Log Plugin Activation Output in Debug.log if WP_DEBUG_LOG is enabled

[lordspace]

I just found out that one of my plugins was producing some output but I couldn't find it in the logs.
I dug deeper and noticed that WordPress starts the output buffering and then loads the plugin file but it doesn't do anything with the output except for it checks for its size.

That's in /wp-admin/includes/plugin.php around line 601

This code allowed me to log the errors (installed it as an MU plugin).

add_action('activated_plugin', 'dbg_log_error', 0, 2);

function dbg_log_error($plugin, $network_wide) {
	if (ob_get_length() == 0) {
		return ;
	}
	
	$buff = '';
	$buff .= "Plugin: $plugin\n";
	$buff .= sprintf("Network Wide: %s\n", $network_wide ? 'No' : 'Yes');
	$buff .= "Output:\n";
	$buff .= ob_get_contents() . "\n\n";
	
	file_put_contents(WP_CONTENT_DIR. '/plugin_activation_error.log', $buff, FILE_APPEND);
}

[johnbillion]

One potential problem I see is that a plugin could be outputting a lot of data when it shouldn't, which could make your debug log incomprehensible.

On a related note, there's ​a plugin which shows the unexpected output on activation.

[lordspace]

Yep. Makes sense.
What if the message is truncated at the first 256 bytes?

[DrewAPicture]

This seems to fall squarely in plugin territory. Thanks for the suggestion. Closing as wontfix.