errors: Make Cause() unwrap *url.Error

internal/errors/errors.go

@@ -1,11 +1,10 @@
 package errors
 
-import "github.com/pkg/errors"
+import (
+	"net/url"
 
-// Cause returns the cause of an error.
+	"github.com/pkg/errors"
-func Cause(err error) error {
+)
-	return errors.Cause(err)
-}
 
 // New creates a new error based on message. Wrapped so that this package does
 // not appear in the stack trace.
@@ -22,3 +21,29 @@ var Wrap = errors.Wrap
 // Wrapf returns an error annotating err with the format specifier. If err is
 // nil, Wrapf returns nil.
 var Wrapf = errors.Wrapf
+
+// Cause returns the cause of an error. It will also unwrap certain errors,
+// e.g. *url.Error returned by the net/http client.
+func Cause(err error) error {
+	type Causer interface {
+		Cause() error
+	}
+
+	for {
+		// unwrap *url.Error
+		if urlErr, ok := err.(*url.Error); ok {
+			err = urlErr.Err
+			continue
+		}
+
+		// if err is a Causer, return the cause for this error.
+		if c, ok := err.(Causer); ok {
+			err = c.Cause()
+			continue
+		}
+
+		break
+	}
+
+	return err
+}