native: Probe availability of abstract socket namespace on Linux

When using a Linux emulation layer like NetBSD or FreeBSD, the abstract
socket namespace may not be available.

Try to bind on an abstract socket upon initialization. If that fails,
don't add the "CAPABILITY_ABSTRACT_NAMESPACE" capability.

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

@@ -19,6 +19,11 @@
 #include "config.h"
 #include "capabilities.h"
 
+#if defined(__linux__)
+  #include <sys/socket.h>
+  #include <sys/un.h>
+#endif
+
 #include "filedescriptors.h"
 #include "init.h"
 #include "reflection.h"
@@ -48,6 +53,30 @@ void init_capabilities(JNIEnv *env CK_UNUSED) {
 void destroy_capabilities(JNIEnv *env CK_UNUSED) {
 }
 
+#if defined(__linux__)
+static bool checkHasAbstractNamespace(void) {
+    bool hasAbstractNamespace = true;
+
+    struct sockaddr_un addr = {
+        .sun_family = AF_UNIX,
+        .sun_path = "\0junixsocket-capability-abstract\0"
+    };
+
+    int sockFd = socket(AF_UNIX, SOCK_STREAM, 0);
+    if(sockFd != -1) {
+        if(bind(sockFd, (struct sockaddr *)&addr, sizeof(struct sockaddr_un)) == -1) {
+            // typically, ENOENT is returned on NetBSD/FreeBSD
+            if(errno != EADDRINUSE) {
+                hasAbstractNamespace = false;
+            }
+        }
+        close(sockFd);
+    }
+
+    return hasAbstractNamespace;
+}
+#endif
+
 /*
  * Class:     org_newsclub_net_unix_NativeUnixSocket
  * Method:    capabilities
@@ -83,7 +112,12 @@ defined(SO_PEERCRED) || defined(SO_PEERID) || defined(__NetBSD__) || defined(__s
         // despite earlier claims [1], it's not supported in Windows 10 (yet) [2]
         // [1] https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/
         // [2] https://github.com/microsoft/WSL/issues/4240
-        capabilities |= CAPABILITY_ABSTRACT_NAMESPACE;
+
+        // Some Linux emulation layers (like NetBSD, FreeBSD) don't support
+        // the abstract socket namespace
+        if(checkHasAbstractNamespace()) {
+            capabilities |= CAPABILITY_ABSTRACT_NAMESPACE;
+        }
 #endif
 
 #if !defined(_WIN32)