net/SocketDescriptor: add GetPeerPidfd()

src/net/SocketDescriptor.hxx

@@ -25,6 +25,7 @@ class StaticSocketAddress;
 class IPv4Address;
 class IPv6Address;
 class UniqueSocketDescriptor;
+class UniqueFileDescriptor;
 
 /**
  * An OO wrapper for a Berkeley or WinSock socket descriptor.
@@ -229,6 +230,16 @@ public:
 	struct ucred GetPeerCredentials() const noexcept;
 #endif
 
+#ifdef __linux__
+	/**
+	 * Get a pidfd for the peer process.  Returns an undefined
+	 * instance on error (with errno set).
+	 *
+	 * Requires Linux 6.5.
+	 */
+	UniqueFileDescriptor GetPeerPidfd() const noexcept;
+#endif // __linux__
+
 	bool SetOption(int level, int name,
 		       const void *value, std::size_t size) const noexcept;