[RFC container v2 2/2] add container console scrollback buffer

[Filip Schauer <f.schauer@proxmox.com>]

Send a scrollback buffer to new clients connecting to a container
console.

This is achieved by replacing dtach and lxc-console with
pve-lxc-console, which serves the console to all clients and replays a
scrollback buffer when a client attaches. For containers with
`cmode: console`, we additionally enable the LXC console ring buffer by
setting `lxc.console.buffer.size` to 32KB. It records the console
output since the container started, and pve-lxc-console reads its data
from there. This way, users who open the console too late still see
what the container printed earlier, even if no console was open at that
time.

The other console modes do not need a ring buffer to avoid losing
data: with `cmode: tty` the kernel buffers the tty output until it is
read, and with `cmode: shell` the shell is only started when the
console is opened for the first time.

This improves the user experience for containers and is especially
useful for many application containers that print important diagnostic
messages at startup, that would otherwise be easily missed by users
opening the container console slightly too late.

This change is backwards-compatible: pve-lxc-console speaks the dtach
protocol on the same socket path, so it can still connect to a dtach
master that is already running for a container. Upgrading also does not
break consoles for already running containers with `cmode: console`.
They were started without a console buffer, so if pve-lxc-console has
to start a new master for them, it falls back to reading the console
data directly.

Signed-off-by: Filip Schauer <f.schauer@proxmox.com>
---
 debian/control      |  1 +
 src/PVE/API2/LXC.pm | 36 +++---------------------------------
 src/PVE/LXC.pm      | 11 +++++++++++
 3 files changed, 15 insertions(+), 33 deletions(-)

diff --git a/debian/control b/debian/control
index 1d77c38..8a2a63f 100644
--- a/debian/control
+++ b/debian/control
@@ -36,6 +36,7 @@ Depends: binutils,
          pve-cluster (>= 4.0-8),
          pve-firewall (>= 6.0.1),
          pve-ha-manager (>= 5.0.3),
+         pve-lxc-console,
          pve-lxc-syscalld,
          proxmox-backup-client (>= 3.2.3-1),
          xz-utils,
diff --git a/src/PVE/API2/LXC.pm b/src/PVE/API2/LXC.pm
index 63d30d1..713f4df 100644
--- a/src/PVE/API2/LXC.pm
+++ b/src/PVE/API2/LXC.pm
@@ -1019,17 +1019,7 @@ __PACKAGE__->register_method({
         my $ticket = PVE::AccessControl::assemble_vnc_ticket($authuser, $authpath, $port);
 
         my $conf = PVE::LXC::Config->load_config($vmid, $node);
-        my $concmd = PVE::LXC::get_console_command($vmid, $conf, -1);
-
-        my $shcmd = [
-            '/usr/bin/dtach',
-            '-A',
-            "/var/run/dtach/vzctlconsole$vmid",
-            '-r',
-            'winch',
-            '-z',
-            @$concmd,
-        ];
+        my $shcmd = PVE::LXC::get_console_dtach_command($vmid, $conf);
 
         my $password;
         if ($param->{websocket}) {
@@ -1150,17 +1140,7 @@ __PACKAGE__->register_method({
         my $ticket = PVE::AccessControl::assemble_vnc_ticket($authuser, $authpath, $port);
 
         my $conf = PVE::LXC::Config->load_config($vmid, $node);
-        my $concmd = PVE::LXC::get_console_command($vmid, $conf, -1);
-
-        my $shcmd = [
-            '/usr/bin/dtach',
-            '-A',
-            "/var/run/dtach/vzctlconsole$vmid",
-            '-r',
-            'winch',
-            '-z',
-            @$concmd,
-        ];
+        my $shcmd = PVE::LXC::get_console_dtach_command($vmid, $conf);
 
         my $realcmd = sub {
             my $upid = shift;
@@ -1292,17 +1272,7 @@ __PACKAGE__->register_method({
 
         die "CT $vmid not running\n" if !PVE::LXC::check_running($vmid);
 
-        my $concmd = PVE::LXC::get_console_command($vmid, $conf);
-
-        my $shcmd = [
-            '/usr/bin/dtach',
-            '-A',
-            "/var/run/dtach/vzctlconsole$vmid",
-            '-r',
-            'winch',
-            '-z',
-            @$concmd,
-        ];
+        my $shcmd = PVE::LXC::get_console_dtach_command($vmid, $conf);
 
         my $title = "CT $vmid";
 
diff --git a/src/PVE/LXC.pm b/src/PVE/LXC.pm
index 78cc4cf..9cd0baf 100644
--- a/src/PVE/LXC.pm
+++ b/src/PVE/LXC.pm
@@ -871,6 +871,9 @@ sub update_lxc_config {
         }
     }
 
+    my $cmode = PVE::LXC::Config->get_cmode($conf);
+    $raw .= "lxc.console.buffer.size = 32KB\n" if ($cmode eq 'console');
+
     my $had_cpuset = 0;
     if (my $lxcconf = $conf->{lxc}) {
         foreach my $entry (@$lxcconf) {
@@ -952,6 +955,14 @@ sub get_console_command {
     return $cmd;
 }
 
+sub get_console_dtach_command {
+    my ($vmid, $conf) = @_;
+
+    my $cmode = PVE::LXC::Config->get_cmode($conf);
+
+    return ['pve-lxc-console', '-m', $cmode, $vmid];
+}
+
 sub get_primary_ips {
     my ($conf) = @_;
 
-- 
2.47.3