time: add runtime sanity check

std::chrono::system_clock.time_since_epoch and time_t(0) are not guaranteed
to use the Unix epoch timestamp, but in practice they almost certainly will.
Any differing behavior will be assumed to be an error, unless certain
platforms prove to consistently deviate, at which point we'll cope with it
by adding offsets.

Do a quick runtime check to verify that
time_t(0) == std::chrono::system_clock's epoch time == unix epoch.

Co-authored-by: Anthony Towns <aj@erisian.com.au>

1 files changed, 43 insertions, 0 deletions

diff --git a/src/util/time.cpp b/src/util/time.cpp
index 295806c54a..9631c115e9 100644
--- a/src/util/time.cpp
+++ b/src/util/time.cpp
@@ -33,6 +33,49 @@ int64_t GetTime()
     return now;
 }
 
+bool ChronoSanityCheck()
+{
+    // std::chrono::system_clock.time_since_epoch and time_t(0) are not guaranteed
+    // to use the Unix epoch timestamp, prior to C++20, but in practice they almost
+    // certainly will. Any differing behavior will be assumed to be an error, unless
+    // certain platforms prove to consistently deviate, at which point we'll cope
+    // with it by adding offsets.
+
+    // Create a new clock from time_t(0) and make sure that it represents 0
+    // seconds from the system_clock's time_since_epoch. Then convert that back
+    // to a time_t and verify that it's the same as before.
+    const time_t time_t_epoch{};
+    auto clock = std::chrono::system_clock::from_time_t(time_t_epoch);
+    if (std::chrono::duration_cast<std::chrono::seconds>(clock.time_since_epoch()).count() != 0) {
+        return false;
+    }
+
+    time_t time_val = std::chrono::system_clock::to_time_t(clock);
+    if (time_val != time_t_epoch) {
+        return false;
+    }
+
+    // Check that the above zero time is actually equal to the known unix timestamp.
+    struct tm epoch;
+#ifdef HAVE_GMTIME_R
+    if (gmtime_r(&time_val, &epoch) == nullptr) {
+#else
+    if (gmtime_s(&epoch, &time_val) != 0) {
+#endif
+        return false;
+    }
+
+    if ((epoch.tm_sec != 0)  ||
+       (epoch.tm_min  != 0)  ||
+       (epoch.tm_hour != 0)  ||
+       (epoch.tm_mday != 1)  ||
+       (epoch.tm_mon  != 0)  ||
+       (epoch.tm_year != 70)) {
+        return false;
+    }
+    return true;
+}
+
 template <typename T>
 T GetTime()
 {