proc_open is in whitelist_functions but not available in App Engine

Question

I am trying to deploy a Laravel application to App Engine Flexible environment with a custom PHP runtime. When I try to generate a PDF on the site (using wkhtmltopdf with the laravel-snappy wrapper) I get the error: (1/1) LogicException

The Process class relies on proc_open, which is not available on your PHP installation.
in Process.php line 143 
at Process->__construct(array(), null, null, null, 60.0)in Process.php line 195
at Process::fromShellCommandline('/app/vendor/h4cc/wkhtmltopdf-amd64/bin/wkhtmltopdf-amd64 --lowquality \'/tmp/knp_snappy5dc8497d489631.19576754.html\' \'/tmp/knp_snappy5dc8497d489f90.93505253.pdf\'', null, null)in AbstractGenerator.php line 520
at AbstractGenerator->executeCommand('/app/vendor/h4cc/wkhtmltopdf-amd64/bin/wkhtmltopdf-amd64 --lowquality \'/tmp/knp_snappy5dc8497d489631.19576754.html\' \'/tmp/knp_snappy5dc8497d489f90.93505253.pdf\'')in AbstractGenerator.php line 176

The thing is, though, I have proc_open and proc_closed listed in the whitelist_functions line in my app.yaml.

Here is a snippet from my app.yaml:

runtime:         custom
env:             flex
api_version:    1

runtime_config:
  document_root:  public
  whitelist_functions: proc_open,proc_close

For the sake of trying other approaches, I've also tried including it in a php.ini:

google_app_engine.enable_functions = "php_sapi_name,php_uname,getmypid,proc_open, proc_close"
google_app_engine.disable_functions ="exec,passthru,shell_exec,show_source,symlink,system"

Yet I still get the error above.

In case it's relevant, I'll also show my Dockerfile:

FROM gcr.io/google-appengine/php

COPY ./ /app
RUN mkdir -p /app/storage/framework/cache \
    mkdir -p /app/storage/framework/cache/data \
    mkdir -p /app/storage/framework/views \
    mkdir -p /app/storage/framework/sessions \
    mkdir -p /app/storage/logs
RUN chmod -R a+rwX /app/storage
RUN chmod a+rwx /app/vendor/h4cc/wkhtmltopdf-amd64/bin/wkhtmltopdf-amd64
ENV DOCUMENT_ROOT /app/public

RUN apt-get update && apt-get -y install libfontconfig1 libxrender1 libxext6 libssl-dev libssl1.0 libjpeg62 libpng16-16

EXPOSE 8080

... and the relevant bits from my composer.json:

"require": {
    "php": ">=7.2.9",
    "laravel/framework": "^6.0",
    "google/cloud": "^0.24.0",
    "superbalist/laravel-google-cloud-storage": "^2.0",
    "barryvdh/laravel-snappy": "^0.4.3",
    "h4cc/wkhtmltopdf-amd64": "0.12.x",
    "h4cc/wkhtmltoimage-amd64": "0.12.x"
},

Am I missing something?

Update: I'm not sure if this is related, or just another error I'll need to deal with next, but if I attempt to run wkhtmltopdf directly from the command line, I get error while loading shared libraries: libssl.so.1.1: cannot open shared object file: No such file or directory But I can't install that because it does not appear to be available for the version of Linux I'm using (Ubuntu 16.04.6).

Answer 1

You can check this example: How to use the image on App Engine Flexible Environment:

If you need any of those functions, you can add an environment variable WHITELIST_FUNCTIONS.

app.yaml:

runtime: php
vm: true
api_version: 1

env_variables:
  WHITELIST_FUNCTIONS: phpinfo,exec