Fix mutex impl detection for critical sections

Fixes #21.

Author: seritools

library/std/src/sys/windows/compat.rs

@@ -25,7 +25,7 @@ use crate::sync::atomic::Ordering;
 use crate::sys::c;
 
 mod version;
-pub use version::{is_windows_nt, supports_async_io};
+pub use version::*;
 
 // This uses a static initializer to preload some imported functions.
 // The CRT (C runtime) executes static initializers before `main`

library/std/src/sys/windows/compat/version.rs

@@ -2,12 +2,22 @@ use crate::sys::c;
 
 static mut IS_NT: bool = true;
 static mut SUPPORTS_ASYNC_IO: bool = true;
+static mut SUPPORTS_TRY_ENTER_CRITICAL_SECTION: bool = true;
 
 pub fn init_windows_version_check() {
-    // according to old MSDN info, the high-order bit is set only on 95/98/ME.
     unsafe {
-        IS_NT = c::GetVersion() < 0x8000_0000;
-        SUPPORTS_ASYNC_IO = IS_NT && c::CancelIo::option().is_some();
+        let version = c::GetVersion();
+        let major = version as u8;
+
+        // according to old MSDN info, the high-order bit is set only on 95/98/ME.
+        let is_nt = version < 0x8000_0000;
+
+        IS_NT = is_nt;
+        SUPPORTS_ASYNC_IO = is_nt && c::CancelIo::option().is_some();
+
+        // at least 9x exports TryEnterCriticalSection, but it doesn't work, so we need to check the
+        // version. MSDN specifies that the function is available on NT4 and later.
+        SUPPORTS_TRY_ENTER_CRITICAL_SECTION = is_nt && major >= 4;
     };
 }
 
@@ -22,3 +32,8 @@ pub fn is_windows_nt() -> bool {
 pub fn supports_async_io() -> bool {
     unsafe { SUPPORTS_ASYNC_IO }
 }
+
+#[inline(always)]
+pub fn supports_try_enter_critical_section() -> bool {
+    unsafe { SUPPORTS_TRY_ENTER_CRITICAL_SECTION }
+}

library/std/src/sys/windows/locks/mutex/compat.rs

@@ -1,4 +1,4 @@
-use crate::sys::c;
+use crate::sys::{c, compat};
 
 #[derive(Debug, PartialEq)]
 pub enum MutexKind {
@@ -15,7 +15,9 @@ pub static mut MUTEX_KIND: MutexKind = MutexKind::SrwLock;
 pub fn init() {
     let kind = if c::TryAcquireSRWLockExclusive::option().is_some() {
         MutexKind::SrwLock
-    } else if c::TryEnterCriticalSection::option().is_some() {
+    } else if compat::supports_try_enter_critical_section()
+        && c::TryEnterCriticalSection::option().is_some()
+    {
         MutexKind::CriticalSection
     } else {
         MutexKind::Legacy