Clamp long timeouts on Windows

src/windows/com.rs

@@ -249,13 +249,21 @@ impl SerialPort for COMPort {
 
     fn set_timeout(&mut self, timeout: Duration) -> Result<()> {
         let milliseconds = timeout.as_millis();
+        // In the way we are setting up timeouts here, a timeout_constant of MAXDWORD gets
+        // rejected. Let's clamp the timeout constant for values of MAXDWORD and above. See remarks
+        // at https://learn.microsoft.com/en-us/windows/win32/api/winbase/ns-winbase-commtimeouts.
+        //
+        // This effectively throws away accuracy for really long timeouts but at least preserves a
+        // long-ish timeout. But just casting to DWORD would result in presumably unexpected short
+        // and non-monotonic timeouts from cutting off the higher bits.
+        let timeout_constant = u128::min(milliseconds, MAXDWORD as u128 - 1) as DWORD;
 
         let mut timeouts = COMMTIMEOUTS {
             ReadIntervalTimeout: MAXDWORD,
             ReadTotalTimeoutMultiplier: MAXDWORD,
-            ReadTotalTimeoutConstant: milliseconds as DWORD,
+            ReadTotalTimeoutConstant: timeout_constant,
             WriteTotalTimeoutMultiplier: 0,
-            WriteTotalTimeoutConstant: milliseconds as DWORD,
+            WriteTotalTimeoutConstant: timeout_constant,
         };
 
         if unsafe { SetCommTimeouts(self.handle, &mut timeouts) } == 0 {