Allow to customize user functions in rule error-strings (#703)

* Allow to customize user functions in rule `error-strings`

* Rollback the Available Rules table format in README

* adds memoization of the rule's configuration

Co-authored-by: chavacava <salvadorcavadini+github@gmail.com>

rule/error-strings.go

@@ -4,6 +4,8 @@ import (
 	"go/ast"
 	"go/token"
 	"strconv"
+	"strings"
+	"sync"
 	"unicode"
 	"unicode/utf8"
 
@@ -11,13 +13,20 @@ import (
 )
 
 // ErrorStringsRule lints given else constructs.
-type ErrorStringsRule struct{}
+type ErrorStringsRule struct {
+	errorFunctions map[string]map[string]struct{}
+	sync.Mutex
+}
 
-// Apply applies the rule to given file.
-func (*ErrorStringsRule) Apply(file *lint.File, _ lint.Arguments) []lint.Failure {
-	var failures []lint.Failure
+func (r *ErrorStringsRule) configure(arguments lint.Arguments) {
+	r.Lock()
+	defer r.Unlock()
 
-	errorFunctions := map[string]map[string]struct{}{
+	if r.errorFunctions != nil {
+		return
+	}
+
+	r.errorFunctions = map[string]map[string]struct{}{
 		"fmt": {
 			"Errorf": {},
 		},
@@ -31,11 +40,33 @@ func (*ErrorStringsRule) Apply(file *lint.File, _ lint.Arguments) []lint.Failure
 		},
 	}
 
+	var invalidCustomFunctions []string
+	for _, argument := range arguments {
+		if functionName, ok := argument.(string); ok {
+			fields := strings.Split(strings.TrimSpace(functionName), ".")
+			if len(fields) != 2 || len(fields[0]) == 0 || len(fields[1]) == 0 {
+				invalidCustomFunctions = append(invalidCustomFunctions, functionName)
+				continue
+			}
+			r.errorFunctions[fields[0]] = map[string]struct{}{fields[1]: {}}
+		}
+	}
+	if len(invalidCustomFunctions) != 0 {
+		panic("found invalid custom function: " + strings.Join(invalidCustomFunctions, ","))
+	}
+}
+
+// Apply applies the rule to given file.
+func (r *ErrorStringsRule) Apply(file *lint.File, arguments lint.Arguments) []lint.Failure {
+	var failures []lint.Failure
+
+	r.configure(arguments)
+
 	fileAst := file.AST
 	walker := lintErrorStrings{
 		file:           file,
 		fileAst:        fileAst,
-		errorFunctions: errorFunctions,
+		errorFunctions: r.errorFunctions,
 		onFailure: func(failure lint.Failure) {
 			failures = append(failures, failure)
 		},