Fix return nil (#15330)

* Fix return nil

A nil error is an interface having nil value but a non-nil type.
This leads to unexpected errors in the code well-documented here:
https://golang.org/doc/faq#nil_error

We fix this by passing an additional parameter to handle it.

Caught using https://github.com/dgryski/semgrep-go

* Bring back auto-generated line

config/file.go

@@ -234,7 +234,7 @@ func (fs *FileStore) RemoveFile(name string) error {
 		return errors.Wrap(err, "failed to remove file")
 	}
 
-	return err
+	return nil
 }
 
 // startWatcher starts a watcher to monitor for external config file changes.

store/layer_generators/main.go

@@ -231,11 +231,15 @@ func generateLayer(name, templateFile string) ([]byte, error) {
 			}
 			return fmt.Sprintf("(%s)", strings.Join(results, ", "))
 		},
-		"genResultsVars": func(results []string) string {
+		"genResultsVars": func(results []string, withNilError bool) string {
 			vars := []string{}
 			for i, typeName := range results {
 				if isError(typeName) {
-					vars = append(vars, "err")
+					if withNilError {
+						vars = append(vars, "nil")
+					} else {
+						vars = append(vars, "err")
+					}
 				} else if i == 0 {
 					vars = append(vars, "result")
 				} else {

store/layer_generators/opentracing_layer.go.tmpl

@@ -53,14 +53,14 @@ func (s *{{$.Name}}{{$substoreName}}Store) {{$index}}({{$element.Params | joinPa
 	{{- if $element.Results | len | eq 0}}
 	s.{{$substoreName}}Store.{{$index}}({{$element.Params | joinParams}})
 	{{else}}
-		{{$element.Results | genResultsVars}} := s.{{$substoreName}}Store.{{$index}}({{$element.Params | joinParams}})
+		{{genResultsVars $element.Results false }} := s.{{$substoreName}}Store.{{$index}}({{$element.Params | joinParams}})
 		{{- if $element.Results | errorPresent }}
 			if {{$element.Results | errorVar}} != nil {
 				span.LogFields(spanlog.Error({{$element.Results | errorVar}}))
 				ext.Error.Set(span, true)
 			}
-		{{end}}	
-		return {{ $element.Results | genResultsVars -}}
+		{{end}}
+		return {{ genResultsVars $element.Results false -}}
 	{{end}}
 }
 {{end}}

store/layer_generators/retry_layer.go.tmpl

@@ -8,7 +8,7 @@ package retrylayer
 
 import (
 	"context"
-	
+
 	"github.com/lib/pq"
 	"github.com/mattermost/mattermost-server/v5/model"
     "github.com/mattermost/mattermost-server/v5/store"
@@ -65,17 +65,17 @@ func (s *{{$.Name}}{{$substoreName}}Store) {{$index}}({{$element.Params | joinPa
             {{else}}
                 tries := 0
                 for {
-                    {{$element.Results | genResultsVars}} := s.{{$substoreName}}Store.{{$index}}({{$element.Params | joinParams}})
+                    {{genResultsVars $element.Results false }} := s.{{$substoreName}}Store.{{$index}}({{$element.Params | joinParams}})
                     if {{$element.Results | errorVar}} == nil {
-                        return {{$element.Results | genResultsVars}}
+                        return {{genResultsVars $element.Results true }}
                     }
                     if !isRepeatableError({{$element.Results | errorVar}}) {
-                        return {{$element.Results | genResultsVars}}
+                        return {{genResultsVars $element.Results false }}
                     }
                     tries++
                     if tries >= 3 {
                         {{$element.Results | errorVar}} = errors.Wrap({{$element.Results | errorVar}}, "giving up after 3 consecutive repeatable transaction failures")
-                        return {{$element.Results | genResultsVars}}
+                        return {{genResultsVars $element.Results false }}
                     }
                 }
             {{end}}

store/layer_generators/timer_layer.go.tmpl

@@ -9,7 +9,7 @@ package timerlayer
 import (
 	"context"
 	timemodule "time"
-	
+
     "github.com/mattermost/mattermost-server/v5/einterfaces"
 	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/mattermost/mattermost-server/v5/store"
@@ -42,7 +42,7 @@ func (s *{{$.Name}}{{$substoreName}}Store) {{$index}}({{$element.Params | joinPa
 	{{if $element.Results | len | eq 0}}
 	s.{{$substoreName}}Store.{{$index}}({{$element.Params | joinParams}})
 	{{else}}
-	{{$element.Results | genResultsVars}} := s.{{$substoreName}}Store.{{$index}}({{$element.Params | joinParams}})
+	{{genResultsVars $element.Results false }} := s.{{$substoreName}}Store.{{$index}}({{$element.Params | joinParams}})
 	{{end}}
 	elapsed := float64(timemodule.Since(start)) / float64(timemodule.Second)
 	if s.Root.Metrics != nil {
@@ -51,7 +51,7 @@ func (s *{{$.Name}}{{$substoreName}}Store) {{$index}}({{$element.Params | joinPa
 			success = "true"
 		}
 		s.Root.Metrics.ObserveStoreMethodDuration("{{$substoreName}}Store.{{$index}}", success, elapsed)
-	{{ with ($element.Results | genResultsVars) -}}
+	{{ with (genResultsVars $element.Results false ) -}}
 	}
 	return {{ . }}
 	{{- else -}}