machinectl: fix race when opening new shells with "machinectl shell"

Previously, we'd allocate the TTY, spawn a service on it, but
immediately start processing the TTY and forwarding it to whatever the
commnd was started on. This is however problematic, as the TTY might get
actually opened only much later by the service. We'll hence first get
EIOs on the master as the other side is still closed, and hence
considered it hung up and terminated the session.

With this change we add a flag to the pty forwarding logic:
PTY_FORWARD_IGNORE_INITIAL_VHANGUP. If set, we'll ignore all hangups
(i.e. EIOs) on the master PTY until the first byte is successfully read.
From that point on we consider a hangup/EIO a regular connection termination. This
way, we handle the race: when we get EIO initially we'll ignore it,
until the connection is properly set up, at which time we start
honouring it.

1 files changed, 5 insertions, 5 deletions

diff --git a/src/machine/machinectl.c b/src/machine/machinectl.c
index 17a186eb0e..e75b183328 100644
--- a/src/machine/machinectl.c
+++ b/src/machine/machinectl.c
@@ -1173,7 +1173,7 @@ static int on_machine_removed(sd_bus_message *m, void *userdata, sd_bus_error *r
         return 0;
 }
 
-static int process_forward(sd_event *event, PTYForward **forward, int master, bool ignore_vhangup, const char *name) {
+static int process_forward(sd_event *event, PTYForward **forward, int master, PTYForwardFlags flags, const char *name) {
         char last_char = 0;
         bool machine_died;
         int ret = 0, r;
@@ -1192,7 +1192,7 @@ static int process_forward(sd_event *event, PTYForward **forward, int master, bo
         sd_event_add_signal(event, NULL, SIGINT, NULL, NULL);
         sd_event_add_signal(event, NULL, SIGTERM, NULL, NULL);
 
-        r = pty_forward_new(event, master, ignore_vhangup, false, forward);
+        r = pty_forward_new(event, master, flags, forward);
         if (r < 0)
                 return log_error_errno(r, "Failed to create PTY forwarder: %m");
 
@@ -1203,7 +1203,7 @@ static int process_forward(sd_event *event, PTYForward **forward, int master, bo
         pty_forward_get_last_char(*forward, &last_char);
 
         machine_died =
-                ignore_vhangup &&
+                (flags & PTY_FORWARD_IGNORE_VHANGUP) &&
                 pty_forward_get_ignore_vhangup(*forward) == 0;
 
         *forward = pty_forward_free(*forward);
@@ -1286,7 +1286,7 @@ static int login_machine(int argc, char *argv[], void *userdata) {
         if (r < 0)
                 return bus_log_parse_error(r);
 
-        return process_forward(event, &forward, master, true, machine);
+        return process_forward(event, &forward, master, PTY_FORWARD_IGNORE_VHANGUP, machine);
 }
 
 static int shell_machine(int argc, char *argv[], void *userdata) {
@@ -1390,7 +1390,7 @@ static int shell_machine(int argc, char *argv[], void *userdata) {
         if (r < 0)
                 return bus_log_parse_error(r);
 
-        return process_forward(event, &forward, master, false, machine);
+        return process_forward(event, &forward, master, PTY_FORWARD_IGNORE_INITIAL_VHANGUP, machine);
 }
 
 static int remove_image(int argc, char *argv[], void *userdata) {