Add a --tty option

If the tty option is passed it will:
  Create a pty.
  Set the parent's stdin to raw mode.
  Set the child's fds to the term.
  Handle copying to/from the term.
  Handle SIGWINCH.

enter.c

@@ -31,6 +31,7 @@
 #include "ns.h"
 #include "outer.h"
 #include "path.h"
+#include "tty.h"
 #include "setarch.h"
 #include "sig.h"
 #include "util.h"
@@ -208,6 +209,11 @@ int enter(struct entry_settings *opts)
 	   cleaner and easier to always fork, regardless of unsharing the PID
 	   namespace. */
 
+	int parentSock = -1, childSock = -1;
+	if (opts->tty) {
+		tty_setup_socketpair(&parentSock, &childSock);
+	}
+
 	pid_t pid = fork();
 	if (pid == -1) {
 		if (errno == ENOMEM && nsactions[NS_PID] >= 0) {
@@ -217,6 +223,9 @@ int enter(struct entry_settings *opts)
 	}
 
 	if (pid) {
+		if (childSock >= 0) {
+			close(childSock);
+		}
 		if (opts->pidfile != NULL) {
 			int pidfile = open(opts->pidfile, O_WRONLY | O_CREAT | O_CLOEXEC | O_NOCTTY , 0666);
 			if (pidfile == -1) {
@@ -260,22 +269,32 @@ int enter(struct entry_settings *opts)
 		sigset_t mask;
 		sigfillset(&mask);
 
+		if (parentSock >= 0) {
+			tty_parent_setup(parentSock);
+		}
+
 		for (;;) {
 
+			int waitflags = WEXITED | WNOHANG;
 			siginfo_t info;
-			sig_wait(&mask, &info);
-			sig_forward(&info, pid);
-
-			if (info.si_signo != SIGCHLD) {
-				continue;
+			if (parentSock < 0) {
+				sig_wait(&mask, &info);
+				sig_forward(&info, pid);
+				if (info.si_signo != SIGCHLD) {
+					continue;
+				}
+			} else {
+				if (!tty_parent_select(pid, &waitflags)) {
+					continue;
+				}
 			}
 
 			/* We might have been run as a process subreaper against our
 			   will -- make sure we only exit when the main child pid
 			   exited. */
 
 			int rc;
-			while ((rc = waitid(P_ALL, 0, &info, WEXITED | WNOHANG)) != -1) {
+			while ((rc = waitid(P_ALL, 0, &info, waitflags)) != -1) {
 				if (info.si_signo != SIGCHLD) {
 					break;
 				}
@@ -308,6 +327,10 @@ int enter(struct entry_settings *opts)
 		}
 	}
 
+	if (parentSock >= 0) {
+		close(parentSock);
+	}
+
 	sigset_t mask;
 	sigemptyset(&mask);
 
@@ -575,6 +598,10 @@ int enter(struct entry_settings *opts)
 		}
 	}
 
+	if (childSock >= 0) {
+		tty_child(childSock);
+	}
+
 	if (opts->init != NULL && opts->init[0] != '\0') {
 
 		if (!pid_unshare && prctl(PR_SET_CHILD_SUBREAPER, 1) == -1) {

enter.h

@@ -83,6 +83,8 @@ struct entry_settings {
 
 	const char *pidfile;
 
+	bool tty;
+
 	int no_copy_hard_limits;
 	int no_fake_devtmpfs;
 	int no_derandomize;

main.c

@@ -62,6 +62,7 @@ enum {
 	OPTION_NO_INIT,
 	OPTION_NO_ENV,
 	OPTION_NO_COPY_HARD_LIMITS,
+	OPTION_TTY,
 };
 
 static void process_nslist_entry(const char **out, const char *share, const char *path, int append_nsname)
@@ -284,6 +285,7 @@ int main(int argc, char *argv[], char *envp[])
 		{ "pidfile",            required_argument, NULL, OPTION_PIDFILE         },
 		{ "ip",                 required_argument, NULL, OPTION_IP              },
 		{ "route",              required_argument, NULL, OPTION_ROUTE           },
+		{ "tty",                no_argument,       NULL, OPTION_TTY             },
 
 		/* Opt-out feature flags */
 		{ "no-copy-hard-limits", no_argument, NULL, OPTION_NO_COPY_HARD_LIMITS  },
@@ -656,6 +658,10 @@ int main(int argc, char *argv[], char *envp[])
 				opts.no_env = 1;
 				break;
 
+			case OPTION_TTY:
+			  opts.tty = 1;
+			  break;
+
 			case 'r':
 				opts.root = optarg;
 				break;

man/bst.1.scd

@@ -328,6 +328,12 @@ _VAR=value_ before the executable to run.
 	the spacetime process. This can cause issues for environment variables like
 	PATH if the rootfs layout is different than the host.
 
+\--tty
+	Allocate a pty for the process.
+
+	By default bst inherits the parent's terminal device (or lack thereof).  Use
+	the --tty option to allocate a new pty for the child process.
+
 # PRIVILEGE MODEL
 
 *bst* strives to be runnable by unprivileged users.  However, some code paths

meson.build

@@ -87,6 +87,7 @@ bst_sources = [
 	'setarch.c',
 	'sig.c',
 	'timens.c',
+	'tty.c',
 	'usage.c',
 	'userns.c',
 ]

test/tty.t

@@ -0,0 +1,5 @@
+#!/usr/bin/env cram.sh
+
+Allocate a PTY for the spacetime
+	$ bst --tty --mount devpts,/dev/pts,devpts,mode=620,ptmxmode=666 tty
+	/dev/pts/0