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implement Exception.Unwrap method
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In order to make exceptions testable with errors.Is, they must implement
the `Unwrap` method to allow access to the underlying error code(s).

This change is addressing the problem of the below code not working as
expected, when testing returned by `Do` method error:

    c, _ := ch.Dial(ctx, ch.Options{...})
    switch err := c.Do(ctx, ch.Query{...}); {
    case err == nil:
    	// ...
    case errors.Is(err, proto.ErrTableIsReadOnly), errors.Is(err, proto.ErrReadonly):
    	// ... <- Never executed, because exception unwraping is not implemented.
    default:
        // ...
    }

Because `Exception` instances are created dynamically, unwrapping can
ignore nested exception instances and collect only the underlying error
codes.
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husio committed Oct 10, 2023
1 parent cce1c00 commit 45cb154
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Showing 2 changed files with 49 additions and 0 deletions.
21 changes: 21 additions & 0 deletions client.go
Original file line number Diff line number Diff line change
Expand Up @@ -131,6 +131,27 @@ func (e *Exception) Error() string {
return fmt.Sprintf("%s: %s: %s", e.Code, e.Name, msg)
}

// Unwrap implements errors.Unwrap interface.
func (e *Exception) Unwrap() []error {
if e == nil {
return nil
}
codes := make([]error, 0, 12)
// Flatten the error tree by collecting all error codes. Only error codes
// matter because they are declared and therefore can be compared using
// errors.Is. Dynamically created Exception instances are not relevant for
// this functionality.
return collectCodes(e, codes)
}

func collectCodes(e *Exception, codes []error) []error {
codes = append(codes, e.Code)
for _, next := range e.Next {
codes = collectCodes(&next, codes)

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}
return codes
}

// AsException finds first *Exception in err chain.
func AsException(err error) (*Exception, bool) {
var e *Exception
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28 changes: 28 additions & 0 deletions client_test.go
Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@ import (
"os"
"testing"

"github.com/go-faster/errors"
"github.com/stretchr/testify/require"
"go.uber.org/zap/zaptest"

Expand Down Expand Up @@ -87,3 +88,30 @@ func TestDial(t *testing.T) {
require.True(t, IsErr(err, proto.ErrUnknownDatabase))
})
}

func TestExceptionUnwrap(t *testing.T) {
flat := &Exception{
Code: proto.ErrReadonly,
Name: "foo",
Message: "bar",
Next: nil,
}

if !errors.Is(flat, proto.ErrReadonly) {
t.Fatal("flat exception must be the error code it represents")
}

nested := &Exception{
Code: proto.ErrAborted,
Name: "foo",
Message: "bar",
Next: []Exception{*flat},
}
if !errors.Is(nested, proto.ErrAborted) {
t.Fatal("nested exception must be the error code it represents")
}
if !errors.Is(nested, proto.ErrReadonly) {
t.Fatal("nested exception must be the error code it wraps")
}

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}

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