Running external worker process in Node.js

This is example how to properly spawn external worker process in Node.js with error checking. This example runs a ImageMagick convert tool to transform .png image files to 8-bit .png file.

Function spawns ‘convert’ with original file as input and temporary destination path as output and checks the result. If everything seems to be in order, it renames the temporary file over original one.

This also prints out all output from the spawned process to stdout for diagnostics.

var fs = require('fs'),
    util = require('util'),
    spawn = require('child_process').spawn;

function package_image( path, next ) {

    // temporary file name. use process PID as part of the name so there
    // wont' be conflicts if two processes run in parallel
    var tmpfile = path + '.tmp.' + process.pid;

    var cmd = 'convert';
    // executes command 'convert path -type Palette png8:path.tmp'
    var convert = spawn(cmd, [
        path,
        '-type', 'Palette',
	'png8:' + tmpfile ]);

    // capture stdout and stderr. Note that convert does not have any output on success
    convert.stdout.setEncoding('utf8');
    convert.stdout.on('data', function (data) {
        console.log( cmd + ': stdout '+ path + ' ' + data.trim() );
    });

    convert.stderr.setEncoding('utf8');
    convert.stderr.on('data', function (data) {
        if (/^execvp\(\)/.test(data)) {
            // we get here if 'convert' command was not found or could
            // not be executed
	    console.log( cmd + ': failed to start: ' + data );
	} else {
	    console.log( cmd + ': stderr '+ path + ' ' + data.trim() );
	}
    });

    // hook on process exit
    convert.on('exit', function( code ) {
        if ( code ) {
	    // 127 means spawn error, command could not be executed
   	    console.log(cmd + ': error '+ path + ' ' + code );
	    return next(code);
	}
        // check if output file exists
        fs.stat( tmpfile, function(err, info) {
	    if ( err ) {
		// no file found? something went wrong. Just ignore
		console.log( cmd + ': output file not found ' + util.inspect(err));
		return next( err );
	    }
	    // check output file size
	    if ( !info.size || !info.isFile() ) {
		console.log( cmd + ': out file 0 bytes or not a file '+ tmpfile);
		fs.unlink( tmpfile ); // remove output file
		return next( true );
	    }

	    // rename temporary file over original one
	    fs.rename( tmpfile, path, function(err) {
	         if ( err ) {
		    fs.unlink( tmpfile );
		    console.log( cmd + ': can not rename '+ tmpfile + ' ' + util.inspect(err));
		 }
                 // done
		 return next( err );
	    });
	});
    });
}

Example of usage

package_image( '/tmp/some.png', function(err) { 
    if ( err ) { 
       console.log('Image conversion failed', err );
    }
});

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