Invalid Argument error on brscan5 device

[Szum123321]

Hi!

I am trying to share my scanner Brother MFC-L2802DN using brscan5 driver over the local network via an arch linux server.
The daemon starts up correctly, but segfaults when a scan is about to start.
What i found is that the second call to sane_open within the same sane_init/sane_exit session leads to an Invalid Argument error.

git commit: 31c3c9e (branch master, rev 321)
build date: 2026-01-03T20:32:45Z
reading access rules from file /etc/airsane/access.conf
start time is 32520.49
reading device options from '/etc/airsane/options.conf'
enumerating  devices...
sane_init(nullptr, nullptr)
sane_get_devices() ...
... sane_get_devices() -> SANE_Status Success
found: brother5:bus1;dev12 (Brother MFC-L2802DN)
stable unique name: brother5:Brother MFC-L2802DN:1
uuid: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
sane_open(brother5:bus1;dev12) -> 0x6785a003b580
[source] := "FlatBed"
[source] := "Automatic Document Feeder(left aligned)"
sane_close(0x6785a003b580)
published as 'Brother MFC-L2802DN'
end time is 32520.68
startup took 0.19 secconds
listening on 127.0.0.1:8090
listening on [::1]:8090
allowing 192.168.0.246, matching rule: local on *
document format requested: image/jpeg
document format used: image/jpeg
job kind: single
using color gamma of 0.56
allowing 192.168.0.246, matching rule: local on *
allowing 192.168.0.246, matching rule: local on *
sane_open(brother5:bus1;dev12) -> SANE_Status Invalid argument

The same thing occurs when testing the setup from a python script.

import sane

sane.init()

dev_handle=sane.get_devices()[0][0]
dev=sane.open(dev_handle)
print(f"First run: {dev.scanner_model}")
dev.close()

dev=sane.open(dev_handle)
print(f"Second run: {dev.scanner_model}")

Produces:

First run: ('Brother', 'MFC-L2802DN')
Traceback (most recent call last):
  File "<python-input-0>", line 10, in <module>
    dev=sane.open(dev_handle)
  File "/usr/lib/python3.13/site-packages/sane.py", line 412, in open
    return SaneDev(devname)
  File "/usr/lib/python3.13/site-packages/sane.py", line 164, in __init__
    d['dev'] = _sane._open(devname)
               ~~~~~~~~~~~^^^^^^^^^
_sane.error: Invalid argument

While the following code works correctly:

import sane

sane.init()

dev_handle=sane.get_devices()[0][0]
dev=sane.open(dev_handle)
print(f"First run: {dev.scanner_model}")
dev.close()
sane.exit()
sane.init()
dev=sane.open(dev_handle)
print(f"Second run: {dev.scanner_model}")

and outputs:

First run: ('Brother', 'MFC-L2802DN')
Second run: ('Brother', 'MFC-L2802DN')

The issue appears to be due to the particular brscan5 scanner driver. So far i tried patching out the reference counting in sanecpp/sanecpp.cpp.
This leads to multiple repeated calls to sane_init and exit, which seems to work fine. I have yet to test the system at any load levels or for a longer span of time than 5 minutes.

I have not fully studied the architecture of this system to be sure that this would patch the behavior, and most certainly, this breaks other vendor's scanners.

I hope to be able to fix this issue permanently, but i understand that breaking other devices is not acceptable. please let me know what are your thought on this issue.

[SimulPiscator]

Thanks for your concise error report.
The refcounting in sanecpp::init is to avoid the lengthy SANE initialization procedure as far as possible. Also, the SANE docs are silent about whether it is ok to call sane_init() multiple times in a row, so it might be forbidden. This is why the refcounting mechanism is necessary.

I guess removing it will not break any of the backends but in default configurations -- with many backends enabled in dll.conf -- starting up airsaned and possibly scanning will take much more time than necesssary.