jdk: just reserve FDs in 'socketpair'

socketpair() is used in libnio/libnet to create "marker FDs", which are
only used as destination FD in dup2(). So it is safe to just reserve
those descriptors. Otherwise, calling functions may use uninitialized
stack variables after successful return of socketpair(), which results
in closing arbtrary fds later on.

Fixes #192

src/app/jdk/lib/jvm/dummies.cpp

@@ -10,6 +10,8 @@ extern "C" {
 #include <signal.h>
 }
 
+#include <libc-plugin/fd_alloc.h>
+
 #if 0
 #define WARN_NOT_IMPL Genode::warning(__func__, " not implemented (jvm) from ", __builtin_return_address(0));
 #else
@@ -99,7 +101,14 @@ ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags)
 
 int socketpair(int domain, int type, int protocol, int sv[2])
 {
-	WARN_NOT_IMPL;
+	/*
+	 * socketpair() is used in libnio/libnet to create "marker FDs", which are
+	 * only used as destination FD in dup2(). So it is safe to just reserve
+	 * those descriptors here.
+	 */
+	sv[0] = Libc::file_descriptor_allocator()->alloc(nullptr, nullptr)->libc_fd;
+	sv[1] = Libc::file_descriptor_allocator()->alloc(nullptr, nullptr)->libc_fd;
+
 	return 0;
 }