all: adopt strings, errors, and os helpers

hscontrol/derp/derp.go

@@ -22,19 +22,12 @@ import (
 )
 
 func loadDERPMapFromPath(path string) (*tailcfg.DERPMap, error) {
-	derpFile, err := os.Open(path)
+	b, err := os.ReadFile(path)
 	if err != nil {
 		return nil, err
 	}
-	defer derpFile.Close()
 
 	var derpMap tailcfg.DERPMap
-
-	b, err := io.ReadAll(derpFile)
-	if err != nil {
-		return nil, err
-	}
-
 	err = yaml.Unmarshal(b, &derpMap)
 
 	return &derpMap, err

hscontrol/grpcv1.go

@@ -420,7 +420,7 @@ func (api headscaleV1APIServer) SetApprovedRoutes(
 }
 
 func validateTag(tag string) error {
-	if strings.Index(tag, "tag:") != 0 {
+	if !strings.HasPrefix(tag, "tag:") {
 		return errors.New("tag must start with the string 'tag:'")
 	}
 	if strings.ToLower(tag) != tag {
@@ -935,7 +935,7 @@ func (api headscaleV1APIServer) AuthReject(
 	}
 
 	authReq.FinishAuth(types.AuthVerdict{
-		Err: fmt.Errorf("auth request rejected"),
+		Err: errors.New("auth request rejected"),
 	})
 
 	return &v1.AuthRejectResponse{}, nil

hscontrol/policy/v2/filter.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"net/netip"
 	"slices"
-	"strconv"
 	"strings"
 	"time"
 
@@ -597,14 +596,7 @@ func groupSourcesByUser(
 // filterRuleKey generates a unique key for merging based on [tailcfg.FilterRule.SrcIPs]
 // and [tailcfg.FilterRule.IPProto].
 func filterRuleKey(rule tailcfg.FilterRule) string {
-	srcKey := strings.Join(rule.SrcIPs, ",")
-
-	protoStrs := make([]string, len(rule.IPProto))
-	for i, p := range rule.IPProto {
-		protoStrs[i] = strconv.Itoa(p)
-	}
-
-	return srcKey + "|" + strings.Join(protoStrs, ",")
+	return fmt.Sprintf("%s|%v", strings.Join(rule.SrcIPs, ","), rule.IPProto)
 }
 
 // mergeFilterRules merges rules with identical [tailcfg.FilterRule.SrcIPs] and