Do not block on accept() if server died

The server may die before connecting to the client. In that case, the
client was blocked indefinitely (until Ctrl+C) on accept().

To avoid the problem, use a pipe and a select() before calling accept(),
so that the blocking call can be interrupted. Once accept() is called,
it is guaranteed not to block.

app/src/server.c

@@ -5,6 +5,7 @@
 #include <inttypes.h>
 #include <libgen.h>
 #include <stdio.h>
+#include <unistd.h>
 #include <SDL2/SDL_thread.h>
 #include <SDL2/SDL_timer.h>
 #include <SDL2/SDL_platform.h>
@@ -337,6 +338,10 @@ static int
 run_wait_server(void *data) {
     struct server *server = data;
     cmd_simple_wait(server->process, NULL); // ignore exit code
+
+    // wake up any net_select_interruptible()
+    close(server->pipe_intr[1]);
+
     LOGD("Server terminated");
     return 0;
 }
@@ -361,10 +366,16 @@ server_start(struct server *server, const char *serial,
         goto error1;
     }
 
+    bool ok = net_pipe(server->pipe_intr);
+    if (!ok) {
+        perror("pipe");
+        goto error2;
+    }
+
     // server will connect to our server socket
     server->process = execute_server(server, params);
     if (server->process == PROCESS_NONE) {
-        goto error2;
+        goto error3;
     }
 
     server->wait_server_thread =
@@ -374,13 +385,16 @@ server_start(struct server *server, const char *serial,
             LOGW("Could not terminate server");
         }
         cmd_simple_wait(server->process, NULL); // ignore exit code
-        goto error2;
+        goto error3;
     }
 
     server->tunnel_enabled = true;
 
     return true;
 
+error3:
+    close(server->pipe_intr[0]);
+    close(server->pipe_intr[1]);
 error2:
     if (!server->tunnel_forward) {
         close_socket(&server->server_socket);
@@ -394,17 +408,32 @@ error1:
 bool
 server_connect_to(struct server *server) {
     if (!server->tunnel_forward) {
+        bool acceptable = net_select_interruptible(server->server_socket,
+                                                   server->pipe_intr[0]);
+        if (!acceptable) {
+            // the process died, accept() would never succeed
+            return false;
+        }
+
         server->video_socket = net_accept(server->server_socket);
         if (server->video_socket == INVALID_SOCKET) {
             return false;
         }
 
+        acceptable = net_select_interruptible(server->server_socket,
+                                              server->pipe_intr[0]);
+        if (!acceptable) {
+            // the process died, accept() would never succeed
+            return false;
+        }
         server->control_socket = net_accept(server->server_socket);
         if (server->control_socket == INVALID_SOCKET) {
             // the video_socket will be cleaned up on destroy
             return false;
         }
 
+        close(server->pipe_intr[0]);
+
         // we don't need the server socket anymore
         close_socket(&server->server_socket);
     } else {

app/src/server.h

@@ -14,6 +14,7 @@ struct server {
     char *serial;
     process_t process;
     SDL_Thread *wait_server_thread;
+    int pipe_intr[2]; // to wake up blocking accept() on process exit
     socket_t server_socket; // only used if !tunnel_forward
     socket_t video_socket;
     socket_t control_socket;

app/src/util/net.c

@@ -1,16 +1,22 @@
 #include "net.h"
 
+#include <assert.h>
+#include <errno.h>
 #include <stdio.h>
 #include <SDL2/SDL_platform.h>
 
 #include "config.h"
+#include "common.h"
 #include "log.h"
 
 #ifdef __WINDOWS__
+# include <io.h>
+# include <winsock2.h>
   typedef int socklen_t;
 #else
-# include <sys/types.h>
+# include <sys/select.h>
 # include <sys/socket.h>
+# include <sys/types.h>
 # include <netinet/in.h>
 # include <arpa/inet.h>
 # include <unistd.h>
@@ -145,3 +151,38 @@ net_close(socket_t socket) {
     return !close(socket);
 #endif
 }
+
+bool
+net_select_interruptible(int fd, int fd_intr) {
+    fd_set rfds;
+
+    FD_ZERO(&rfds);
+    FD_SET(fd, &rfds);
+    FD_SET(fd_intr, &rfds);
+
+    int nfds = MAX(fd, fd_intr) + 1;
+
+    // use select() because it's available on supported platforms
+    int r = select(nfds, &rfds, NULL, NULL, NULL);
+    if (r == -1) {
+        // failure
+        return false;
+    }
+    assert(r > 0);
+    if (FD_ISSET(fd_intr, &rfds)) {
+        // interrupted is set
+        return false;
+    }
+
+    assert(FD_ISSET(fd, &rfds));
+    return true;
+}
+
+bool
+net_pipe(int fds[static 2]) {
+#ifdef __WINDOWS__
+    return !_pipe(fds, 4096, 0);
+#else
+    return !pipe(fds);
+#endif
+}

app/src/util/net.h

@@ -54,4 +54,12 @@ net_shutdown(socket_t socket, int how);
 bool
 net_close(socket_t socket);
 
+// wait for fd or fd_intr to be readable
+// return true if fd is readable and fd_intr is not
+bool
+net_select_interruptible(int fd, int fd_intr);
+
+bool
+net_pipe(int fd[static 2]);
+
 #endif