Ignore global error variables

Error variables are variables starting with 'err' or 'Err'.

This implements #2.

check_no_globals.go

@@ -10,6 +10,22 @@ import (
 	"strings"
 )
 
+func isWhitelisted(i *ast.Ident) bool {
+	return i.Name == "_" || looksLikeError(i)
+}
+
+// looksLikeError checks if the AST identifier starts with 'err' or 'Err'.
+//
+// See the following issue on how to further improve this func:
+// https://github.com/leighmcculloch/gochecknoglobals/issues/5
+func looksLikeError(i *ast.Ident) bool {
+	prefix := "err"
+	if i.IsExported() {
+		prefix = "Err"
+	}
+	return strings.HasPrefix(i.Name, prefix)
+}
+
 func checkNoGlobals(rootPath string, includeTests bool) ([]string, error) {
 	const recursiveSuffix = string(filepath.Separator) + "..."
 	recursive := false
@@ -55,11 +71,11 @@ func checkNoGlobals(rootPath string, includeTests bool) ([]string, error) {
 			line := fset.Position(genDecl.TokPos).Line
 			valueSpec := genDecl.Specs[0].(*ast.ValueSpec)
 			for i := 0; i < len(valueSpec.Names); i++ {
-				name := valueSpec.Names[i].Name
-				if name == "_" {
+				vn := valueSpec.Names[i]
+				if isWhitelisted(vn) {
 					continue
 				}
-				message := fmt.Sprintf("%s:%d %s is a global variable", filename, line, name)
+				message := fmt.Sprintf("%s:%d %s is a global variable", filename, line, vn.Name)
 				messages = append(messages, message)
 			}
 		}

check_no_globals_test.go

@@ -79,6 +79,19 @@ func TestCheckNoGlobals(t *testing.T) {
 			path:         "testdata/6",
 			wantMessages: nil,
 		},
+		{
+			path: "testdata/7",
+			wantMessages: []string{
+				"testdata/7/code.go:8 myVar is a global variable",
+				// "testdata/7/code.go:13 errFakeErrorUnexported is a global variable",
+				// "testdata/7/code.go:14 ErrFakeErrorExported is a global variable",
+				"testdata/7/code.go:17 myErrVar is a global variable",
+				"testdata/7/code.go:18 myVarErr is a global variable",
+				"testdata/7/code.go:19 myVarError is a global variable",
+				"testdata/7/code.go:20 customErr is a global variable",
+				"testdata/7/code.go:29 declaredErr is a global variable",
+			},
+		},
 		{
 			path:         ".",
 			wantMessages: nil,
@@ -93,6 +106,14 @@ func TestCheckNoGlobals(t *testing.T) {
 				"testdata/4/subpkg/code_1.go:3 myVar is a global variable",
 				"testdata/5/code.go:3 myVar1 is a global variable",
 				"testdata/5/code.go:3 myVar2 is a global variable",
+				"testdata/7/code.go:8 myVar is a global variable",
+				// "testdata/7/code.go:13 errFakeErrorUnexported is a global variable",
+				// "testdata/7/code.go:14 ErrFakeErrorExported is a global variable",
+				"testdata/7/code.go:17 myErrVar is a global variable",
+				"testdata/7/code.go:18 myVarErr is a global variable",
+				"testdata/7/code.go:19 myVarError is a global variable",
+				"testdata/7/code.go:20 customErr is a global variable",
+				"testdata/7/code.go:29 declaredErr is a global variable",
 			},
 		},
 	}

testdata/7/code.go

@@ -0,0 +1,34 @@
+package code
+
+import (
+	"errors"
+)
+
+// Those are not errors
+var myVar = 1
+
+// Those are fake errors which are currently not detected
+// because they start with 'err' or 'Err' and we don't
+// check if such a variable implements the error interface.
+var errFakeErrorUnexported = 1
+var ErrFakeErrorExported = 1
+
+// Those errors are not named correctly
+var myErrVar = errors.New("myErrVar")
+var myVarErr = errors.New("myVarErr")
+var myVarError = errors.New("myVarErr")
+var customErr = customError{"customErr"}
+
+// Those are actual errors which should be ignored
+var errUnexported = errors.New("errUnexported")
+var ErrExported = errors.New("ErrExported")
+var errCustomUnexported = customError{"errCustomUnexported"}
+var ErrCustomExported = customError{"ErrCustomExported"}
+
+// Those actual errors have a declared error type
+var declaredErr error = errors.New("declaredErr")
+var errDeclared error = errors.New("errDeclared")
+
+type customError struct{ e string }
+
+func (e *customError) Error() string { return e.e }