From d08819797a823bea5ab7544881e670b1599af0a6 Mon Sep 17 00:00:00 2001
From: Jason2866 <24528715+Jason2866@users.noreply.github.com>
Date: Thu, 16 Apr 2026 11:53:16 +0200
Subject: [PATCH 1/2] Refactor COM port read error handling

Removed error handling for GetOverlappedResult and added a note about ov->hEvent.
---
 src/serialport_win.cpp | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/src/serialport_win.cpp b/src/serialport_win.cpp
index 7e79216a..511b81d8 100644
--- a/src/serialport_win.cpp
+++ b/src/serialport_win.cpp
@@ -442,12 +442,6 @@ void __stdcall ReadIOCompletion(DWORD errorCode, DWORD bytesTransferred, OVERLAP
   }
 
   DWORD lastError;
-  if (!GetOverlappedResult(int2handle(baton->fd), ov, &bytesTransferred, TRUE)) {
-    lastError = GetLastError();
-    ErrorCodeToString("Reading from COM port (GetOverlappedResult)", lastError, baton->errorString);
-    baton->complete = true;
-    return;
-  }
   if (bytesTransferred) {
     baton->bytesToRead -= bytesTransferred;
     baton->bytesRead += bytesTransferred;
@@ -455,7 +449,7 @@ void __stdcall ReadIOCompletion(DWORD errorCode, DWORD bytesTransferred, OVERLAP
   }
   if (!baton->bytesToRead) {
     baton->complete = true;
-    CloseHandle(ov->hEvent);
+    // Note: ov->hEvent is a baton pointer, not a Windows handle — do not CloseHandle
     return;
   }
 

From fcf05f3f01cd72033c60cd488f59b41e49f6004a Mon Sep 17 00:00:00 2001
From: Jason2866 <24528715+Jason2866@users.noreply.github.com>
Date: Sat, 18 Apr 2026 13:06:28 +0200
Subject: [PATCH 2/2] Refactor WriteIOCompletion and WriteThread error handling

Updated error handling and comments in WriteIOCompletion and WriteThread functions to clarify usage of bytesTransferred and GetOverlappedResult.
---
 src/serialport_win.cpp | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/src/serialport_win.cpp b/src/serialport_win.cpp
index 511b81d8..7f8bac6f 100644
--- a/src/serialport_win.cpp
+++ b/src/serialport_win.cpp
@@ -326,18 +326,28 @@ bool IsClosingHandle(int fd) {
 
 void __stdcall WriteIOCompletion(DWORD errorCode, DWORD bytesTransferred, OVERLAPPED* ov) {
   WriteBaton* baton = static_cast<WriteBaton*>(ov->hEvent);
-  DWORD bytesWritten;
-  if (!GetOverlappedResult(int2handle(baton->fd), ov, &bytesWritten, TRUE)) {
-    errorCode = GetLastError();
-    ErrorCodeToString("Writing to COM port (GetOverlappedResult)", errorCode, baton->errorString);
+
+  if (errorCode) {
+    ErrorCodeToString("Writing to COM port (WriteIOCompletion)", errorCode, baton->errorString);
     baton->complete = true;
     return;
   }
-  if (bytesWritten) {
-    baton->offset += bytesWritten;
+
+  // bytesTransferred is already provided by the APC completion callback.
+  // Do NOT call GetOverlappedResult here — MSDN explicitly states:
+  //   "Do not use GetOverlappedResult for I/O operations that use
+  //    ReadFileEx or WriteFileEx completion routines."
+  // The overlapped's hEvent holds a WriteBaton pointer (not a Windows event
+  // handle), so GetOverlappedResult fails with ERROR_INVALID_HANDLE on
+  // drivers that inspect hEvent (e.g. usbser.sys used by USB serial chips).
+  if (bytesTransferred) {
+    baton->offset += bytesTransferred;
     if (baton->offset >= baton->bufferLength) {
       baton->complete = true;
     }
+  } else {
+    // Zero-byte completion with no error — avoid hanging in WriteThread.
+    baton->complete = true;
   }
 }
 
@@ -441,6 +451,14 @@ void __stdcall ReadIOCompletion(DWORD errorCode, DWORD bytesTransferred, OVERLAP
     return;
   }
 
+  // bytesTransferred is already provided by the APC completion callback.
+  // Do NOT call GetOverlappedResult here — MSDN explicitly states:
+  //   "Do not use GetOverlappedResult for I/O operations that use
+  //    ReadFileEx or WriteFileEx completion routines."
+  // The overlapped's hEvent holds a ReadBaton pointer (not a Windows event
+  // handle), so GetOverlappedResult fails with ERROR_INVALID_HANDLE on
+  // drivers that inspect hEvent (e.g. usbser.sys used by ESP32 native USB).
+
   DWORD lastError;
   if (bytesTransferred) {
     baton->bytesToRead -= bytesTransferred;