Detect encoding and BOM in content (#6727) (#6765)

Detect and remove a decoded BOM when showing content.
Restore the previous encoding and BOM when updating content.
On error keep as UTF-8 encoding.

Signed-off-by: Andrew Thornton <art27@cantab.net>

3 files changed, 127 insertions, 7 deletions

diff --git a/modules/base/tool.go b/modules/base/tool.go
index b069e5fae..4dd7ba59b 100644
--- a/modules/base/tool.go
+++ b/modules/base/tool.go
@@ -5,6 +5,7 @@
 package base
 
 import (
+	"bytes"
 	"crypto/md5"
 	"crypto/rand"
 	"crypto/sha1"
@@ -32,6 +33,9 @@ import (
 	"github.com/gogits/chardet"
 )
 
+// UTF8BOM is the utf-8 byte-order marker
+var UTF8BOM = []byte{'\xef', '\xbb', '\xbf'}
+
 // EncodeMD5 encodes string to md5 hex value.
 func EncodeMD5(str string) string {
 	m := md5.New()
@@ -87,6 +91,14 @@ func DetectEncoding(content []byte) (string, error) {
 	return result.Charset, err
 }
 
+// RemoveBOMIfPresent removes a UTF-8 BOM from a []byte
+func RemoveBOMIfPresent(content []byte) []byte {
+	if len(content) > 2 && bytes.Equal(content[0:3], UTF8BOM) {
+		return content[3:]
+	}
+	return content
+}
+
 // BasicAuthDecode decode basic auth string
 func BasicAuthDecode(encoded string) (string, string, error) {
 	s, err := base64.StdEncoding.DecodeString(encoded)
diff --git a/modules/templates/helper.go b/modules/templates/helper.go
index ce077d1a9..84952d14e 100644
--- a/modules/templates/helper.go
+++ b/modules/templates/helper.go
@@ -267,7 +267,7 @@ func ToUTF8WithErr(content []byte) (string, error) {
 	if err != nil {
 		return "", err
 	} else if charsetLabel == "UTF-8" {
-		return string(content), nil
+		return string(base.RemoveBOMIfPresent(content)), nil
 	}
 
 	encoding, _ := charset.Lookup(charsetLabel)
@@ -277,19 +277,21 @@ func ToUTF8WithErr(content []byte) (string, error) {
 
 	// If there is an error, we concatenate the nicely decoded part and the
 	// original left over. This way we won't lose data.
-	result, n, err := transform.String(encoding.NewDecoder(), string(content))
+	result, n, err := transform.Bytes(encoding.NewDecoder(), content)
 	if err != nil {
-		result = result + string(content[n:])
+		result = append(result, content[n:]...)
 	}
 
-	return result, err
+	result = base.RemoveBOMIfPresent(result)
+
+	return string(result), err
 }
 
 // ToUTF8WithFallback detects the encoding of content and coverts to UTF-8 if possible
 func ToUTF8WithFallback(content []byte) []byte {
 	charsetLabel, err := base.DetectEncoding(content)
 	if err != nil || charsetLabel == "UTF-8" {
-		return content
+		return base.RemoveBOMIfPresent(content)
 	}
 
 	encoding, _ := charset.Lookup(charsetLabel)
@@ -304,7 +306,7 @@ func ToUTF8WithFallback(content []byte) []byte {
 		return append(result, content[n:]...)
 	}
 
-	return result
+	return base.RemoveBOMIfPresent(result)
 }
 
 // ToUTF8 converts content to UTF8 encoding and ignore error
diff --git a/modules/uploader/update.go b/modules/uploader/update.go
index 08caf11ee..fe85cbf47 100644
--- a/modules/uploader/update.go
+++ b/modules/uploader/update.go
@@ -5,15 +5,85 @@
 package uploader
 
 import (
+	"bytes"
 	"fmt"
 	"strings"
 
+	"golang.org/x/net/html/charset"
+	"golang.org/x/text/transform"
+
 	"code.gitea.io/git"
 	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/lfs"
+	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 )
 
