Using QSharedMemory for SingleApplication might stop working as of Qt 6.6 about singleapplication HOT 8 CLOSED

Technically I found a workaround for releasing QSharedMemory after a process has crashed. On Windows it's not a problem, but on Linux by initializing it, then deleting forces the kernel to clean up the shared memory. This way the moment e initialize it again it's the first time it's initialized. Take a look at the code below.

Lockfiles on the other hand suffer from the same problem when an instance has crashed. Originally I tried implementing it with a lockfile, but found that to be too unreliable. But maybe we can give it another look.

Another option would be to handle the SingleAplication functionality completely through QLocalSocket. If there is a primary instance to connect to - use it. Otherwise create one. I think we can use the fact that you can't bind two programs to the same address for single instance protection.

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One issue when using local sockets that I know of is if an app is busy and doesn't reply to the message in time. It can happen if the main event loop is blocked doing a task. To account for this we'll have to always start a separate thread for the SingleApplication QLocalServer.

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I can confirm, it looks like SingleApplication doesn't work anymore with Qt 6.6...

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@emericg Try #177

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Works great on Linux, thanks, I'll try on other platforms tomorrow!

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Fixed in #177 and released in v3.5.0.

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Thanks @jonaski

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Thiago just posted on the Qt dev mailing-list, see https://lists.qt-project.org/pipermail/development/2023-November/044680.html and https://codereview.qt-project.org/c/qt/qtbase/+/518039
Shortly summarized:
QSharedMemory is flawed by design and unless someone else steps up to fix QSharedMemory, it will be deprecated in Qt 6 and removed in Qt 7.
I would like to dig into this myself, but I'm already working 4-7 hours every day on open source work in addition to a full time job (which is non-Qt), and I'm already way behind on other stuff so I can't find the time now.

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