Fix AF_UNIX listen() w/ zero backlog.

In https://github.com/google/gvisor/commit/f075522849fa a check to increase zero
to a minimum backlog length was removed from sys_socket.go to bring it in parity
with linux and then in tcp/endpoint.go we bump backlog by 1. But this broke
calling listen on a AF_UNIX socket w/ a zero backlog as in linux it does allow 1
connection even with a zero backlog.

This was caught by a php runtime test socket_abstract_path.phpt.

PiperOrigin-RevId: 369974744

1 files changed, 46 insertions, 0 deletions

diff --git a/test/syscalls/linux/socket_unix_unbound_abstract.cc b/test/syscalls/linux/socket_unix_unbound_abstract.cc
index 8b1762000..dd3d25450 100644
--- a/test/syscalls/linux/socket_unix_unbound_abstract.cc
+++ b/test/syscalls/linux/socket_unix_unbound_abstract.cc
@@ -72,6 +72,52 @@ TEST_P(UnboundAbstractUnixSocketPairTest, BindNothing) {
               SyscallSucceeds());
 }
 
+TEST_P(UnboundAbstractUnixSocketPairTest, ListenZeroBacklog) {
+  SKIP_IF((GetParam().type & SOCK_DGRAM) != 0);
+  auto sockets = ASSERT_NO_ERRNO_AND_VALUE(NewSocketPair());
+  struct sockaddr_un addr = {};
+  addr.sun_family = AF_UNIX;
+  constexpr char kPath[] = "\x00/foo_bar";
+  memcpy(addr.sun_path, kPath, sizeof(kPath));
+  ASSERT_THAT(bind(sockets->first_fd(),
+                   reinterpret_cast<struct sockaddr*>(&addr), sizeof(addr)),
+              SyscallSucceeds());
+  ASSERT_THAT(listen(sockets->first_fd(), 0 /* backlog */), SyscallSucceeds());
+  ASSERT_THAT(connect(sockets->second_fd(),
+                      reinterpret_cast<struct sockaddr*>(&addr), sizeof(addr)),
+              SyscallSucceeds());
+  auto sockets2 = ASSERT_NO_ERRNO_AND_VALUE(NewSocketPair());
+  {
+    // Set the FD to O_NONBLOCK.
+    int opts;
+    int orig_opts;
+    ASSERT_THAT(opts = fcntl(sockets2->first_fd(), F_GETFL), SyscallSucceeds());
+    orig_opts = opts;
+    opts |= O_NONBLOCK;
+    ASSERT_THAT(fcntl(sockets2->first_fd(), F_SETFL, opts), SyscallSucceeds());
+
+    ASSERT_THAT(
+        connect(sockets2->first_fd(), reinterpret_cast<struct sockaddr*>(&addr),
+                sizeof(addr)),
+        SyscallFailsWithErrno(EAGAIN));
+  }
+  {
+    // Set the FD to O_NONBLOCK.
+    int opts;
+    int orig_opts;
+    ASSERT_THAT(opts = fcntl(sockets2->second_fd(), F_GETFL),
+                SyscallSucceeds());
+    orig_opts = opts;
+    opts |= O_NONBLOCK;
+    ASSERT_THAT(fcntl(sockets2->second_fd(), F_SETFL, opts), SyscallSucceeds());
+
+    ASSERT_THAT(
+        connect(sockets2->second_fd(),
+                reinterpret_cast<struct sockaddr*>(&addr), sizeof(addr)),
+        SyscallFailsWithErrno(EAGAIN));
+  }
+}
+
 TEST_P(UnboundAbstractUnixSocketPairTest, GetSockNameFullLength) {
   auto sockets = ASSERT_NO_ERRNO_AND_VALUE(NewSocketPair());