libutil: add generic unix socketpair wrapper

previously we used this only for SSH, but other uses may appear soon.

Change-Id: Ibe9666d63aaea07525ebad57decda88b11964cc0

lix/libstore/ssh.cc

@@ -51,24 +51,7 @@ void SSH::addCommonSSHOpts(Strings & args)
 
 std::unique_ptr<SSH::Connection> SSH::startCommand(const std::string & command)
 {
-    int sp[2];
+    auto [parent, child] = SocketPair::stream();
-    // only linux and bsd support SOCK_CLOEXEC in socketpair type.
-#if __linux__ || __FreeBSD__
-    constexpr int sock_type = SOCK_STREAM | SOCK_CLOEXEC;
-#else
-    constexpr int sock_type = SOCK_STREAM;
-#endif
-    if (socketpair(AF_UNIX, sock_type, 0, sp) < 0) {
-        throw SysError("socketpair() for ssh");
-    }
-
-    AutoCloseFD parent(sp[0]), child(sp[1]);
-#if !(__linux__ || __FreeBSD__)
-    if (fcntl(parent.get(), F_SETFD, O_CLOEXEC) < 0 || fcntl(child.get(), F_SETFD, O_CLOEXEC) < 0) {
-        throw SysError("making socketpair O_CLOEXEC");
-    }
-#endif
-
     auto conn = std::make_unique<Connection>();
     ProcessOptions options;
     options.dieWithParent = false;

lix/libutil/file-descriptor.cc

@@ -9,6 +9,7 @@
 #include <cerrno>
 #include <fcntl.h>
 #include <sys/poll.h>
+#include <sys/socket.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 
@@ -327,4 +328,26 @@ void resetBlockingState(int fd, FdBlockingState prevState)
         throw SysError("resetBlockingState");
     }
 }
+
+SocketPair SocketPair::stream()
+{
+    int sp[2];
+#ifdef SOCK_CLOEXEC
+    constexpr int sock_type = SOCK_STREAM | SOCK_CLOEXEC;
+#else
+    constexpr int sock_type = SOCK_STREAM;
+#endif
+    if (socketpair(AF_UNIX, sock_type, 0, sp) < 0) {
+        throw SysError("socketpair()");
+    }
+
+    AutoCloseFD a(sp[0]), b(sp[1]);
+#ifndef SOCK_CLOEXEC
+    closeOnExec(a.get());
+    closeOnExec(b.get());
+#endif
+
+    return {std::move(a), std::move(b)};
+}
+
 }

lix/libutil/file-descriptor.hh

@@ -83,6 +83,18 @@ public:
     void close();
 };
 
+struct SocketPair
+{
+    /** The two sides of the socket pair. */
+    AutoCloseFD a, b;
+
+    /** Create a unix stream socket pair with the `O_CLOEXEC` set on both ends. */
+    static SocketPair stream();
+
+private:
+    SocketPair(AutoCloseFD a, AutoCloseFD b) : a(std::move(a)), b(std::move(b)) {}
+};
+
 /**
  * Close all file descriptors except stdio fds (ie 0, 1, 2).
  * Good practice in child processes.