[Memory Engine] MemTablet creation and compatibility handling in BE

[decster]

After adding meta, BE now can create MemTablet, and put it into TabletManager, but MemTablet is not compatible with Tablet, a lot of code may break. This CL contains the following changes:

When TabletManager::get_tablet is called, only return Tablet, if the underlying tablet is MemTablet, return an error, this keeps the initial code changes small.

For methods/functionalities Tablet&MemTablet both have, refactor/extract them to BaseTablet, and change the usage to TabletManager::get_base_tablet, and it will gradually remove errors introduced by step1, this is a long going process.

Resolves #3669

[morningman]

Hi @decster , in your comment, you said When TabletManager::get_tablet is called, only return Tablet, if the underlying tablet is MemTablet, return an error, this keeps the initial code changes small.

But i didn't see the code change related to it?

[decster]

Hi @decster , in your comment, you said When TabletManager::get_tablet is called, only return Tablet, if the underlying tablet is MemTablet, return an error, this keeps the initial code changes small.

But i didn't see the code change related to it?

the related change is in tablet_manager TabletManager::_get_tablet_unlocked

    if (ret->is_memory()) {
        LOG(FATAL) << "_get_tablet_unlocked get MemTablet";
        return TabletSharedPtr();
    }

[morningman]

Hi @decster , in your comment, you said When TabletManager::get_tablet is called, only return Tablet, if the underlying tablet is MemTablet, return an error, this keeps the initial code changes small.
But i didn't see the code change related to it?

the related change is in tablet_manager TabletManager::_get_tablet_unlocked

    if (ret->is_memory()) {
        LOG(FATAL) << "_get_tablet_unlocked get MemTablet";
        return TabletSharedPtr();
    }

OK, I see~

[morningman]

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