errors: add an error for nonexistent objects

Depending on the storage backend used, we can return any of several
errors if an object doesn't exist.  For consistency, let's add an error
specifically for this condition and place it into a new errors package
so that it can be referred to from both the main package and the pack
package without import cycles.  Add a function in addition to quickly
determine if the error is due to a missing object.

Author: bk2204

errors/errors.go

@@ -0,0 +1,28 @@
+package errors
+
+import (
+	"fmt"
+)
+
+// noSuchObject is an error type that occurs when no object with a given object
+// ID is available.
+type noSuchObject struct {
+	oid []byte
+}
+
+// Error implements the error.Error() function.
+func (e *noSuchObject) Error() string {
+	return fmt.Sprintf("gitobj: no such object: %x", e.oid)
+}
+
+// NoSuchObject creates a new error representing a missing object with a given
+// object ID.
+func NoSuchObject(oid []byte) error {
+	return &noSuchObject{oid: oid}
+}
+
+// IsNoSuchObject indicates whether an error is a noSuchObject and is non-nil.
+func IsNoSuchObject(e error) bool {
+	err, ok := e.(*noSuchObject)
+	return ok && err != nil
+}

errors/errors_test.go

@@ -0,0 +1,24 @@
+package errors
+
+import (
+	"encoding/hex"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNoSuchObjectTypeErrFormatting(t *testing.T) {
+	sha := "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+	oid, err := hex.DecodeString(sha)
+	assert.NoError(t, err)
+
+	err = NoSuchObject(oid)
+
+	assert.Equal(t, "gitobj: no such object: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", err.Error())
+	assert.Equal(t, IsNoSuchObject(err), true)
+}
+
+func TestIsNoSuchObjectNilHandling(t *testing.T) {
+	assert.Equal(t, IsNoSuchObject((*noSuchObject)(nil)), false)
+	assert.Equal(t, IsNoSuchObject(nil), false)
+}