Fetch should return a unique error type when ref not found

It can be useful for callers to distinguish between an error of
"couldn't find remote ref" and some other error like "network error".

Creating an explicit error type for this allows consumers to
determine the kind of error using the errors.Is and errors.As
interface added in go1.13

2 files changed, 16 insertions, 1 deletions

diff --git a/remote.go b/remote.go
index 89b3051..d88e8e6 100644
--- a/remote.go
+++ b/remote.go
@@ -32,6 +32,19 @@ var (
 	ErrExactSHA1NotSupported = errors.New("server does not support exact SHA1 refspec")
 )
 
+type NoMatchingRefSpecError struct {
+	refSpec config.RefSpec
+}
+
+func (e NoMatchingRefSpecError) Error() string {
+	return fmt.Sprintf("couldn't find remote ref %q", e.refSpec.Src())
+}
+
+func (e NoMatchingRefSpecError) Is(target error) bool {
+	_, ok := target.(NoMatchingRefSpecError)
+	return ok
+}
+
 const (
 	// This describes the maximum number of commits to walk when
 	// computing the haves to send to a server, for each ref in the
@@ -751,7 +764,7 @@ func doCalculateRefs(
 	})
 
 	if !matched && !s.IsWildcard() {
-		return fmt.Errorf("couldn't find remote ref %q", s.Src())
+		return NoMatchingRefSpecError{refSpec: s}
 	}
 
 	return err
diff --git a/remote_test.go b/remote_test.go
index c6ea9ea..3446f1a 100644
--- a/remote_test.go
+++ b/remote_test.go
@@ -3,6 +3,7 @@ package git
 import (
 	"bytes"
 	"context"
+	"errors"
 	"io"
 	"io/ioutil"
 	"os"
@@ -145,6 +146,7 @@ func (s *RemoteSuite) TestFetchNonExistantReference(c *C) {
 	})
 
 	c.Assert(err, ErrorMatches, "couldn't find remote ref.*")
+	c.Assert(errors.Is(err, NoMatchingRefSpecError{}), Equals, true)
 }
 
 func (s *RemoteSuite) TestFetchContext(c *C) {