native: Do not process ancillary messages upon recvmsg error

Previously, when trying to receive ancillary messages, a failing call to
recvmsg (returning -1 with some errno) would still attempt to parse
ancillary messages stored in the receive buffer.

This would, under some circumstances and relatively sporadically, for
example when using nonblocking sockets, return duplicate file
descriptors (e.g, see
FileDescriptorsTest.testSendRecvFileDescriptorsChannelNonBlocking)

Detect the error case and skip parsing of ancillary messages.

junixsocket-native/src/main/c/receive.c

@@ -130,7 +130,7 @@ static ssize_t recvmsg_wrapper(JNIEnv * env, int handle, jbyte *buf, jint length
     controlLen = msg.msg_controllen;
     control = msg.msg_control;
 
-    if(controlLen <= 0 || control == NULL || ancSupp == NULL) {
+    if(count < 0 || controlLen <= 0 || control == NULL || ancSupp == NULL) {
         return count;
     }
 

src/site/markdown/changelog.md

@@ -15,6 +15,7 @@ artifact (`<type>pom</type>`); see [Add junixsocket to your project](dependency.
 ### _(2024-XX-XX)_ **junixsocket 2.10.1**
 
 - Fix left-over temporary library files on Windows
+- Fix duplicate file descriptors being received sporadically for non-blocking sockets and upon error
 - Fix a flaky selftest when VSOCK is not supported
 - Improve interoperability with exotic Linux/Java combinations
 - Add support for loongarch64 Linux