spawn/IProtocol: add commands for copying stdin to stdout/stderr

This reduces some overhead and fixes an assertion failure that can
occur when "tty" is set because previously, stdin/stdout fds were not
the same if they had been transferred through a local socket.

src/spawn/Client.cxx

@@ -290,8 +290,21 @@ Serialize(SpawnSerializer &s, const PreparedChildProcess &p)
 	}
 
 	s.CheckWriteFd(SpawnExecCommand::STDIN, p.stdin_fd);
-	s.CheckWriteFd(SpawnExecCommand::STDOUT, p.stdout_fd);
-	s.CheckWriteFd(SpawnExecCommand::STDERR, p.stderr_fd);
+
+	if (p.stdout_fd.IsDefined()) {
+		if (p.stdout_fd == p.stdin_fd)
+			s.Write(SpawnExecCommand::STDOUT_IS_STDIN);
+		else
+			s.WriteFd(SpawnExecCommand::STDOUT, p.stdout_fd);
+	}
+
+	if (p.stderr_fd.IsDefined()) {
+		if (p.stderr_fd == p.stdin_fd)
+			s.Write(SpawnExecCommand::STDERR_IS_STDIN);
+		else
+			s.WriteFd(SpawnExecCommand::STDERR, p.stderr_fd);
+	}
+
 	s.CheckWriteFd(SpawnExecCommand::CONTROL, p.control_fd);
 
 	s.CheckWriteFd(SpawnExecCommand::RETURN_STDERR,

src/spawn/IProtocol.hxx

@@ -26,7 +26,9 @@ enum class SpawnExecCommand : uint8_t {
 	UMASK,
 	STDIN,
 	STDOUT,
+	STDOUT_IS_STDIN,
 	STDERR,
+	STDERR_IS_STDIN,
 	STDERR_PATH,
 	RETURN_STDERR,
 	RETURN_PIDFD,

src/spawn/Server.cxx

@@ -542,10 +542,18 @@ SpawnServerConnection::HandleExecMessage(SpawnPayload payload,
 			p.SetStdout(fds.Get().Steal());
 			break;
 
+		case SpawnExecCommand::STDOUT_IS_STDIN:
+			p.stdout_fd = p.stdin_fd;
+			break;
+
 		case SpawnExecCommand::STDERR:
 			p.SetStderr(fds.Get().Steal());
 			break;
 
+		case SpawnExecCommand::STDERR_IS_STDIN:
+			p.stderr_fd = p.stdin_fd;
+			break;
+
 		case SpawnExecCommand::STDERR_PATH:
 			p.stderr_path = payload.ReadString();
 			break;