key too long on macos

[flaviotordini]

With Qt 6.6.0 on macOS version 12.3.1 Intel I get this error on startup:

[SingleApplication::SingleApplication] SingleApplication: Unable to create block.
[SingleApplication::abortSafely] SingleApplication:  QSharedMemory::KeyError "QSharedMemoryPrivate::initKey: unable to set key on lock (QSystemSemaphore::handle: key too long)"

[flaviotordini]

I don't know the details, but in general shouldn't this lib allow a second instance rather than completely failing?

[itay-grudev]

Just to clarify, which version of the kl library are you using?

[flaviotordini]

Ah sorry, the latest version: 3.5.0

[itay-grudev]

@jonaski Can you help me with this one, as this is likely related to #177?

[jonaski]

It's using shm_open(), this is not a problem on Linux and other unix variants because it uses PATH_MAX, but on macOS the maximum key length is PSHMNAMLEN (31). While the base64 encoded sha256 we use is 44.
See: https://github.com/qt/qtbase/blob/368ea559ebe842500fcad962d02308c7cb2f5632/src/corelib/ipc/qsharedmemory_posix.cpp#L70
I tested the old constructor taking QString instead of QNativeIpcKey, and it's as broken as on Linux (as I explained in the issue), so switching back is not an alternative.
The only simple fix I see now is to use a different base64 encoded string or hash specifically for macOS.

[jonaski]

Unfortunately there seem to be another issue on macOS too, once the application is started and you quit normally, you still can't start it again, so the memory is not released, Qt bug perhaps.

[itay-grudev]

@jonaski Does that mean that even with the fix in #180 it won't work?

[jonaski]

It solves the issue related to the key size, so the application starts, but there are still issues since the shared memory isn't released.

[itay-grudev]

@jonaski I documented it as a known issue in the release notes.

Known issues:

Using Qt 6.6+ on macOS once the application is started and you quit normally, you still can't start it again. This is likely due to a Qt bug. We are still looking for a workaround.

[jonaski]

If someone with a mac has a chance to dig into this more, there is a small test project attached to the Qt bug report on https://bugreports.qt.io/browse/QTBUG-111855