+func detectEncodingAndBOM(entry *git.TreeEntry, repo *models.Repository) (string, bool) {
+	reader, err := entry.Blob().DataAsync()
+	if err != nil {
+		// return default
+		return "UTF-8", false
+	}
+	defer reader.Close()
+	buf := make([]byte, 1024)
+	n, err := reader.Read(buf)
+	if err != nil {
+		// return default
+		return "UTF-8", false
+	}
+	buf = buf[:n]
+
+	if setting.LFS.StartServer {
+		meta := lfs.IsPointerFile(&buf)
+		if meta != nil {
+			meta, err = repo.GetLFSMetaObjectByOid(meta.Oid)
+			if err != nil && err != models.ErrLFSObjectNotExist {
+				// return default
+				return "UTF-8", false
+			}
+		}
+		if meta != nil {
+			dataRc, err := lfs.ReadMetaObject(meta)
+			if err != nil {
+				// return default
+				return "UTF-8", false
+			}
+			defer dataRc.Close()
+			buf = make([]byte, 1024)
+			n, err = dataRc.Read(buf)
+			if err != nil {
+				// return default
+				return "UTF-8", false
+			}
+			buf = buf[:n]
+		}
+
+	}
+
+	encoding, err := base.DetectEncoding(buf)
+	if err != nil {
+		// just default to utf-8 and no bom
+		return "UTF-8", false
+	}
+	if encoding == "UTF-8" {
+		return encoding, bytes.Equal(buf[0:3], base.UTF8BOM)
+	}
+	charsetEncoding, _ := charset.Lookup(encoding)
+	if charsetEncoding == nil {
+		return "UTF-8", false
+	}
+
+	result, n, err := transform.String(charsetEncoding.NewDecoder(), string(buf))
+
+	if n > 2 {
+		return encoding, bytes.Equal([]byte(result)[0:3], base.UTF8BOM)
+	}
+
+	return encoding, false
+}
+
 // UpdateRepoFileOptions holds the repository file update options
 type UpdateRepoFileOptions struct {
 	LastCommitID string
@@ -45,12 +115,29 @@ func UpdateRepoFile(repo *models.Repository, doer *models.User, opts *UpdateRepo
 		return fmt.Errorf("UpdateRepoFile: %v", err)
 	}
 
+	encoding := "UTF-8"
+	bom := false
+
 	if opts.IsNewFile {
 		for _, file := range filesInIndex {
 			if file == opts.NewTreeName {
 				return models.ErrRepoFileAlreadyExist{FileName: opts.NewTreeName}
 			}
 		}
+	} else {
+		gitRepo, err := git.OpenRepository(t.basePath)
+		if err != nil {
+			return err
+		}
+		tree, err := gitRepo.GetTree("HEAD")
+		if err != nil {
+			return err
+		}
+		entry, err := tree.GetTreeEntryByPath(opts.OldTreeName)
+		if err != nil {
+			return err
+		}
+		encoding, bom = detectEncodingAndBOM(entry, repo)
 	}
 
 	//var stdout string
@@ -72,9 +159,28 @@ func UpdateRepoFile(repo *models.Repository, doer *models.User, opts *UpdateRepo
 	}
 
 	content := opts.Content
+	if bom {
+		content = string(base.UTF8BOM) + content
+	}
+	if encoding != "UTF-8" {
+		charsetEncoding, _ := charset.Lookup(encoding)
+		if charsetEncoding != nil {
+			result, _, err := transform.String(charsetEncoding.NewEncoder(), string(content))
+			if err != nil {
+				// Look if we can't encode back in to the original we should just stick with utf-8
+				log.Error(4, "Error re-encoding %s (%s) as %s - will stay as UTF-8: %v", opts.NewTreeName, opts.OldTreeName, encoding, err)
+				result = content
+			}
+			content = result
+		} else {
+			log.Error(4, "Unknown encoding: %s", encoding)
+		}
+	}
+	// Reset the opts.Content with the re-encoded and BOM'd content
+	opts.Content = content
 	var lfsMetaObject *models.LFSMetaObject
 
-	if filename2attribute2info[opts.NewTreeName] != nil && filename2attribute2info[opts.NewTreeName]["filter"] == "lfs" {
+	if setting.LFS.StartServer && filename2attribute2info[opts.NewTreeName] != nil && filename2attribute2info[opts.NewTreeName]["filter"] == "lfs" {
 		// OK so we are supposed to LFS this data!
 		oid, err := models.GenerateLFSOid(strings.NewReader(opts.Content))
 		if err != nil {