Defer to num_threads for OS support

.github/workflows/build.yaml

@@ -156,9 +156,9 @@ jobs:
       matrix:
         rust: ["1.53", stable]
         os:
-          - { name: Ubuntu, value: ubuntu-20.04, has_local_offset: true }
-          - { name: Windows, value: windows-latest, has_local_offset: true }
-          - { name: MacOS, value: macOS-latest, has_local_offset: false }
+          - { name: Ubuntu, value: ubuntu-20.04 }
+          - { name: Windows, value: windows-latest }
+          - { name: MacOS, value: macOS-latest }
 
     steps:
       - name: Checkout sources
@@ -203,24 +203,6 @@ jobs:
             --features macros
         if: matrix.os.has_local_offset == true
 
-      # Due to the soundness bug on Unix-like platforms, adding the local-offset flag doesn't
-      # actually add any new behavior. Since we know nothing changes, we can unconditionally enable
-      # the flag.
-      - name: Check feature powerset
-        uses: actions-rs/cargo@v1
-        with:
-          command: hack
-          args: |
-            check
-            --feature-powerset
-            --optional-deps
-            --group-features serde,rand
-            --group-features formatting,parsing
-            --group-features serde-human-readable,serde-well-known
-            --exclude-features default,quickcheck-dep,time-macros,itoa
-            --features macros,local-offset
-        if: matrix.os.has_local_offset == false
-
       - name: Test
         uses: actions-rs/cargo@v1
         with:

Cargo.toml

@@ -44,11 +44,9 @@ rand = { version = "0.8.4", optional = true, default-features = false }
 serde = { version = "1.0.126", optional = true, default-features = false }
 time-macros = { version = "=0.2.3", path = "time-macros", optional = true }
 
-[target.'cfg(any(target_os = "linux", target_os = "freebsd", unsound_local_offset))'.dependencies]
+[target.'cfg(target_family = "unix")'.dependencies]
 libc = "0.2.98"
-
-[target.'cfg(any(target_os = "linux", target_os = "freebsd"))'.dependencies]
-num_threads = "0.1.0"
+num_threads = "0.1.2"
 
 [dev-dependencies]
 rand = { version = "0.8.4", default-features = false }

src/sys/local_offset_at/unix.rs

@@ -1,20 +1,10 @@
 //! Get the system's UTC offset on Unix.
-#[cfg(any(target_os = "linux", target_os = "freebsd", unsound_local_offset))]
+
 use core::convert::TryInto;
-#[cfg(any(target_os = "linux", target_os = "freebsd", unsound_local_offset))]
 use core::mem::MaybeUninit;
 
 use crate::{OffsetDateTime, UtcOffset};
 
-/// Obtain the system's UTC offset.
-// This fallback is used whenever an operating system doesn't have a method below for determine if a
-// process is single threaded.
-#[cfg(not(any(target_os = "linux", target_os = "freebsd", 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.
 ///
 /// # Safety
@@ -24,7 +14,6 @@ pub(super) fn local_offset_at(_datetime: OffsetDateTime) -> Option<UtcOffset> {
 /// This method will remain `unsafe` until `std::env::set_var` is deprecated or has its behavior
 /// altered. This method is, on its own, safe. It is the presence of a safe, unsound way to set
 /// environment variables that makes it unsafe.
-#[cfg(any(target_os = "linux", target_os = "freebsd", unsound_local_offset))]
 unsafe fn timestamp_to_tm(timestamp: i64) -> Option<libc::tm> {
     extern "C" {
         #[cfg_attr(target_os = "netbsd", link_name = "__tzset50")]
@@ -56,8 +45,10 @@ unsafe fn timestamp_to_tm(timestamp: i64) -> Option<libc::tm> {
 
 /// Convert a `libc::tm` to a `UtcOffset`. Returns `None` on any error.
 // `tm_gmtoff` extension
-#[cfg(any(target_os = "linux", target_os = "freebsd", unsound_local_offset))]
-#[cfg(not(any(target_os = "solaris", target_os = "illumos")))]
+#[cfg(not(all(
+    unsound_local_offset,
+    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(
@@ -69,8 +60,10 @@ fn tm_to_offset(tm: libc::tm) -> Option<UtcOffset> {
 }
 
 /// Convert a `libc::tm` to a `UtcOffset`. Returns `None` on any error.
-#[cfg(unsound_local_offset)]
-#[cfg(any(target_os = "solaris", target_os = "illumos"))]
+#[cfg(all(
+    unsound_local_offset,
+    any(target_os = "solaris", target_os = "illumos")
+))]
 fn tm_to_offset(tm: libc::tm) -> Option<UtcOffset> {
     use core::convert::TryFrom;
 
@@ -107,12 +100,13 @@ fn tm_to_offset(tm: libc::tm) -> Option<UtcOffset> {
 }
 
 /// Obtain the system's UTC offset.
-#[cfg(any(target_os = "linux", target_os = "freebsd", unsound_local_offset))]
 pub(super) fn local_offset_at(datetime: OffsetDateTime) -> Option<UtcOffset> {
     // Ensure that the process is single-threaded unless the user has explicitly opted out of this
     // check. This is to prevent issues with the environment being mutated by a different thread in
     // the process while execution of this function is taking place, which can cause a segmentation
     // fault by dereferencing a dangling pointer.
+    // If the `num_threads` crate is incapable of determining the number of running threads, then
+    // we conservatively return `None` to avoid a soundness bug.
     if !cfg!(unsound_local_offset) && num_threads::is_single_threaded() != Some(true) {
         return None;
     }