Add more golangci linters and fix warnings.

.golangci.yml

@@ -8,15 +8,18 @@ linters:
   enable:
     - deadcode
     - errcheck
+    - errname
+    - errorlint
     - gocritic
     - gofmt
-    - golint
     - gosec
     - gosimple
     - govet
     - ineffassign
     - misspell
+    - revive
     - staticcheck
     - structcheck
     - unconvert
+    - unused
     - varcheck

cmd/gmailctl/cmd/edit_cmd.go

@@ -177,7 +177,8 @@ func spawnEditor(path string) error {
 		if err == nil {
 			return nil
 		}
-		if _, ok := err.(*exec.ExitError); ok {
+		var exitErr *exec.ExitError
+		if errors.As(err, exitErr) {
 			return errAbort
 		}
 	}

internal/engine/apply/apply.go

@@ -51,7 +51,7 @@ type FetchAPI interface {
 func FromAPI(api FetchAPI) (GmailConfig, error) {
 	l, err := api.ListLabels()
 	if err != nil {
-		return GmailConfig{}, fmt.Errorf("listing labels from Gmail: %v", err)
+		return GmailConfig{}, fmt.Errorf("listing labels from Gmail: %w", err)
 	}
 	f, err := api.ListFilters()
 	if err != nil {

internal/engine/export/xml/marshal.go

@@ -19,7 +19,7 @@ func NewWithTime(now func() time.Time) Exporter {
 	return Exporter{now}
 }
 
-var defaultNow func() time.Time = time.Now
+var defaultNow = time.Now
 
 type xmlDoc struct {
 	XMLName     xml.Name  `xml:"feed"`

internal/engine/rimport/jsonnet.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"bytes"
 	"encoding/json"
+	"errors"
 	"io"
 	"regexp"
 )
@@ -54,7 +55,7 @@ func MarshalJsonnet(v interface{}, w io.Writer, header string) error {
 		line, _, err = reader.ReadLine()
 	}
 
-	if err == io.EOF {
+	if errors.Is(err, io.EOF) {
 		return writer.Flush()
 	}
 	return err

internal/errors/err_test.go

@@ -8,24 +8,24 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
-type errType struct {
+type someError struct {
 	a int
 }
 
-func (errType) Error() string { return "errType" }
+func (someError) Error() string { return "someError" }
 
 func TestAnnotated(t *testing.T) {
 	err1 := errors.New("error1")
-	err2 := errType{3}
+	err2 := someError{3}
 	wrapped := WithCause(err2, err1)
 
 	// The error is both err1 and err2
-	assert.Equal(t, "error1: errType", wrapped.Error())
+	assert.Equal(t, "error1: someError", wrapped.Error())
 	assert.True(t, errors.Is(wrapped, err1))
 	assert.True(t, errors.Is(wrapped, err2))
 
 	// Contents are preserved.
-	var et errType
+	var et someError
 	assert.True(t, errors.As(wrapped, &et))
 	assert.Equal(t, err2, et)