UI: Detect and restore encoding and BOM in content (#6727)

* detect and remove a decoded BOM

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

* Restore the previous encoding and BOM

* On error keep as UTF-8

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

* create remove BOM function

* Deal with LFSed content

* Update modules/repofiles/update.go

* Fix final LFS bug

* Keep LFS sections referring to opts.Content

3 files changed, 114 insertions, 7 deletions

diff --git a/modules/base/tool.go b/modules/base/tool.go
index 97fd87e85c..d99ed2bab4 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"
@@ -36,6 +37,9 @@ 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()
@@ -91,6 +95,14 @@ 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/repofiles/update.go b/modules/repofiles/update.go
index e9b3077535..8bc3b50ae0 100644
--- a/modules/repofiles/update.go
+++ b/modules/repofiles/update.go
@@ -5,13 +5,19 @@
 package repofiles
 
 import (
+	"bytes"
 	"fmt"
 	"path"
 	"strings"
 
+	"golang.org/x/net/html/charset"
+	"golang.org/x/text/transform"
+
 	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/lfs"
+	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/sdk/gitea"
 )
@@ -37,6 +43,70 @@ type UpdateRepoFileOptions struct {
 	Committer    *IdentityOptions
 }
 
+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
+}
+
 // CreateOrUpdateRepoFile adds or updates a file in the given repository
 func CreateOrUpdateRepoFile(repo *models.Repository, doer *models.User, opts *UpdateRepoFileOptions) (*gitea.FileResponse, error) {
 	// If no branch name is set, assume master
@@ -118,6 +188,9 @@ func CreateOrUpdateRepoFile(repo *models.Repository, doer *models.User, opts *Up
 		opts.LastCommitID = commit.ID.String()
 	}
 
+	encoding := "UTF-8"
+	bom := false
+
 	if !opts.IsNewFile {
 		fromEntry, err := commit.GetTreeEntryByPath(fromTreePath)
 		if err != nil {
@@ -151,6 +224,7 @@ func CreateOrUpdateRepoFile(repo *models.Repository, doer *models.User, opts *Up
 			// haven't been made. We throw an error if one wasn't provided.
 			return nil, models.ErrSHAOrCommitIDNotProvided{}
 		}
+		encoding, bom = detectEncodingAndBOM(fromEntry, repo)
 	}
 
 	// For the path where this file will be created/updated, we need to make
@@ -235,9 +309,28 @@ func CreateOrUpdateRepoFile(repo *models.Repository, doer *models.User, opts *Up
 	}
 
 	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("Error re-encoding %s (%s) as %s - will stay as UTF-8: %v", opts.TreePath, opts.FromTreePath, encoding, err)
+				result = content
+			}
+			content = result
+		} else {
+			log.Error("Unknown encoding: %s", encoding)
+		}
+	}
+	// Reset the opts.Content to our adjusted content to ensure that LFS gets the correct content
+	opts.Content = content
 	var lfsMetaObject *models.LFSMetaObject
 
-	if filename2attribute2info[treePath] != nil && filename2attribute2info[treePath]["filter"] == "lfs" {
+	if setting.LFS.StartServer && filename2attribute2info[treePath] != nil && filename2attribute2info[treePath]["filter"] == "lfs" {
 		// OK so we are supposed to LFS this data!
 		oid, err := models.GenerateLFSOid(strings.NewReader(opts.Content))
 		if err != nil {
diff --git a/modules/templates/helper.go b/modules/templates/helper.go
index d3eb8c48b8..b6c5cc5945 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