exif_image_metadata.go

package main

// I'm a total beginner in Go, please have mercy ;-)

// Reads a JSON input object with an "url" field that points to an image file
// Uses exiftool to extract file metadata
// Outputs a JSON document with that metadata + request information
import (
	"bufio"
	"encoding/json"
	"log"
	"os"
	"os/exec"
	"strings"
	"time"
)

const EXIF_TOOL string = "exiftool"

// TODO json package only accesses exported fields like with names that
// begin with an uppercase letter - how to specify lowercased keys?
// (json: tag like Metadata here is ugly)
type OutputFormat struct {
	Request struct {
		AssetID              string
		InputFile            string
		Processor            string
		Info                 string
		DurationMilliseconds time.Duration
	}
	Output struct {
		Metadata map[string]string `json:"metadata"`
	}
}

func getInputURI() string {
	dec := json.NewDecoder(os.Stdin)
	for {
		var v map[string]interface{}
		if err := dec.Decode(&v); err != nil {
			panic("JSON parsing error")
		}
		return v["url"].(string)
	}
}

func main() {
	var start = time.Now()
	var inputURI = getInputURI()
	var inputFile = strings.TrimPrefix(inputURI, "file://")
	log.Print("Input file ", inputFile)

	cmd := exec.Command(EXIF_TOOL, inputFile)
	pipe, err := cmd.StdoutPipe()
	if err != nil {
		panic(err)
	}
	scanner := bufio.NewScanner(pipe)

	if err := cmd.Start(); err != nil {
		panic(err)
	}

	var m map[string]string
	m = make(map[string]string)
	for scanner.Scan() {
		fields := strings.Split(scanner.Text(), ":")
		m[strings.Trim(fields[0], " ")] = strings.Trim(fields[1], " ")
	}

	output := &OutputFormat{}
	output.Request.AssetID = inputURI
	output.Request.InputFile = inputFile
	output.Request.Processor = EXIF_TOOL
	output.Request.Info = "Image metadata generated by " + EXIF_TOOL
	output.Request.DurationMilliseconds = time.Since(start) / 1000000
	output.Output.Metadata = m

	enc := json.NewEncoder(os.Stdout)
	if err := enc.Encode(&output); err != nil {
		panic(err)
	}
}