Open files lazily when serving content

Changelog: performance

1 files changed, 80 insertions, 0 deletions

diff --git a/internal/serving/disk/lazy.go b/internal/serving/disk/lazy.go
new file mode 100644
index 00000000..9b58457c
--- /dev/null
+++ b/internal/serving/disk/lazy.go
@@ -0,0 +1,80 @@
+package disk
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+
+	"gitlab.com/gitlab-org/gitlab-pages/internal/vfs"
+)
+
+var (
+	// Make sure lazyFile is a vfs.File to support vfs.ServeCompressedFile
+	// if the file is compressed.
+	// This should always be satisfied because root.Open always returns
+	// a vfs.File.
+	_ vfs.File = &lazyFile{}
+
+	// Make sure lazyFile is a ReadSeeker to support http.ServeContent
+	// if the file is not compressed.
+	// Note: lazyFile.Seek only works if the underlying root.Open returns
+	// a vfs.SeekableFile which is the case if the file is not compressed.
+	_ io.ReadSeeker = &lazyFile{}
+
+	// ErrInvalidSeeker is returned if lazyFile.Seek is called and the
+	// underlying file is not seekable
+	ErrInvalidSeeker = errors.New("file is not seekable")
+)
+
+type lazyFile struct {
+	f    vfs.File
+	err  error
+	load func() (vfs.File, error)
+}
+
+func lazyOpen(ctx context.Context, root vfs.Root, fullPath string) lazyFile {
+	lf := lazyFile{
+		load: func() (vfs.File, error) {
+			return root.Open(ctx, fullPath)
+		},
+	}
+
+	return lf
+}
+
+func (lf lazyFile) Read(p []byte) (int, error) {
+	if lf.f == nil && lf.err == nil {
+		lf.f, lf.err = lf.load()
+	}
+
+	if lf.err != nil {
+		return 0, lf.err
+	}
+
+	return lf.f.Read(p)
+}
+
+func (lf lazyFile) Close() error {
+	if lf.f != nil {
+		return lf.f.Close()
+	}
+
+	return nil
+}
+
+func (lf lazyFile) Seek(offset int64, whence int) (int64, error) {
+	if lf.f == nil && lf.err == nil {
+		lf.f, lf.err = lf.load()
+	}
+
+	if lf.err != nil {
+		return 0, lf.err
+	}
+
+	if sf, ok := lf.f.(io.ReadSeeker); ok {
+		return sf.Seek(offset, whence)
+	}
+
+	return 0, fmt.Errorf("unable to seek from %T: %w", lf.f, ErrInvalidSeeker)
+}