repeated calls to "logs" emits empty logs

[jochumdev]

Migrated from https://gitlab.com/r3j0/incus-compose/-/issues/23

Bug

incus-compose logs (without --follow) returns correct output on the first
call but nothing on subsequent calls for running containers.

Root cause

The Incus server's GET /1.0/instances/{name}/console handler calls liblxc
with WriteToLogFile: true. This advances a write cursor in the LXC ring
buffer — each call only returns bytes written after the previous cursor
position. The first call drains all buffered output; subsequent calls return
nothing until new output arrives.

For stopped containers the server reads from the accumulated
ConsoleBufferLogPath() file, which always contains the full history — so
logs works correctly after stop.

Workaround

Use --follow to stream continuously:

incus-compose logs --follow web-1

What we cannot fix client-side

The cursor is advanced server-side. There is no reset or read-from-start
option in the current GET /console REST API.

Potential upstream fix

A reset_cursor or read_from_start query parameter on GET /console would
let clients always read from the beginning of the ring buffer, matching
docker logs semantics.

Until then, --tail (not yet implemented) could partially help by
reading from the log file for stopped instances.

[jochumdev]

This is fixed in lxc/incus#3467 we have to wait for 7.2 to be released.