gitea/modules/typesniffer/typesniffer.go

// Copyright 2021 The Gitea Authors. All rights reserved.
// SPDX-License-Identifier: MIT

package typesniffer

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"io"
	"net/http"
	"regexp"
	"slices"
	"strings"

	"code.gitea.io/gitea/modules/util"
)

// Use at most this many bytes to determine Content Type.
const sniffLen = 1024

const (
	MimeTypeImageSvg  = "image/svg+xml"
	MimeTypeImageAvif = "image/avif"

	MimeTypeApplicationOctetStream = "application/octet-stream"
)

var (
	svgComment       = regexp.MustCompile(`(?s)<!--.*?-->`)
	svgTagRegex      = regexp.MustCompile(`(?si)\A\s*(?:(<!DOCTYPE\s+svg([\s:]+.*?>|>))\s*)*<svg\b`)
	svgTagInXMLRegex = regexp.MustCompile(`(?si)\A<\?xml\b.*?\?>\s*(?:(<!DOCTYPE\s+svg([\s:]+.*?>|>))\s*)*<svg\b`)
)

// SniffedType contains information about a blobs type.
type SniffedType struct {
	contentType string
}

// IsText etects if content format is plain text.
func (ct SniffedType) IsText() bool {
	return strings.Contains(ct.contentType, "text/")
}

// IsImage detects if data is an image format
func (ct SniffedType) IsImage() bool {
	return strings.Contains(ct.contentType, "image/")
}

// IsSvgImage detects if data is an SVG image format
func (ct SniffedType) IsSvgImage() bool {
	return strings.Contains(ct.contentType, MimeTypeImageSvg)
}

// IsPDF detects if data is a PDF format
func (ct SniffedType) IsPDF() bool {
	return strings.Contains(ct.contentType, "application/pdf")
}

// IsVideo detects if data is an video format
func (ct SniffedType) IsVideo() bool {
	return strings.Contains(ct.contentType, "video/")
}

// IsAudio detects if data is an video format
func (ct SniffedType) IsAudio() bool {
	return strings.Contains(ct.contentType, "audio/")
}

// IsRepresentableAsText returns true if file content can be represented as
// plain text or is empty.
func (ct SniffedType) IsRepresentableAsText() bool {
	return ct.IsText() || ct.IsSvgImage()
}

// IsBrowsableBinaryType returns whether a non-text type can be displayed in a browser
func (ct SniffedType) IsBrowsableBinaryType() bool {
	return ct.IsImage() || ct.IsSvgImage() || ct.IsPDF() || ct.IsVideo() || ct.IsAudio()
}

// GetMimeType returns the mime type
func (ct SniffedType) GetMimeType() string {
	return strings.SplitN(ct.contentType, ";", 2)[0]
}

// https://en.wikipedia.org/wiki/ISO_base_media_file_format#File_type_box
func detectFileTypeBox(data []byte) (brands []string, found bool) {
	if len(data) < 12 {
		return nil, false
	}
	boxSize := int(binary.BigEndian.Uint32(data[:4]))
	if boxSize < 12 || boxSize > len(data) {
		return nil, false
	}
	tag := string(data[4:8])
	if tag != "ftyp" {
		return nil, false
	}
	brands = append(brands, string(data[8:12]))
	for i := 16; i+4 <= boxSize; i += 4 {
		brands = append(brands, string(data[i:i+4]))
	}
	return brands, true
}

// DetectContentType extends http.DetectContentType with more content types. Defaults to text/unknown if input is empty.
func DetectContentType(data []byte) SniffedType {
	if len(data) == 0 {
		return SniffedType{"text/unknown"}
	}

	ct := http.DetectContentType(data)

	if len(data) > sniffLen {
		data = data[:sniffLen]
	}

	// SVG is unsupported by http.DetectContentType, https://github.com/golang/go/issues/15888
	detectByHTML := strings.Contains(ct, "text/plain") || strings.Contains(ct, "text/html")
	detectByXML := strings.Contains(ct, "text/xml")
	if detectByHTML || detectByXML {
		dataProcessed := svgComment.ReplaceAll(data, nil)
		dataProcessed = bytes.TrimSpace(dataProcessed)
		if detectByHTML && svgTagRegex.Match(dataProcessed) ||
			detectByXML && svgTagInXMLRegex.Match(dataProcessed) {
			ct = MimeTypeImageSvg
		}
	}

	if strings.HasPrefix(ct, "audio/") && bytes.HasPrefix(data, []byte("ID3")) {
		// The MP3 detection is quite inaccurate, any content with "ID3" prefix will result in "audio/mpeg".
		// So remove the "ID3" prefix and detect again, if result is text, then it must be text content.
		// This works especially because audio files contain many unprintable/invalid characters like `0x00`
		ct2 := http.DetectContentType(data[3:])
		if strings.HasPrefix(ct2, "text/") {
			ct = ct2
		}
	}

	fileTypeBrands, found := detectFileTypeBox(data)
	if found && slices.Contains(fileTypeBrands, "avif") {
		ct = MimeTypeImageAvif
	}

	if ct == "application/ogg" {
		dataHead := data
		if len(dataHead) > 256 {
			dataHead = dataHead[:256] // only need to do a quick check for the file header
		}
		if bytes.Contains(dataHead, []byte("theora")) || bytes.Contains(dataHead, []byte("dirac")) {
			ct = "video/ogg" // ogg is only used for some video formats, and it's not popular
		} else {
			ct = "audio/ogg" // for most cases, it is used as an audio container
		}
	}
	return SniffedType{ct}
}

// DetectContentTypeFromReader guesses the content type contained in the reader.
func DetectContentTypeFromReader(r io.Reader) (SniffedType, error) {
	buf := make([]byte, sniffLen)
	n, err := util.ReadAtMost(r, buf)
	if err != nil {
		return SniffedType{}, fmt.Errorf("DetectContentTypeFromReader io error: %w", err)
	}
	buf = buf[:n]

	return DetectContentType(buf), nil
}