Move local offset function to proper sys directory

src/lib.rs

@@ -310,6 +310,7 @@ mod rand;
 #[cfg_attr(__time_03_docs, doc(cfg(feature = "serde")))]
 #[allow(missing_copy_implementations, missing_debug_implementations)]
 pub mod serde;
+mod sys;
 #[cfg(test)]
 mod tests;
 mod time;

src/sys/local_offset_at/imp.rs

@@ -0,0 +1,7 @@
+//! A fallback for any OS not covered.
+
+use crate::{OffsetDateTime, UtcOffset};
+
+pub(super) fn local_offset_at(_datetime: OffsetDateTime) -> Option<UtcOffset> {
+    None
+}

src/sys/local_offset_at/mod.rs

@@ -0,0 +1,13 @@
+//! A method to obtain the local offset from UTC.
+
+#[cfg_attr(target_family = "windows", path = "windows.rs")]
+#[cfg_attr(target_family = "unix", path = "unix.rs")]
+mod imp;
+
+use crate::{OffsetDateTime, UtcOffset};
+
+/// Attempt to obtain the system's UTC offset. If the offset cannot be determined, `None` is
+/// returned.
+pub(crate) fn local_offset_at(datetime: OffsetDateTime) -> Option<UtcOffset> {
+    imp::local_offset_at(datetime)
+}

src/sys/local_offset_at/unix.rs

@@ -0,0 +1,112 @@
+//! Get the system's UTC offset on Unix.
+
+#[cfg(unsound_local_offset)]
+use core::convert::TryInto;
+#[cfg(unsound_local_offset)]
+use core::mem::MaybeUninit;
+
+use crate::{OffsetDateTime, UtcOffset};
+
+/// Obtain the system's UTC offset.
+// See #293 for details.
+#[cfg(not(unsound_local_offset))]
+#[allow(clippy::missing_const_for_fn)]
+pub(super) fn local_offset_at(_datetime: OffsetDateTime) -> Option<UtcOffset> {
+    None
+}
+
+/// Convert the given Unix timestamp to a `libc::tm`. Returns `None` on any error.
+#[cfg(unsound_local_offset)]
+fn timestamp_to_tm(timestamp: i64) -> Option<libc::tm> {
+    extern "C" {
+        #[cfg_attr(target_os = "netbsd", link_name = "__tzset50")]
+        fn tzset();
+    }
+
+    // The exact type of `timestamp` beforehand can vary, so this conversion is necessary.
+    #[allow(clippy::useless_conversion)]
+    let timestamp = timestamp.try_into().ok()?;
+
+    let mut tm = MaybeUninit::uninit();
+
+    // Update timezone information from system. `localtime_r` does not do this for us.
+    //
+    // Safety: tzset is thread-safe.
+    #[allow(unsafe_code)]
+    unsafe {
+        tzset();
+    }
+
+    // Safety: We are calling a system API, which mutates the `tm` variable. If a null
+    // pointer is returned, an error occurred.
+    #[allow(unsafe_code)]
+    let tm_ptr = unsafe { libc::localtime_r(&timestamp, tm.as_mut_ptr()) };
+
+    if tm_ptr.is_null() {
+        None
+    } else {
+        // Safety: The value was initialized, as we no longer have a null pointer.
+        #[allow(unsafe_code)]
+        {
+            Some(unsafe { tm.assume_init() })
+        }
+    }
+}
+
+/// Convert a `libc::tm` to a `UtcOffset`. Returns `None` on any error.
+// `tm_gmtoff` extension
+#[cfg(unsound_local_offset)]
+#[cfg(not(any(target_os = "solaris", target_os = "illumos")))]
+fn tm_to_offset(tm: libc::tm) -> Option<UtcOffset> {
+    let seconds: i32 = tm.tm_gmtoff.try_into().ok()?;
+    UtcOffset::from_hms(
+        (seconds / 3_600) as _,
+        ((seconds / 60) % 60) as _,
+        (seconds % 60) as _,
+    )
+    .ok()
+}
+
+/// Convert a `libc::tm` to a `UtcOffset`. Returns `None` on any error.
+#[cfg(unsound_local_offset)]
+#[cfg(any(target_os = "solaris", target_os = "illumos"))]
+fn tm_to_offset(tm: libc::tm) -> Option<UtcOffset> {
+    use core::convert::TryFrom;
+
+    use crate::Date;
+
+    let mut tm = tm;
+    if tm.tm_sec == 60 {
+        // Leap seconds are not currently supported.
+        tm.tm_sec = 59;
+    }
+
+    let local_timestamp =
+        Date::from_ordinal_date(1900 + tm.tm_year, u16::try_from(tm.tm_yday).ok()? + 1)
+            .ok()?
+            .with_hms(
+                tm.tm_hour.try_into().ok()?,
+                tm.tm_min.try_into().ok()?,
+                tm.tm_sec.try_into().ok()?,
+            )
+            .ok()?
+            .assume_utc()
+            .unix_timestamp();
+
+    let diff_secs: i32 = (local_timestamp - datetime.unix_timestamp())
+        .try_into()
+        .ok()?;
+
+    UtcOffset::from_hms(
+        (diff_secs / 3_600) as _,
+        ((diff_secs / 60) % 60) as _,
+        (diff_secs % 60) as _,
+    )
+    .ok()
+}
+
+/// Obtain the system's UTC offset.
+#[cfg(unsound_local_offset)]
+pub(super) fn local_offset_at(datetime: OffsetDateTime) -> Option<UtcOffset> {
+    tm_to_offset(timestamp_to_tm(datetime.unix_timestamp())?)
+}

