always truncate when SyncWrites is enabled

[Stebalien]

This will delete values that were in the process of being "put" when we crashed
but that's likely expected behavior anyways. Without this, ipfs on badger may
refuse to start after a crash.

part of ipfs/kubo#5275

[schomatis]

We're assuming here (and it seems reasonable) that SyncWrites wasn't modified after the crash.

[magik6k]

I've been running few nodes with similar patch for quite some time now, it even survived few crashes

[Stebalien]

We're assuming here (and it seems reasonable) that SyncWrites wasn't modified after the crash.

An alternative is to have a truncate config option and set it to true by default (like syncWrites). Actually, maybe we should do that instead. Gateways/caches will likely want to keep truncate enabled while disabling sync writes. Thoughts @schomatis @magik6k?

[schomatis]

An alternative is to have a truncate config option

Yes, I agree, that was the intention behind ipfs/kubo#5275.

set it to true by default

I would prefer to set it to false by default, just to be conservative here, and document the possibility that a truncate might be needed after a crash and how the user can set it to true, but I don't really feel that strong about it, true is also ok.

[Stebalien]

I would prefer to set it to false by default, just to be conservative here, and document the possibility that a truncate might be needed after a crash and how the user can set it to true, but I don't really feel that strong about it, true is also ok.

Given the semantics of our datastore, I'd prefer to set it to true (and document it). I'm pretty sure all (or almost all) filesystems/databases behave this way. Basically:

1. If there is a bug in badger, we can lose data anyways.
2. If there isn't a bug in badger, we haven't told the user that we've persisted the data so we're free to do anything we want.

Are there any cases I'm missing?

[schomatis]

If there isn't a bug in badger, we haven't told the user that we've persisted the data so we're free to do anything we want.

If syncWrites is true by default that seems fair.