chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/models/markdown.go

@@ -1,7 +1,6 @@
 package models
 
 import (
-	"errors"
 	"fmt"
 	"strings"
 
@@ -109,8 +108,6 @@ func (s *Servers) toMarkdown(vpnProvider string) (formatted string, err error) {
 	return formatted, nil
 }
 
-var ErrMarkdownHeadersNotDefined = errors.New("markdown headers not defined")
-
 func getMarkdownHeaders(vpnProvider string) (headers []string, err error) {
 	switch vpnProvider {
 	case providers.Airvpn:
@@ -169,6 +166,6 @@ func getMarkdownHeaders(vpnProvider string) (headers []string, err error) {
 	case providers.Windscribe:
 		return []string{regionHeader, cityHeader, hostnameHeader, vpnHeader}, nil
 	default:
-		return nil, fmt.Errorf("%w: for %s", ErrMarkdownHeadersNotDefined, vpnProvider)
+		return nil, fmt.Errorf("markdown headers not defined: for %s", vpnProvider)
 	}
 }

internal/models/markdown_test.go

@@ -15,12 +15,10 @@ func Test_Servers_ToMarkdown(t *testing.T) {
 		provider   string
 		servers    Servers
 		formatted  string
-		errWrapped error
 		errMessage string
 	}{
 		"unsupported_provider": {
 			provider:   "unsupported",
-			errWrapped: ErrMarkdownHeadersNotDefined,
 			errMessage: "getting markdown headers: markdown headers not defined: for unsupported",
 		},
 		providers.Cyberghost: {
@@ -58,9 +56,10 @@ func Test_Servers_ToMarkdown(t *testing.T) {
 			markdown, err := testCase.servers.toMarkdown(testCase.provider)
 
 			assert.Equal(t, testCase.formatted, markdown)
-			assert.ErrorIs(t, err, testCase.errWrapped)
-			if testCase.errWrapped != nil {
+			if testCase.errMessage != "" {
 				assert.EqualError(t, err, testCase.errMessage)
+			} else {
+				assert.NoError(t, err)
 			}
 		})
 	}

internal/models/server.go

@@ -38,27 +38,18 @@ type Server struct {
 	IPs         []netip.Addr `json:"ips,omitempty"`
 }
 
-var (
-	ErrVPNFieldEmpty           = errors.New("vpn field is empty")
-	ErrHostnameFieldEmpty      = errors.New("hostname field is empty")
-	ErrIPsFieldEmpty           = errors.New("ips field is empty")
-	ErrNoNetworkProtocol       = errors.New("both TCP and UDP fields are false for OpenVPN")
-	ErrNetworkProtocolSet      = errors.New("no network protocol should be set")
-	ErrWireguardPublicKeyEmpty = errors.New("wireguard public key field is empty")
-)
-
 func (s *Server) HasMinimumInformation() (err error) {
 	switch {
 	case s.VPN == "":
-		return fmt.Errorf("%w", ErrVPNFieldEmpty)
+		return errors.New("vpn field is empty")
 	case len(s.IPs) == 0:
-		return fmt.Errorf("%w", ErrIPsFieldEmpty)
+		return errors.New("ips field is empty")
 	case s.VPN == vpn.Wireguard && (s.TCP || s.UDP):
-		return fmt.Errorf("%w", ErrNetworkProtocolSet)
+		return errors.New("no network protocol should be set")
 	case s.VPN == vpn.OpenVPN && !s.TCP && !s.UDP:
-		return fmt.Errorf("%w", ErrNoNetworkProtocol)
+		return errors.New("both TCP and UDP fields are false for OpenVPN")
 	case s.VPN == vpn.Wireguard && s.WgPubKey == "":
-		return fmt.Errorf("%w", ErrWireguardPublicKeyEmpty)
+		return errors.New("wireguard public key field is empty")
 	default:
 		return nil
 	}

internal/models/servers.go

@@ -3,7 +3,6 @@ package models
 import (
 	"bytes"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"math"
 	"reflect"
@@ -158,8 +157,6 @@ type Servers struct {
 	Servers   []Server `json:"servers,omitempty"`
 }
 
-var ErrServersFormatNotSupported = errors.New("servers format not supported")
-
 func (s *Servers) Format(vpnProvider, format string) (formatted string, err error) {
 	switch format {
 	case "markdown":
@@ -167,7 +164,7 @@ func (s *Servers) Format(vpnProvider, format string) (formatted string, err erro
 	case "json":
 		return s.toJSON()
 	default:
-		return "", fmt.Errorf("%w: %s", ErrServersFormatNotSupported, format)
+		return "", fmt.Errorf("servers format not supported: %s", format)
 	}
 }
 

internal/models/servers_test.go

@@ -16,7 +16,6 @@ func Test_AllServers_MarshalJSON(t *testing.T) {
 	testCases := map[string]struct {
 		allServers *AllServers
 		dataString string
-		errWrapped error
 		errMessage string
 	}{
 		"no provider": {
@@ -58,16 +57,18 @@ func Test_AllServers_MarshalJSON(t *testing.T) {
 			t.Parallel()
 
 			data, err := testCase.allServers.MarshalJSON()
-			assert.ErrorIs(t, err, testCase.errWrapped)
-			if err != nil {
+			if testCase.errMessage != "" {
 				assert.EqualError(t, err, testCase.errMessage)
+			} else {
+				assert.NoError(t, err)
 			}
 			require.Equal(t, testCase.dataString, string(data))
 
 			data, err = json.Marshal(testCase.allServers)
-			assert.ErrorIs(t, err, testCase.errWrapped)
-			if err != nil {
+			if testCase.errMessage != "" {
 				assert.EqualError(t, err, testCase.errMessage)
+			} else {
+				assert.NoError(t, err)
 			}
 			require.Equal(t, testCase.dataString, string(data))
 
@@ -87,7 +88,6 @@ func Test_AllServers_UnmarshalJSON(t *testing.T) {
 	testCases := map[string]struct {
 		dataString string
 		allServers AllServers
-		errWrapped error
 		errMessage string
 	}{
 		"empty": {
@@ -131,9 +131,10 @@ func Test_AllServers_UnmarshalJSON(t *testing.T) {
 
 			err := json.Unmarshal(data, &allServers)
 
-			assert.ErrorIs(t, err, testCase.errWrapped)
-			if err != nil {
+			if testCase.errMessage != "" {
 				assert.EqualError(t, err, testCase.errMessage)
+			} else {
+				assert.NoError(t, err)
 			}
 			assert.Equal(t, testCase.allServers, allServers)
 		})