-util.h"
+#include "random-util.h"
 
 static void test_parse_sec(void) {
         usec_t u;
@@ -201,6 +202,48 @@ static void test_usec_sub(void) {
         assert_se(usec_sub(USEC_INFINITY, 5) == USEC_INFINITY);
 }
 
+static void test_format_timestamp(void) {
+        unsigned i;
+
+        for (i = 0; i < 100; i++) {
+                char buf[MAX(FORMAT_TIMESTAMP_MAX, FORMAT_TIMESPAN_MAX)];
+                usec_t x, y;
+
+                random_bytes(&x, sizeof(x));
+                x = x % (2147483600 * USEC_PER_SEC) + 1;
+
+                assert_se(format_timestamp(buf, sizeof(buf), x));
+                log_info("%s", buf);
+                assert_se(parse_timestamp(buf, &y) >= 0);
+                assert_se(x / USEC_PER_SEC == y / USEC_PER_SEC);
+
+                assert_se(format_timestamp_utc(buf, sizeof(buf), x));
+                log_info("%s", buf);
+                assert_se(parse_timestamp(buf, &y) >= 0);
+                assert_se(x / USEC_PER_SEC == y / USEC_PER_SEC);
+
+                assert_se(format_timestamp_us(buf, sizeof(buf), x));
+                log_info("%s", buf);
+                assert_se(parse_timestamp(buf, &y) >= 0);
+                assert_se(x == y);
+
+                assert_se(format_timestamp_us_utc(buf, sizeof(buf), x));
+                log_info("%s", buf);
+                assert_se(parse_timestamp(buf, &y) >= 0);
+                assert_se(x == y);
+
+                assert_se(format_timestamp_relative(buf, sizeof(buf), x));
+                log_info("%s", buf);
+                assert_se(parse_timestamp(buf, &y) >= 0);
+
+                /* The two calls above will run with a slightly different local time. Make sure we are in the same
+                 * range however, but give enough leeway that this is unlikely to explode. And of course,
+                 * format_timestamp_relative() scales the accuracy with the distance from the current time up to one
+                 * month, cover for that too. */
+                assert_se(y > x ? y - x : x - y <= USEC_PER_MONTH + USEC_PER_DAY);
+        }
+}
+
 int main(int argc, char *argv[]) {
         uintmax_t x;
 
@@ -214,6 +257,7 @@ int main(int argc, char *argv[]) {
         test_get_timezones();
         test_usec_add();
         test_usec_sub();
+        test_format_timestamp();
 
         /* Ensure time_t is signed */
         assert_cc((time_t) -1 < (time_t) 1);