Using bufio reader for Stdin, otherwise the first 3 bytes are consumed and file gets "corrupted" (stdin is not seekable?)

analyzer/analyzer.go

@@ -1,6 +1,7 @@
 package analyzer
 
 import (
+	"bufio"
 	"compress/gzip"
 	"context"
 	"io"
@@ -136,12 +137,14 @@ func (ac Config) Analyze(ctx context.Context, imageName string, opts ...types.Do
 
 func (ac Config) AnalyzeFile(ctx context.Context, f *os.File) (fileMap extractor.FileMap, err error) {
 	var r io.Reader
-	r = f
-	if utils.IsGzip(f) {
-		r, err = gzip.NewReader(f)
+	br := bufio.NewReader(f)
+	if utils.IsGzip(br) {
+		r, err = gzip.NewReader(br)
 		if err != nil {
 			return nil, xerrors.Errorf("failed to open gzip: %w", err)
 		}
+	} else {
+		r = br
 	}
 	fileMap, err = ac.Extractor.ExtractFromFile(ctx, r, RequiredFilenames())
 	if err != nil {

utils/utils.go

@@ -1,8 +1,8 @@
 package utils
 
 import (
+	"bufio"
 	"fmt"
-	"io"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -37,11 +37,10 @@ func IsCommandAvailable(name string) bool {
 	return true
 }
 
-func IsGzip(f *os.File) bool {
+func IsGzip(f *bufio.Reader) bool {
 	buf := make([]byte, 3)
-	n, _ := f.Read(buf)
-	defer f.Seek(0, io.SeekStart)
-	if n < 3 {
+	buf, err := f.Peek(3)
+	if err != nil {
 		return false
 	}
 	return buf[0] == 0x1F && buf[1] == 0x8B && buf[2] == 0x8

utils/utils_test.go

@@ -1,6 +1,7 @@
 package utils
 
 import (
+	"bufio"
 	"os"
 	"testing"
 )
@@ -23,7 +24,7 @@ func TestIsGzip(t *testing.T) {
 				t.Fatalf("unknown error: %s", err)
 			}
 
-			got := IsGzip(f)
+			got := IsGzip(bufio.NewReader(f))
 			if got != tt.want {
 				t.Errorf("got %t, want %t", got, tt.want)
 			}