fix: sentinel errors should not have stack traces

[mitar]

Problem

While debugging a program which is using badger I noticed that stack traces returned from my code (which uses errors with stack traces) are useless. I found the reason: badger uses sentinel errors (great!) but it creates them with stack traces at the "global init time" (e.g., var ErrFoo = errors.New(...)) and not at "return from function time". Because of that I was seeing stack traces like:

github.com/dgraph-io/badger/v4.init
	/go/pkg/mod/github.com/dgraph-io/badger/v4@v4.2.0/compaction.go:41
runtime.doInit1
	/usr/local/go/src/runtime/proc.go:6740
runtime.doInit
	/usr/local/go/src/runtime/proc.go:6707
runtime.main
	/usr/local/go/src/runtime/proc.go:249
runtime.goexit
	/usr/local/go/src/runtime/asm_amd64.s:1650

Instead of a stack trace which would show me where the call which is returning sentinel error is made.

Solution

Ideally, one would create all sentinel errors without stack traces (standard errors package works great for that) and then when the error happens in a function, you return errors.WithStack(ErrFoobar) instead of just ErrFoobar. This means that sentinel error is then annotated with a stack trace at the place where the error happens. But if you do that, then you have to check for sentinel errors with errors.Is instead of just comparing errors with equality (==). That might be a breaking change for people who are not using errors.Is as they should. Because badger's own codebase uses equality instead of errors.Is I decided to not do this in this PR, but instead:

Just make sentinel errors without stack traces. This allows me to annotate with a stack trace inside my program (otherwise my program does not do so because if finds an existing stack trace and assumes it is a deeper one) and I get at least that part of the stack trace. Equality still works as usual.

I suggest this is merged this way and in v5 of badger errors.WithStack and required errors.Is are introduced.

Side note

Some time ago I made this errors package for Go which fixes various issues in github.com/pkg/errors and also has direct support for sentinel (in that package called "base") errors. It also has some features badger has in its y package for dealing with errors. You could consider adopting that package to have a comprehensive errors handling in badger.

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