src/sys/local_offset_at/windows.rs

@@ -0,0 +1,117 @@
+//! Get the system's UTC offset on Windows.
+
+use core::convert::TryInto;
+use core::mem::MaybeUninit;
+
+// ffi: WINAPI FILETIME struct
+#[repr(C)]
+#[allow(non_snake_case)]
+struct FileTime {
+    dwLowDateTime: u32,
+    dwHighDateTime: u32,
+}
+
+// ffi: WINAPI SYSTEMTIME struct
+#[repr(C)]
+#[allow(non_snake_case)]
+struct SystemTime {
+    wYear: u16,
+    wMonth: u16,
+    wDayOfWeek: u16,
+    wDay: u16,
+    wHour: u16,
+    wMinute: u16,
+    wSecond: u16,
+    wMilliseconds: u16,
+}
+
+#[link(name = "kernel32")]
+extern "system" {
+    // https://docs.microsoft.com/en-us/windows/win32/api/timezoneapi/nf-timezoneapi-systemtimetofiletime
+    fn SystemTimeToFileTime(lpSystemTime: *const SystemTime, lpFileTime: *mut FileTime) -> i32;
+
+    // https://docs.microsoft.com/en-us/windows/win32/api/timezoneapi/nf-timezoneapi-systemtimetotzspecificlocaltime
+    fn SystemTimeToTzSpecificLocalTime(
+        lpTimeZoneInformation: *const std::ffi::c_void, // We only pass `nullptr` here
+        lpUniversalTime: *const SystemTime,
+        lpLocalTime: *mut SystemTime,
+    ) -> i32;
+}
+
+/// Convert a `SYSTEMTIME` to a `FILETIME`. Returns `None` if any error occurred.
+fn systemtime_to_filetime(systime: &SystemTime) -> Option<FileTime> {
+    let mut ft = MaybeUninit::uninit();
+
+    // Safety: `SystemTimeToFileTime` is thread-safe. We are only assuming initialization if
+    // the call succeeded.
+    #[allow(unsafe_code)]
+    {
+        if 0 == unsafe { SystemTimeToFileTime(systime, ft.as_mut_ptr()) } {
+            // failed
+            None
+        } else {
+            Some(unsafe { ft.assume_init() })
+        }
+    }
+}
+
+/// Convert a `FILETIME` to an `i64`, representing a number of seconds.
+fn filetime_to_secs(filetime: &FileTime) -> i64 {
+    /// FILETIME represents 100-nanosecond intervals
+    const FT_TO_SECS: i64 = 10_000_000;
+    ((filetime.dwHighDateTime as i64) << 32 | filetime.dwLowDateTime as i64) / FT_TO_SECS
+}
+
+/// Convert an [`OffsetDateTime`] to a `SYSTEMTIME`.
+fn offset_to_systemtime(datetime: OffsetDateTime) -> SystemTime {
+    let (_, month, day_of_month) = datetime.to_offset(UtcOffset::UTC).date().to_calendar_date();
+    SystemTime {
+        wYear: datetime.year() as _,
+        wMonth: month as _,
+        wDay: day_of_month as _,
+        wDayOfWeek: 0, // ignored
+        wHour: datetime.hour() as _,
+        wMinute: datetime.minute() as _,
+        wSecond: datetime.second() as _,
+        wMilliseconds: datetime.millisecond(),
+    }
+}
+
+/// Obtain the system's UTC offset.
+pub(super) fn local_offset_at(datetime: OffsetDateTime) -> Option<UtcOffset> {
+    // This function falls back to UTC if any system call fails.
+    let systime_utc = offset_to_systemtime(datetime.to_offset(UtcOffset::UTC));
+
+    // Safety: `local_time` is only read if it is properly initialized, and
+    // `SystemTimeToTzSpecificLocalTime` is thread-safe.
+    #[allow(unsafe_code)]
+    let systime_local = unsafe {
+        let mut local_time = MaybeUninit::uninit();
+
+        if 0 == SystemTimeToTzSpecificLocalTime(
+            core::ptr::null(), // use system's current timezone
+            &systime_utc,
+            local_time.as_mut_ptr(),
+        ) {
+            // call failed
+            return None;
+        } else {
+            local_time.assume_init()
+        }
+    };
+
+    // Convert SYSTEMTIMEs to FILETIMEs so we can perform arithmetic on them.
+    let ft_system = systemtime_to_filetime(&systime_utc)?;
+    let ft_local = systemtime_to_filetime(&systime_local)?;
+
+    let diff_secs: i32 = (filetime_to_secs(&ft_local) - filetime_to_secs(&ft_system))
+        .try_into()
+        .ok()?;
+
+    UtcOffset::from_hms(
+        (diff_secs / 3_600) as _,
+        ((diff_secs / 60) % 60) as _,
+        (diff_secs % 60) as _,
+    )
+    .ok()
+}

src/sys/mod.rs

@@ -0,0 +1,7 @@
+//! Functions with a common interface that rely on system calls.
+
+#[cfg(feature = "local-offset")]
+mod local_offset_at;
+
+#[cfg(feature = "local-offset")]
+pub(crate) use local_offset_at::local_offset_at;