How to readFile() with async / execFile() Node js - node.js

Question

Thanks in advance.
I'm creating an Electron-Create-React-App using electron-forge on Windows 10 Pro and am stuck with using async functions with execFile and readFile().
I want to achieve the following:-
main process - Receive a buffer of a screen capture (video) from the renderer process.
Create a temporary file and write the buffer to a .mp4 file.
Crop the video (based on x:y:width:height) using ffmpeg (installed in Electron as a binary).
Output = .mp4 file in temporary directory
Read the cropped .mp4 file using fs.readFile() (as a base64 encoded buffer)
Send the buffer to another renderer screen.
Delete temp file.
Q: I've managed to do most of it but cannot access the cropped .mp4 file in the temp directory.
I've tried the following:-
Electron main process
const fs = require('fs').promises
const path = require('path')
ipcMain.on("capture:buffer", async (video_object) => {
const {x_pos, y_pos, window_width, window_height, buffer} = video_object
try {
const dir = await fs.mkdtemp(await fs.realpath(os.tmpdir()) + path.sep)
const captured_video_file_path = path.join(dir, "screen_capture_video.mp4")
// This works
await fs.writeFile(captured_video_file_path, buffer, (error, stdout, stderr) => {
if (error) {
console.log(error)
}
console.log("Screen Capture File written")
})
// This also works
execFile(`${ffmpeg.path}`,
['-i', `${captured_video_file_path}`, '-vf',
`crop=${window_width}:${window_height}:${x_pos}:${y_pos}`,
`${path.join(dir,'cropped_video.mp4')}`],
(error, stdout, stderr) => {
if (error) {
console.log(error.message)
}
if (stderr) {
console.log(stderr)
}
console.log("Cropped File created")
})
// This code onwards doesn't work
await fs.readFile(path.join(dir, "cropped_video.mp4"), 'base64', (error, data) => {
if (error) {
console.log(error)
}
// To renderer
mainWindow.webContents.send("main:video_buffer", Buffer.from(data))
})
} catch (error) {
console.log(error)
} finally {
fs.rmdir(dir, {recursive: true})
}
})
When trying to read the file i get the following error :-
[Error: ENOENT: no such file or directory, open 'C:\Users\XXXX\XXXXX\XXXXX\temp\temp_eYGMCR\cropped_video.mp4']
I've checked that the correct path exists with console.log.
I suspect it is a 'simple' issue with using async / execFile() properly but don't know exactly where I am making a silly mistake.
Any help would be appreciated.
Thanks.

Answer 1

Because at that time of calling fs.readFile, execFile may not be done yet.
Untested, but you may want to create a promise and wait for execFile to be completed before proceeding and see whether it works.
await new Promise( resolve => {
execFile(`${ffmpeg.path}`,
['-i', `${captured_video_file_path}`, '-vf',
`crop=${window_width}:${window_height}:${x_pos}:${y_pos}`,
`${path.join(dir,'cropped_video.mp4')}`],
(error, stdout, stderr) => {
if (error) {
console.log(error.message)
}
if (stderr) {
console.log(stderr)
}
console.log("Cropped File created")
resolve() //this tells `await` it's ready to move on
})
})

Answer 2

Thanks for the pointers guys.
Here's the solution I found.
Another big problem with safely creating and removing temporary directories in Electron is fs.rmdir() doesn't work when using Electron-Forge / Builder due to an issue with ASAR files.
(ASAR files are used to package Electron apps).
const fsPromises = require('fs').promises
ipcMain.on("capture:buffer", async (video_object) => {
const {x_pos, y_pos, window_width, window_height, buffer} = video_object
const temp_dir = await fsPromises.mkdtemp(await fsPromises.realpath(os.tmpdir()) + path.sep)
const captured_video_file_path = path.join(dir, "screen_capture_video.mp4")
try {
await fsPromises.writeFile(captured_video_file_path, buffer)
}
catch (error) {console.error}
// note no callback req'd as per jfriends advice
let child_object =
execFile(`${ffmpeg.path}`,
['-i', `${captured_video_file_path}`, '-vf',
`crop=${window_width}:${window_height}:${x_pos}:${y_pos}`,
`${path.join(dir,'cropped_video.mp4')}`],
(error, stdout, stderr) => {
if (error) {
console.log(error.message)
}
if (stderr) {
console.log(stderr)
}
console.log("Cropped File created")
})
child_object.on("close", async
() => {
try { video_buffer = await fsPromises.readFile(path.join(dir, "cropped_video.mp4")
// To renderer
mainWindow.webContents.send("main:video_buffer", video_buffer)
} catch (error) {
log(error)
} finally {
process.noAsar = true
fs.rmdir(temp_directory, {recursive: true}, (error) => {if (error) {log(error)}})
console.log("Done !!!")
process.noASAR = false
}
})