Always throw the right exception in connect

When `nix::connect` is called with a socket path that's too long, it
forks a process that will `chdir` to the directory of the socket path
and call `::connect` with the relative path (which is hopefully
short-enough).

That works fairly well, except that an exception raised in this
subprocess won't be forwarded to the parent process. Instead the logic
will just notice that the subprocess exited with a non-zero error code,
and throw a generic `Error`. In particular, any failure in the
`::connect` call should throw a `SysError` with the correct error code,
but that's not the case.

Some places try to catch this `SysError` and look at its error code (to
potentially restart for example). But this doesn't work since the
actual error that gets thrown isn't a `SysError`.

Fix that by forwarding the `errno` in case something gets wrong (by
setting the subprocess exit code to it), and throwing a `SysError` with
the right error code in the parent process.

src/libutil/util.cc

@@ -1817,6 +1817,7 @@ void connect(int fd, const std::string & path)
 
     if (path.size() + 1 >= sizeof(addr.sun_path)) {
         Pid pid = startProcess([&]() {
+            try {
                 Path dir = dirOf(path);
                 if (chdir(dir.c_str()) == -1)
                     throw SysError("chdir to '%s' failed", dir);
@@ -1827,9 +1828,21 @@ void connect(int fd, const std::string & path)
                 if (connect(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
                     throw SysError("cannot connect to socket at '%s'", path);
                 _exit(0);
+            } catch (SysError & e) {
+                // If this raised a `SysError`, we want to rethrow it in the
+                // parent process.
+                // For that, we need to transmit the associated error code,
+                // which we do by setting it as the return code for the process.
+                _exit(e.errNo);
+            } catch (std::exception &) {
+                _exit(-1);
+            }
         });
         int status = pid.wait();
-        if (status != 0)
+        if (status > 0) {
+            errno = status;
+            throw SysError("cannot connect to socket at '%s'", path);
+        } else if (status < 0)
             throw Error("cannot connect to socket at '%s'", path);
     } else {
         memcpy(addr.sun_path, path.c_str(), path.size() + 1);