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, 11 insertions, 1 deletions

diff --git a/src/shared/ptyfwd.h b/src/shared/ptyfwd.h
index 6f84e4036a..9b3214221b 100644
--- a/src/shared/ptyfwd.h
+++ b/src/shared/ptyfwd.h
@@ -27,7 +27,17 @@
 
 typedef struct PTYForward PTYForward;
 
-int pty_forward_new(sd_event *event, int master, bool ignore_vhangup, bool read_only, PTYForward **f);
+typedef enum PTYForwardFlags {
+        PTY_FORWARD_READ_ONLY = 1,
+
+        /* Continue reading after hangup? */
+        PTY_FORWARD_IGNORE_VHANGUP = 2,
+
+        /* Continue reading after hangup but only if we never read anything else? */
+        PTY_FORWARD_IGNORE_INITIAL_VHANGUP = 4,
+} PTYForwardFlags;
+
+int pty_forward_new(sd_event *event, int master, PTYForwardFlags flags, PTYForward **f);
 PTYForward *pty_forward_free(PTYForward *f);
 
 int pty_forward_get_last_char(PTYForward *f, char *ch);