markdown: fix json format parse issue

We can't use json meta parser's remaining buffered data as the markdown body
because it may not contain the entire original content.
Now we adopt the way like toml and yaml parser's way to extract the meta content
at first.

Also when spilting the meta data and content body, additional io.Copy is
unnecessary.

Fix issue #355
Signed-off-by: Tw <tw19881113@gmail.com>

middleware/markdown/metadata.go

 package markdown
 
 import (
-	"bufio"
 	"bytes"
 	"encoding/json"
 	"fmt"
-	"io"
 
 	"github.com/BurntSushi/toml"
 	"gopkg.in/yaml.v2"
@@ -73,23 +71,20 @@ type JSONMetadataParser struct {
 
 // Parse the metadata
 func (j *JSONMetadataParser) Parse(b []byte) ([]byte, error) {
+	b, markdown, err := extractMetadata(j, b)
+	if err != nil {
+		return markdown, err
+	}
 	m := make(map[string]interface{})
 
 	// Read the preceding JSON object
 	decoder := json.NewDecoder(bytes.NewReader(b))
 	if err := decoder.Decode(&m); err != nil {
-		return b, err
+		return markdown, err
 	}
 	j.metadata.load(m)
 
-	// Retrieve remaining bytes after decoding
-	buf := make([]byte, len(b))
-	n, err := decoder.Buffered().Read(buf)
-	if err != nil {
-		return b, err
-	}
-
-	return buf[:n], nil
+	return markdown, nil
 }
 
 // Metadata returns parsed metadata.  It should be called
@@ -183,43 +178,29 @@ func (y *YAMLMetadataParser) Closing() []byte {
 // It returns the metadata, the remaining bytes (markdown), and an error, if any.
 func extractMetadata(parser MetadataParser, b []byte) (metadata []byte, markdown []byte, err error) {
 	b = bytes.TrimSpace(b)
-	reader := bufio.NewReader(bytes.NewBuffer(b))
-
-	// Read first line, which should indicate metadata or not
-	line, err := reader.ReadBytes('\n')
-	if err != nil || !bytes.Equal(bytes.TrimSpace(line), parser.Opening()) {
+	openingLine := append(parser.Opening(), '\n')
+	closingLine := append(parser.Closing(), '\n')
+	if !bytes.HasPrefix(b, openingLine) {
 		return nil, b, fmt.Errorf("first line missing expected metadata identifier")
 	}
-
-	// buffer for metadata contents
-	metaBuf := bytes.Buffer{}
-
-	// Read remaining lines until closing identifier is found
-	for {
-		line, err := reader.ReadBytes('\n')
-		if err != nil && err != io.EOF {
-			return nil, nil, err
+	metaStart := len(openingLine)
+	if _, ok := parser.(*JSONMetadataParser); ok {
+		metaStart = 0
 	}
-
-		// if closing identifier found, the remaining bytes must be markdown content
-		if bytes.Equal(bytes.TrimSpace(line), parser.Closing()) {
-			break
-		}
-
-		// if file ended, by this point no closing identifier was found
-		if err == io.EOF {
+	metaEnd := bytes.Index(b[metaStart:], closingLine)
+	if metaEnd == -1 {
 		return nil, nil, fmt.Errorf("metadata not closed ('%s' not found)", parser.Closing())
 	}
-
-		metaBuf.Write(line)
-		metaBuf.WriteString("\r\n")
+	metaEnd += metaStart
+	if _, ok := parser.(*JSONMetadataParser); ok {
+		metaEnd += len(closingLine)
 	}
-
-	// By now, the rest of the buffer contains markdown content
-	contentBuf := new(bytes.Buffer)
-	io.Copy(contentBuf, reader)
-
-	return metaBuf.Bytes(), contentBuf.Bytes(), nil
+	metadata = b[metaStart:metaEnd]
+	markdown = b[metaEnd:]
+	if _, ok := parser.(*JSONMetadataParser); !ok {
+		markdown = b[metaEnd+len(closingLine):]
+	}
+	return metadata, markdown, nil
 }
 
 // findParser finds the parser using line that contains opening identifier