Improve situation around ErrSumNotSupported.

The number that's missing is now reported in the error.
This is done using the golang error wrap feature.

We're keeping a constant value for ErrSumNotSupported for
compatibility and change avoidance reasons.

Code that cares about this exact error will still have to change;
it is now necessary to use `errors.Is` to detect it.

The text in the ErrSumNotSupported value is also updated,
because the text no longer made sense given an open registry system.
As a target-of-opportunity fix, it also now follows golang normative
conventions for error messages (no capitalization, no punctuation).

registry.go

@@ -5,6 +5,7 @@ import (
 	"crypto/sha1"
 	"crypto/sha256"
 	"crypto/sha512"
+	"fmt"
 	"hash"
 )
 
@@ -50,11 +51,11 @@ func Register(indicator uint64, hasherFactory func() hash.Hash) {
 // Other hash implementations can be made available by using the Register function.
 // The 'go-mulithash/register/*' packages can also be imported to gain more common hash functions.
 //
-// If an error is returned, it will be ErrSumNotSupported.
+// If an error is returned, it will match `errors.Is(err, ErrSumNotSupported)`.
 func GetHasher(indicator uint64) (hash.Hash, error) {
 	factory, exists := registry[indicator]
 	if !exists {
-		return nil, ErrSumNotSupported // REVIEW: it's unfortunate that this error doesn't say what code was missing.  Also "NotSupported" is a bit of a misnomer now.
+		return nil, fmt.Errorf("unknown multihash code %d (0x%x): %w", indicator, indicator, ErrSumNotSupported)
 	}
 	return factory(), nil
 }

sum.go

@@ -5,7 +5,7 @@ import (
 )
 
 // ErrSumNotSupported is returned when the Sum function code is not implemented
-var ErrSumNotSupported = errors.New("Function not implemented. Complain to lib maintainer.")
+var ErrSumNotSupported = errors.New("no such hash registered")
 
 var ErrLenTooLarge = errors.New("requested length was too large for digest")
 

sum_test.go

@@ -3,6 +3,7 @@ package multihash_test
 import (
 	"bytes"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"runtime"
 	"testing"
@@ -132,8 +133,9 @@ func TestTooLargeLength(t *testing.T) {
 func TestBasicSum(t *testing.T) {
 	for code, name := range multihash.Codes {
 		_, err := multihash.Sum([]byte("test"), code, -1)
-		switch err {
-		case multihash.ErrSumNotSupported, nil:
+		switch {
+		case errors.Is(err, multihash.ErrSumNotSupported):
+		case err == nil:
 		default:
 			t.Errorf("unexpected error for %s: %s", name, err)
 		}