Expose UDP_MAX_SEGMENTS via System

Sources/CNIOLinux/include/CNIOLinux.h

@@ -30,7 +30,7 @@
 #include <errno.h>
 #include <pthread.h>
 #include <netinet/ip.h>
-#include <netinet/udp.h>
+#include <linux/udp.h>
 #include <linux/vm_sockets.h>
 #include <fcntl.h>
 #include <fts.h>
@@ -143,5 +143,7 @@ extern const unsigned int CNIOLinux_RENAME_EXCHANGE;
 extern const unsigned long CNIOLinux_UTIME_OMIT;
 extern const unsigned long CNIOLinux_UTIME_NOW;
 
+extern const long CNIOLinux_UDP_MAX_SEGMENTS;
+
 #endif
 #endif

Sources/CNIOLinux/shim.c

@@ -215,4 +215,9 @@ const int CNIOLinux_AT_EMPTY_PATH = AT_EMPTY_PATH;
 const unsigned long CNIOLinux_UTIME_OMIT = UTIME_OMIT;
 const unsigned long CNIOLinux_UTIME_NOW = UTIME_NOW;
 
+
+#ifdef UDP_MAX_SEGMENTS
+const long CNIOLinux_UDP_MAX_SEGMENTS = UDP_MAX_SEGMENTS;
+#endif
+const long CNIOLinux_UDP_MAX_SEGMENTS = -1;
 #endif

Sources/NIOCore/Utilities.swift

@@ -257,4 +257,15 @@ extension System {
     /// The option can be enabled by setting the ``ChannelOptions/Types/DatagramReceiveOffload`` channel option.
     public static let supportsUDPReceiveOffload: Bool = false
     #endif
+
+    /// Returns the UDP maximum segment count if the platform supports and defines it.
+    public static var udpMaxSegments: Int? {
+        #if os(Linux)
+        let maxSegments = CNIOLinux_UDP_MAX_SEGMENTS
+        if maxSegments != -1 {
+            return maxSegments
+        }
+        #endif
+        return nil
+    }
 }

Tests/NIOPosixTests/DatagramChannelTests.swift

@@ -1495,7 +1495,10 @@ class DatagramChannelTests: XCTestCase {
     func testWriteBufferAtGSOSegmentCountLimit() throws {
         try XCTSkipUnless(System.supportsUDPSegmentationOffload, "UDP_SEGMENT (GSO) is not supported on this platform")
 
-        var segments = 64
+        let udpMaxSegments = System.udpMaxSegments ?? 64
+
+        var segments = udpMaxSegments
+
         let segmentSize = 10
         let didSet = self.firstChannel.setOption(.datagramSegmentSize, value: CInt(segmentSize))
         XCTAssertNoThrow(try didSet.wait())
@@ -1514,7 +1517,7 @@ class DatagramChannelTests: XCTestCase {
             self.firstChannel = try self.buildChannel(group: self.group)
             let didSet = self.firstChannel.setOption(.datagramSegmentSize, value: CInt(segmentSize))
             XCTAssertNoThrow(try didSet.wait())
-            segments = 61
+            segments = udpMaxSegments - 3
             try send(byteCount: segments * segmentSize)
         }
 
@@ -1525,13 +1528,16 @@ class DatagramChannelTests: XCTestCase {
     func testWriteBufferAboveGSOSegmentCountLimitShouldError() throws {
         try XCTSkipUnless(System.supportsUDPSegmentationOffload, "UDP_SEGMENT (GSO) is not supported on this platform")
 
+        // commonly 64 or 128 on systems which may or may not define UDP_MAX_SEGMENTS, pick the larger to ensure failure
+        let udpMaxSegments = System.udpMaxSegments ?? 128
+
         let segmentSize = 10
         let didSet = self.firstChannel.setOption(.datagramSegmentSize, value: CInt(segmentSize))
         XCTAssertNoThrow(try didSet.wait())
 
-        let buffer = self.firstChannel.allocator.buffer(repeating: 1, count: segmentSize * 65)
+        let buffer = self.firstChannel.allocator.buffer(repeating: 1, count: segmentSize * udpMaxSegments + 1)
         let writeData = AddressedEnvelope(remoteAddress: self.secondChannel.localAddress!, data: buffer)
-        // The kernel limits messages to a maximum of 64 segments; any more should result in an error.
+        // The kernel limits messages to a maximum of UDP_MAX_SEGMENTS segments; any more should result in an error.
         XCTAssertThrowsError(try self.firstChannel.writeAndFlush(NIOAny(writeData)).wait()) {
             XCTAssert($0 is IOError)
         }