Expose the fd in grpc_fd and grpc_tcp.

Fixes a bug where grpc_fd::released was not initialized.

src/core/iomgr/fd_posix.c

@@ -101,6 +101,7 @@ static grpc_fd *alloc_fd(int fd) {
   r->read_watcher = r->write_watcher = NULL;
   r->on_done_closure = NULL;
   r->closed = 0;
+  r->released = 0;
   return r;
 }
 
@@ -210,6 +211,14 @@ static int has_watchers(grpc_fd *fd) {
          fd->inactive_watcher_root.next != &fd->inactive_watcher_root;
 }
 
+int grpc_fd_wrapped_fd(grpc_fd *fd) {
+  if (fd->released || fd->closed) {
+    return -1;
+  } else {
+    return fd->fd;
+  }
+}
+
 void grpc_fd_orphan(grpc_exec_ctx *exec_ctx, grpc_fd *fd, grpc_closure *on_done,
                     int *release_fd, const char *reason) {
   fd->on_done_closure = on_done;

src/core/iomgr/fd_posix.h

@@ -105,6 +105,9 @@ struct grpc_fd {
    This takes ownership of closing fd. */
 grpc_fd *grpc_fd_create(int fd, const char *name);
 
+/* Return the wrapped fd, or -1 if it has been released or closed. */
+int grpc_fd_wrapped_fd(grpc_fd *fd);
+
 /* Releases fd to be asynchronously destroyed.
    on_done is called when the underlying file descriptor is definitely close()d.
    If on_done is NULL, no callback will be made.

src/core/iomgr/tcp_posix.c

@@ -473,6 +473,12 @@ grpc_endpoint *grpc_tcp_create(grpc_fd *em_fd, size_t slice_size,
   return &tcp->base;
 }
 
+int grpc_tcp_fd(grpc_endpoint *ep) {
+  grpc_tcp *tcp = (grpc_tcp *)ep;
+  GPR_ASSERT(ep->vtable == &vtable);
+  return grpc_fd_wrapped_fd(tcp->em_fd);
+}
+
 void grpc_tcp_destroy_and_release_fd(grpc_exec_ctx *exec_ctx, grpc_endpoint *ep,
                                      int *fd, grpc_closure *done) {
   grpc_tcp *tcp = (grpc_tcp *)ep;

src/core/iomgr/tcp_posix.h

@@ -56,6 +56,12 @@ extern int grpc_tcp_trace;
 grpc_endpoint *grpc_tcp_create(grpc_fd *fd, size_t read_slice_size,
                                const char *peer_string);
 
+/* Return the tcp endpoint's fd, or -1 if this is not available. Does not
+   release the fd.
+   Requires: ep must be a tcp endpoint.
+ */
+int grpc_tcp_fd(grpc_endpoint *ep);
+
 /* Destroy the tcp endpoint without closing its fd. *fd will be set and done
  * will be called when the endpoint is destroyed.
  * Requires: ep must be a tcp endpoint and fd must not be NULL. */