Remove Socket.SendFile 2GB limitation on Windows by partitioning large files

src/libraries/System.Net.Sockets/src/System.Net.Sockets.csproj

@@ -179,6 +179,8 @@
              Link="Common\Interop\Windows\Kernel32\Interop.SetFileCompletionNotificationModes.cs" />
     <Compile Include="$(CommonPath)Interop\Windows\Kernel32\Interop.HandleInformation.cs"
              Link="Common\Interop\Windows\Kernel32\Interop.HandleInformation.cs" />
+    <Compile Include="$(CommonPath)Interop\Windows\Kernel32\Interop.SetFilePointerEx.cs"
+             Link="Common\Interop\Windows\Kernel32\Interop.SetFilePointerEx.cs" />
     <Compile Include="$(CommonPath)System\Net\CompletionPortHelper.Windows.cs"
              Link="Common\System\Net\CompletionPortHelper.Windows.cs" />
   </ItemGroup>

src/libraries/System.Net.Sockets/src/System/Net/Sockets/SocketPal.Windows.cs

@@ -295,8 +295,69 @@ public static unsafe SocketError SendFile(SafeSocketHandle handle, SafeFileHandl
             fixed (byte* prePinnedBuffer = preBuffer)
             fixed (byte* postPinnedBuffer = postBuffer)
             {
-                bool success = TransmitFileHelper(handle, fileHandle, null, (IntPtr)prePinnedBuffer, preBuffer.Length, (IntPtr)postPinnedBuffer, postBuffer.Length, flags);
-                return success ? SocketError.Success : GetLastSocketError();
+                // Get file length if we have a file
+                long fileLength = 0;
+                if (fileHandle is not null)
+                {
+                    fileLength = RandomAccess.GetLength(fileHandle);
+                }
+
+                // If file length exceeds int.MaxValue, we need to partition the sends
+                if (fileLength > int.MaxValue)
+                {
+                    // Separate behavior/performance flags from terminal flags
+                    // Behavior flags (WriteBehind, UseSystemThread, UseKernelApc) apply to all operations
+                    // Terminal flags (Disconnect, ReuseSocket) apply only to the final operation
+                    TransmitFileOptions behaviorFlags = flags & ~(TransmitFileOptions.Disconnect | TransmitFileOptions.ReuseSocket);
+
+                    // Send preBuffer if present
+                    if (preBuffer.Length > 0)
+                    {
+                        bool success = TransmitFileHelper(handle, null, null, (IntPtr)prePinnedBuffer, preBuffer.Length, IntPtr.Zero, 0, behaviorFlags);
+                        if (!success)
+                        {
+                            return GetLastSocketError();
+                        }
+                    }
+
+                    // Send file in chunks of int.MaxValue bytes
+                    long offset = 0;
+                    long remaining = fileLength;
+                    while (remaining > 0)
+                    {
+                        int chunkSize = (int)Math.Min(remaining, int.MaxValue);
+
+                        // Set the file pointer to the current offset
+                        if (!Interop.Kernel32.SetFilePointerEx(fileHandle!, offset, out _, 0 /* FILE_BEGIN */))
+                        {
+                            return GetLastSocketError();
+                        }
+
+                        bool success = TransmitFileHelper(handle, fileHandle, null, IntPtr.Zero, 0, IntPtr.Zero, 0, behaviorFlags, chunkSize);
+                        if (!success)
+                        {
+                            return GetLastSocketError();
+                        }
+
+                        offset += chunkSize;
+                        remaining -= chunkSize;
+                    }
+
+                    // Send postBuffer with all flags (behavior + terminal flags apply to last operation)
+                    bool finalSuccess = TransmitFileHelper(handle, null, null, IntPtr.Zero, 0, (IntPtr)postPinnedBuffer, postBuffer.Length, flags);
+                    if (!finalSuccess)
+                    {
+                        return GetLastSocketError();
+                    }
+
+                    return SocketError.Success;
+                }
+                else
+                {
+                    // File is small enough, use single call
+                    bool success = TransmitFileHelper(handle, fileHandle, null, (IntPtr)prePinnedBuffer, preBuffer.Length, (IntPtr)postPinnedBuffer, postBuffer.Length, flags);
+                    return success ? SocketError.Success : GetLastSocketError();
+                }
             }
         }
 
@@ -1004,7 +1065,8 @@ private static unsafe bool TransmitFileHelper(
             int preBufferLength,
             IntPtr pinnedPostBuffer,
             int postBufferLength,
-            TransmitFileOptions flags)
+            TransmitFileOptions flags,
+            int numberOfBytesToWrite = 0)
         {
             bool needTransmitFileBuffers = false;
             Interop.Mswsock.TransmitFileBuffers transmitFileBuffers = default;
@@ -1027,14 +1089,14 @@ private static unsafe bool TransmitFileHelper(
             IntPtr fileHandlePtr = IntPtr.Zero;
             try
             {
-                if (fileHandle != null)
+                if (fileHandle is not null)
                 {
                     fileHandle.DangerousAddRef(ref releaseRef);
                     fileHandlePtr = fileHandle.DangerousGetHandle();
                 }
 
                 return Interop.Mswsock.TransmitFile(
-                    socket, fileHandlePtr, 0, 0, overlapped,
+                    socket, fileHandlePtr, numberOfBytesToWrite, 0, overlapped,
                     needTransmitFileBuffers ? &transmitFileBuffers : null, flags);
             }
             finally

src/libraries/System.Net.Sockets/tests/FunctionalTests/SendFile.cs

@@ -484,7 +484,6 @@ protected SendFile_NonParallel(ITestOutputHelper output) : base(output)
         {
         }
 
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/42534", TestPlatforms.Windows)]
         [OuterLoop("Creates and sends a file several gigabytes long")]
         [Fact]
         public async Task GreaterThan2GBFile_SendsAllBytes()