Add a "fork" of musl as //fusl.

fusl/src/misc/forkpty.c

Index: fusl/src/misc/forkpty.c
diff --git a/fusl/src/misc/forkpty.c b/fusl/src/misc/forkpty.c
new file mode 100644
index 0000000000000000000000000000000000000000..caf13adbf3f9a22aeee9d5cff4d5fe97432fb282
--- /dev/null
+++ b/fusl/src/misc/forkpty.c
@@ -0,0 +1,57 @@
+#include <pty.h>
+#include <utmp.h>
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/wait.h>
+#include <pthread.h>
+
+int forkpty(int *pm, char *name, const struct termios *tio, const struct winsize *ws)
+{
+	int m, s, ec=0, p[2], cs;
+	pid_t pid=-1;
+	sigset_t set, oldset;
+
+	if (openpty(&m, &s, name, tio, ws) < 0) return -1;
+
+	sigfillset(&set);
+	pthread_sigmask(SIG_BLOCK, &set, &oldset);
+	pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
+
+	if (pipe2(p, O_CLOEXEC)) {
+		close(s);
+		goto out;
+	}
+
+	pid = fork();
+	if (!pid) {
+		close(m);
+		close(p[0]);
+		if (login_tty(s)) {
+			write(p[1], &errno, sizeof errno);
+			_exit(127);
+		}
+		close(p[1]);
+		pthread_setcancelstate(cs, 0);
+		pthread_sigmask(SIG_SETMASK, &oldset, 0);
+		return 0;
+	}
+	close(s);
+	close(p[1]);
+	if (read(p[0], &ec, sizeof ec) > 0) {
+		int status;
+		waitpid(pid, &status, 0);
+		pid = -1;
+		errno = ec;
+	}
+	close(p[0]);
+
+out:
+	if (pid > 0) *pm = m;
+	else close(m);
+
+	pthread_setcancelstate(cs, 0);
+	pthread_sigmask(SIG_SETMASK, &oldset, 0);
+
+	return pid;
+}