make Error compatible with functions in standard package

[disksing]

Signed-off-by: disksing i@disksing.com

The current Equals is not friendly to multi-level wrap errors: it always unwraps the bottom-level error and then compares them.

For example, if the bottom level returns os.NotExists, we cannot wrap it with kv.ErrNotExists.Wrap(err), because if we did that, we would not able to determine it later with kv.ErrNotExists.Equals(err) (the reason is Equals will recursively unwraps out os.NotExists then compare)

Now we deal this problem by throwing away the underlying error and just return kv.ErrNotExists, which is undoubtedly inappropriate because some information is lost and there is no way to tell if the error is os.ErrorNotExists anymore.

Go1.13 introduced the error wrapping mechanism in the standard package (https://golang.org/doc/go1.13#error_wrapping) and provides errors.Is / errors.As functions for error determination.

This PR adds two methods to make Error compatible with standard error wrapping.

• Unwrap() returns the next level of the error chain without recursion, so that Is/As can be checked for each level.
• Is() is used to ensure that all instances of Error with the same ID are judged to be equal.

There are 2 cases need to be noted:

e1 := fooError{}
e2 := Normalize("e2", RFCCodeText("e2"))
e21 := e2.Wrap(e1)

Since the ID of both e2 and e21 are "e2", so errors.Is(e21, e2) and errors.Is(e2, e21) are both true.

e1 := fooError{}
e2 := Normalize("e2", RFCCodeText("e2"))
e3 := Normalize("e3", RFCCodeText("e3"))
e21 := e2.Wrap(e1)

e3x := e3.Wrap(e1)
ok := errors.As(e21, &e3x)

In this case, although e3x and e21 have different IDs, the are of the same type (both are *Error), so As returns true and e3x.ID() == "e2" afterward. Due to limitation of errors.As, this issue cannot be avoided by implementing an As function for Error.

[lysu]

LGTM

[YuJuncen]

LGTM

[disksing]

Sorry, I found out that this PR may be somehow incomplete now. Please DNM until I get it clear.

[disksing]

Added Unwrap for pkg/errors types. Borrowed from upstream.

https://github.com/pkg/errors/blob/5dd12d0cfe7f152f80558d591504ce685299311e/errors.go#L162-L163

https://github.com/pkg/errors/blob/5dd12d0cfe7f152f80558d591504ce685299311e/errors.go#L247-L248

Please take a look again. @lysu @YuJuncen

[lysu]

LGTM

[disksing]

@lysu @YuJuncen could we merge it now? it should have no side effects.