Allow markdown files to read from the LFS (#5787)

This PR makes it possible for the markdown renderer to render images and media straight from the LFS.

Fix #5746

Signed-off-by: Andrew Thornton [art27@cantab.net](mailto:art27@cantab.net)

modules/lfs/pointers.go

@@ -0,0 +1,69 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package lfs
+
+import (
+	"io"
+	"strconv"
+	"strings"
+
+	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/base"
+	"code.gitea.io/gitea/modules/setting"
+)
+
+// ReadPointerFile will return a partially filled LFSMetaObject if the provided reader is a pointer file
+func ReadPointerFile(reader io.Reader) (*models.LFSMetaObject, *[]byte) {
+	if !setting.LFS.StartServer {
+		return nil, nil
+	}
+
+	buf := make([]byte, 1024)
+	n, _ := reader.Read(buf)
+	buf = buf[:n]
+
+	if isTextFile := base.IsTextFile(buf); !isTextFile {
+		return nil, nil
+	}
+
+	return IsPointerFile(&buf), &buf
+}
+
+// IsPointerFile will return a partially filled LFSMetaObject if the provided byte slice is a pointer file
+func IsPointerFile(buf *[]byte) *models.LFSMetaObject {
+	if !setting.LFS.StartServer {
+		return nil
+	}
+
+	headString := string(*buf)
+	if !strings.HasPrefix(headString, models.LFSMetaFileIdentifier) {
+		return nil
+	}
+
+	splitLines := strings.Split(headString, "\n")
+	if len(splitLines) < 3 {
+		return nil
+	}
+
+	oid := strings.TrimPrefix(splitLines[1], models.LFSMetaFileOidPrefix)
+	size, err := strconv.ParseInt(strings.TrimPrefix(splitLines[2], "size "), 10, 64)
+	if len(oid) != 64 || err != nil {
+		return nil
+	}
+
+	contentStore := &ContentStore{BasePath: setting.LFS.ContentPath}
+	meta := &models.LFSMetaObject{Oid: oid, Size: size}
+	if !contentStore.Exists(meta) {
+		return nil
+	}
+
+	return meta
+}
+
+// ReadMetaObject will read a models.LFSMetaObject and return a reader
+func ReadMetaObject(meta *models.LFSMetaObject) (io.ReadCloser, error) {
+	contentStore := &ContentStore{BasePath: setting.LFS.ContentPath}
+	return contentStore.Get(meta, 0)
+}