fileutil: improve control flow readability of purgeFile function

Signed-off-by: redwrasse <mail@redwrasse.io>

client/pkg/fileutil/purge.go

@@ -17,7 +17,6 @@ package fileutil
 import (
 	"os"
 	"path/filepath"
-	"sort"
 	"strings"
 	"time"
 
@@ -53,26 +52,35 @@ func purgeFile(lg *zap.Logger, dirname string, suffix string, max uint, interval
 		zap.Uint("max", max),
 		zap.Duration("interval", interval))
 
+	readDirWithSuffix := func(dirname string) ([]string, error) {
+		fnames, err := ReadDir(dirname)
+		if err != nil {
+			return nil, err
+		}
+		// filter in place (ref. https://go.dev/wiki/SliceTricks#filtering-without-allocating)
+		fnamesWithSuffix := fnames[:0]
+		for _, fname := range fnames {
+			if strings.HasSuffix(fname, suffix) {
+				fnamesWithSuffix = append(fnamesWithSuffix, fname)
+			}
+		}
+
+		return fnamesWithSuffix, nil
+	}
+
 	go func() {
 		if donec != nil {
 			defer close(donec)
 		}
 		for {
-			fnames, err := ReadDir(dirname)
+			fnamesWithSuffix, err := readDirWithSuffix(dirname)
 			if err != nil {
 				errC <- err
 				return
 			}
-			newfnames := make([]string, 0)
-			for _, fname := range fnames {
-				if strings.HasSuffix(fname, suffix) {
-					newfnames = append(newfnames, fname)
-				}
-			}
-			sort.Strings(newfnames)
-			fnames = newfnames
-			for len(newfnames) > int(max) {
-				f := filepath.Join(dirname, newfnames[0])
+			nPurged := 0
+			for nPurged < len(fnamesWithSuffix)-int(max) {
+				f := filepath.Join(dirname, fnamesWithSuffix[nPurged])
 				var l *LockedFile
 				if flock {
 					l, err = TryLockFile(f, os.O_WRONLY, PrivateFileMode)
@@ -94,11 +102,12 @@ func purgeFile(lg *zap.Logger, dirname string, suffix string, max uint, interval
 					}
 				}
 				lg.Info("purged", zap.String("path", f))
-				newfnames = newfnames[1:]
+				nPurged++
 			}
+
 			if purgec != nil {
-				for i := 0; i < len(fnames)-len(newfnames); i++ {
-					purgec <- fnames[i]
+				for i := 0; i < nPurged; i++ {
+					purgec <- fnamesWithSuffix[i]
 				}
 			}
 			select {