[PATCH qemu-server v2 0/1] fix #7590: apply timeout to QEMU start fork

[Samuel Rufinatscha <s.rufinatscha@proxmox.com>]

The qmstart task forks a child process that enters the VM scope, starts
the helper processes and launches QEMU. qmstart then waits for the
child to report success or failure. While regular starts already apply
$start_timeout to the QEMU command, the wait for the forked child has
no timeout. Statefile starts do not use the QEMU command timeout
either, leaving both waits unbounded.

Timeout handling currently:

  Start type                       QEMU command timeout    forked startup timeout
  ------------------------------------------------------------------------------
  regular                          $start_timeout          none
  statefile                        none                    none

Timeout handling after this series:

  Start type                       QEMU command timeout    fork startup timeout
  ------------------------------------------------------------------------------
  regular                          $start_timeout          2 * $start_timeout (account for helpers)
  statefile with --timeout N       none                    N
  statefile without --timeout      none                    24 hours (as fallback)

Regular starts retain the existing QEMU command timeout and is twice
for the outer timeout to account for VM scope and helper setup.
Statefile starts continue to have no separate QEMU
command timeout. If --timeout N is specified, N is used as the deadline
for the complete forked startup, otherwise a 24 hour fallback is used.
Values can be changed if needed.

Tested on a PVE 9.2 node:
- the statefile/FIFO timeout released the configuration lock and left
  no QEMU process or VM scope behind, using the reproducer from [1]
- the existing regular QEMU timeout still fired before the outer
  safeguard
- an immediate QEMU startup error performed the same scope cleanup
- successful startup and shutdown continued to work

Note: the separate D-state waitpid() FIXME in
run_fork_with_timeout() [2] concerns the timeout handling itself
potentially blocking indefinitely and is therefore out of scope for
this series.

[0] https://lore.proxmox.com/pve-devel/20260625114500.159384-1-s.rufinatscha@proxmox.com/
[1] https://bugzilla.proxmox.com/show_bug.cgi?id=7590
[2] https://git.proxmox.com/?p=pve-common.git;a=blob;f=src/PVE/Tools.pm;h=8e7646dea43e092fc9ec76b493b418a7ca0eaee2;hb=5054082fe492429fc37574985c2ca812af9a3125#l604

Samuel Rufinatscha (1):
  fix #7590: qemu-server: apply timeout to QEMU start fork

 src/PVE/QemuServer.pm | 93 +++++++++++++++++++++++++++++++++++--------
 1 file changed, 77 insertions(+), 16 deletions(-)

-- 
2.47.3