fix: replace the filemap implementation to not buffer in memory

This filemap is used to generate installer image layer with artifacts.

Previous dumb implementation buffered in memory which leads to extensive
memory usage.

See siderolabs/image-factory#77

Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>

pkg/imager/filemap/filemap.go

@@ -7,7 +7,6 @@ package filemap
 
 import (
 	"archive/tar"
-	"bytes"
 	"io"
 	"os"
 	"path/filepath"
@@ -52,57 +51,64 @@ func Walk(sourceBasePath, imageBasePath string) ([]File, error) {
 	return filemap, err
 }
 
+func build(filemap []File) io.ReadCloser {
+	pr, pw := io.Pipe()
+
+	go func() {
+		pw.CloseWithError(func() error {
+			w := tar.NewWriter(pw)
+
+			for _, entry := range filemap {
+				if err := func(entry File) error {
+					in, err := os.Open(entry.SourcePath)
+					if err != nil {
+						return err
+					}
+
+					defer in.Close() //nolint:errcheck
+
+					st, err := in.Stat()
+					if err != nil {
+						return err
+					}
+
+					if err = w.WriteHeader(&tar.Header{
+						Name: entry.ImagePath,
+						Size: st.Size(),
+					}); err != nil {
+						return err
+					}
+
+					_, err = io.Copy(w, in)
+					if err != nil {
+						return err
+					}
+
+					return in.Close()
+				}(entry); err != nil {
+					return err
+				}
+			}
+
+			return w.Close()
+		}())
+	}()
+
+	return pr
+}
+
 // Layer creates a layer from a single file map.
 //
 // These layers are reproducible and consistent.
 //
 // A filemap is a path -> file content map representing a file system.
 func Layer(filemap []File) (v1.Layer, error) {
-	b := &bytes.Buffer{}
-	w := tar.NewWriter(b)
-
 	sort.Slice(filemap, func(i, j int) bool {
 		return filemap[i].ImagePath < filemap[j].ImagePath
 	})
 
-	for _, entry := range filemap {
-		if err := func(entry File) error {
-			in, err := os.Open(entry.SourcePath)
-			if err != nil {
-				return err
-			}
-
-			defer in.Close() //nolint:errcheck
-
-			st, err := in.Stat()
-			if err != nil {
-				return err
-			}
-
-			if err = w.WriteHeader(&tar.Header{
-				Name: entry.ImagePath,
-				Size: st.Size(),
-			}); err != nil {
-				return err
-			}
-
-			_, err = io.Copy(w, in)
-			if err != nil {
-				return err
-			}
-
-			return in.Close()
-		}(entry); err != nil {
-			return nil, err
-		}
-	}
-
-	if err := w.Close(); err != nil {
-		return nil, err
-	}
-
 	// Return a new copy of the buffer each time it's opened.
 	return tarball.LayerFromOpener(func() (io.ReadCloser, error) {
-		return io.NopCloser(bytes.NewReader(b.Bytes())), nil
+		return build(filemap), nil
 	})
 }