Good news : update worked flawlessly but bad news : i cannot reinstall any extension as this command for example :
/var/www/flarum# sudo -u flarum php7.3 composer.phar require clarkwinkelmann/flarum-ext-comicsans
gives this output :
Using version ^1.0 for clarkwinkelmann/flarum-ext-comicsans
./composer.json has been updated
Running composer update clarkwinkelmann/flarum-ext-comicsans
Loading composer repositories with package information
Updating dependencies
Lock file operations: 1 install, 0 updates, 0 removals
- Locking clarkwinkelmann/flarum-ext-comicsans (1.0.0)
Installation failed, reverting ./composer.json and ./composer.lock to their original content.
[ErrorException]
file_put_contents(./composer.lock): failed to open stream: Permission denied
require [--dev] [--dry-run] [--prefer-source] [--prefer-dist] [--fixed] [--no-suggest] [--no-progress] [--no-update] [--no-install] [--no-scripts] [--update-no-dev] [-w|--update-with-dependencies] [-W|--update-with-all-dependencies] [--with-dependencies] [--with-all-dependencies] [--ignore-platform-req IGNORE-PLATFORM-REQ] [--ignore-platform-reqs] [--prefer-stable] [--prefer-lowest] [--sort-packages] [-o|--optimize-autoloader] [-a|--classmap-authoritative] [--apcu-autoloader] [--apcu-autoloader-prefix APCU-AUTOLOADER-PREFIX] [--] [<packages>]...