util: Clear FD_CLOEXEC on child socket before exec

Explicitly clear FD_CLOEXEC on the child's socket before calling exec,
so the fd survives into the spawned process regardless of how the socket
was created. Previously this relied on socketpair() not setting
FD_CLOEXEC by default, which is not guaranteed if the caller creates
sockets with SOCK_CLOEXEC or if the flag gets set by other means.

Author: ryanofsky

src/mp/util.cpp

@@ -15,7 +15,7 @@
 #include <pthread.h>
 #include <sstream>
 #include <string>
-#include <sys/types.h>
+#include <fcntl.h>
 #include <sys/resource.h>
 #include <sys/socket.h>
 #include <sys/wait.h>
@@ -158,6 +158,16 @@ std::tuple<ProcessId, SocketId> SpawnProcess(ConnectInfoToArgsFn&& connect_info_
             }
         }
 
+        // Explicitly clear FD_CLOEXEC on the child's socket before calling
+        // exec, so the fd survives into the spawned process regardless of how
+        // the socket was created.
+        int flags = fcntl(fds[0], F_GETFD);
+        if (flags == -1) throw std::system_error(errno, std::system_category(), "fcntl F_GETFD");
+        if (flags & FD_CLOEXEC) {
+            flags &= ~FD_CLOEXEC;
+            if (fcntl(fds[0], F_SETFD, flags) == -1) throw std::system_error(errno, std::system_category(), "fcntl F_SETFD");
+        }
+
         execvp(argv[0], argv.data());
         // NOTE: perror() is not async-signal-safe; calling it here in a
         // post-fork child may deadlock in multithreaded parents.