Prepare mmsghdr structure properly. (#2346)

Motivation:

According to the Linux man page the msg_len field supposed to be used to return a number of bytes sent for the particular message.
It does not make a sense to initialize it with a size of the message.

Modifications:

Change msg_leg field initialization, use 0 instead of message size.

Result:

Use sendmmsg() call properly.

Co-authored-by: Cory Benfield <[email protected]>

Author: ser-0xff

Sources/NIOPosix/PendingDatagramWritesManager.swift

@@ -139,7 +139,7 @@ private func doPendingDatagramWriteVectorOperation(pending: PendingDatagramWrite
                              msg_control: controlMessageBytePointer.baseAddress,
                              msg_controllen: .init(controlMessageBytePointer.count),
                              msg_flags: 0)
-            msgs[c] = MMsgHdr(msg_hdr: msg, msg_len: CUnsignedInt(toWriteForThisBuffer))
+            msgs[c] = MMsgHdr(msg_hdr: msg, msg_len: 0)
         }
         c += 1
     }

Tests/NIOPosixTests/PendingDatagramWritesManagerTests.swift

@@ -155,7 +155,7 @@ class PendingDatagramWritesManagerTests: XCTestCase {
                         XCTAssertEqual(expected[multiState].map { numericCast($0.0) }, ptrs.map { $0.msg_hdr.msg_iov.pointee.iov_len },
                                        "in vector write \(multiState) (overall \(everythingState)), \(expected[multiState]) byte counts expected but \(ptrs.map { $0.msg_hdr.msg_iov.pointee.iov_len }) actual",
                                        file: (file), line: line)
-                        XCTAssertEqual(expected[multiState].map { $0.0 }, ptrs.map { Int($0.msg_len) },
+                        XCTAssertEqual(ptrs.map { Int($0.msg_len) }, Array(repeating: 0, count: ptrs.count),
                                        "in vector write \(multiState) (overall \(everythingState)), \(expected[multiState]) byte counts expected but \(ptrs.map { $0.msg_len }) actual",
                             file: (file), line: line)
                         XCTAssertEqual(expected[multiState].map { $0.1 }, ptrs.map { SocketAddress($0.msg_hdr.msg_name.assumingMemoryBound(to: sockaddr.self)) }, "in vector write \(multiState) (overall \(everythingState)), \(expected[multiState].map { $0.1 }) addresses expected but \(ptrs.map { SocketAddress($0.msg_hdr.msg_name.assumingMemoryBound(to: sockaddr.self)) }) actual",