set COMMTIMEOUTS for POSIX-like read timeout behavior on Windows

serial-windows/src/com.rs

@@ -167,10 +167,19 @@ impl SerialDevice for COMPort {
     fn set_timeout(&mut self, timeout: Duration) -> core::Result<()> {
         let milliseconds = timeout.as_secs() * 1000 + timeout.subsec_nanos() as u64 / 1_000_000;
 
+        // populate COMMTIMEOUTS struct
+        // https://docs.microsoft.com/en-us/windows/win32/devio/time-outs
+        // https://docs.microsoft.com/en-us/windows/win32/api/winbase/ns-winbase-commtimeouts
         let timeouts = COMMTIMEOUTS {
-            ReadIntervalTimeout: 0,
-            ReadTotalTimeoutMultiplier: 0,
+            // return as soon as bytes become available (like POSIX would) and
+            // block up to given duration otherwise
+            // https://docs.microsoft.com/en-us/windows/win32/api/winbase/ns-winbase-commtimeouts#remarks
+            ReadIntervalTimeout: MAXDWORD,
+            ReadTotalTimeoutMultiplier: MAXDWORD,
             ReadTotalTimeoutConstant: milliseconds as DWORD,
+            // block without timeout until write is complete
+            // MAXDWORD is *not* a reserved WriteTotalTimeoutMultiplier
+            // value, i.e., setting it incurs a long write timeout
             WriteTotalTimeoutMultiplier: 0,
             WriteTotalTimeoutConstant: 0,
         };

serial-windows/src/ffi.rs

@@ -18,6 +18,8 @@ pub type LPWSTR = *mut WCHAR;
 
 pub type HANDLE = *mut LPVOID;
 
+pub const MAXDWORD: DWORD = 0xFFFFFFFF;
+
 pub const GENERIC_READ: DWORD = 0x80000000;
 pub const GENERIC_WRITE: DWORD = 0x40000000;
 pub const OPEN_EXISTING: DWORD = 3;