Add external markup render support (#2570)

* add external markup render support

* bug fixed

* refacotr codes and fix wrong error log

* fix comments and add check to prevent leaks

* add check for config file and improve the example

* check file close error

* use ioutil.TempFile instead uuid

* correct Render -> Parser

* improve warning when incorrect markup setting

* fix typos

modules/markup/external/external.go

@@ -0,0 +1,88 @@
+// Copyright 2017 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 external
+
+import (
+	"bytes"
+	"io"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"strings"
+
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/markup"
+	"code.gitea.io/gitea/modules/setting"
+)
+
+// RegisterParsers registers all supported third part parsers according settings
+func RegisterParsers() {
+	for _, parser := range setting.ExternalMarkupParsers {
+		if parser.Enabled && parser.Command != "" && len(parser.FileExtensions) > 0 {
+			markup.RegisterParser(&Parser{parser})
+		}
+	}
+}
+
+// Parser implements markup.Parser for external tools
+type Parser struct {
+	setting.MarkupParser
+}
+
+// Name returns the external tool name
+func (p *Parser) Name() string {
+	return p.MarkupName
+}
+
+// Extensions returns the supported extensions of the tool
+func (p *Parser) Extensions() []string {
+	return p.FileExtensions
+}
+
+// Render renders the data of the document to HTML via the external tool.
+func (p *Parser) Render(rawBytes []byte, urlPrefix string, metas map[string]string, isWiki bool) []byte {
+	var (
+		bs       []byte
+		buf      = bytes.NewBuffer(bs)
+		rd       = bytes.NewReader(rawBytes)
+		commands = strings.Fields(p.Command)
+		args     = commands[1:]
+	)
+
+	if p.IsInputFile {
+		// write to temp file
+		f, err := ioutil.TempFile("", "gitea_input")
+		if err != nil {
+			log.Error(4, "%s create temp file when rendering %s failed: %v", p.Name(), p.Command, err)
+			return []byte("")
+		}
+		defer os.Remove(f.Name())
+
+		_, err = io.Copy(f, rd)
+		if err != nil {
+			f.Close()
+			log.Error(4, "%s write data to temp file when rendering %s failed: %v", p.Name(), p.Command, err)
+			return []byte("")
+		}
+
+		err = f.Close()
+		if err != nil {
+			log.Error(4, "%s close temp file when rendering %s failed: %v", p.Name(), p.Command, err)
+			return []byte("")
+		}
+		args = append(args, f.Name())
+	}
+
+	cmd := exec.Command(commands[0], args...)
+	if !p.IsInputFile {
+		cmd.Stdin = rd
+	}
+	cmd.Stdout = buf
+	if err := cmd.Run(); err != nil {
+		log.Error(4, "%s render run command %s %v failed: %v", p.Name(), commands[0], args, err)
+		return []byte("")
+	}
+	return buf.Bytes()
+}