eval-expression on (setq helm-debug t) does not change describe-variable output

Question

I'm running this version of Emacs:

GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.11) of 2019-09-23, modified by Debian

Installed on Ubuntu 19.10 via apt, and not built from source.

I'm trying to set the helm-debug variable so that I can see what Helm is doing to debug a problem (helm-locate is not showing files that exist in a directory whose parent element has .git in it).

So in the past, I could use M-: (mapped to eval-expression) and type in an expression, which I did with this:

(progn (setq helm-debug 'xxx) (message "helm-debug == %S" helm-debug))

And I get what I expect to see in the message area of:

"helm-debug == xxx"

And, I go back in and run M-: again to check on the value like this:

helm-debug

I see xxx as the resulting value. All good so far.

But then, when I type C-h v (mapped to describe-variable), and then type in helm-debug, and I see this:

helm-debug is a variable defined in ‘helm.el’.
Its value is nil

Documentation:
If non-‘nil’, write log message to ‘helm-debug-buffer’.
Default is ‘nil’, which disables writing log messages because the
size of ‘helm-debug-buffer’ grows quickly.

Why is it reporting nil, when I have clearly set it inside the eval-expression call?

For reference, the defvar for this variable resides in ~/.emacs.d/elpa/helm-core-20191013.626/helm.el in my setup (meaning Helm mode has been recently upgraded)

(defvar helm-debug nil
  "If non-`nil', write log message to `helm-debug-buffer'.
Default is `nil', which disables writing log messages because the
size of `helm-debug-buffer' grows quickly.")

Answer 1

It seems it's intended:

helm-internal calls 
  helm-log-save-maybe calls 
    (setq helm-debug nil)

thus whenever a helm session ends, helm-debug will be reset to nil. I guess it's because the log buffer can grow too quickly.

Instead of changing helm-debug directly, according to helm's own help (you can get it by type C-h m within any helm session, I have pasted the related section below), you can use C-h C-d:

Debugging Helm

Helm exposes the special variable helm-debug: setting it to non-nil will enable Helm logging in a special outline-mode buffer. Helm resets the variable to nil at the end of each session.

For convenience, C-h C-d allows you to turn on debugging for this session only. To avoid accumulating log entries while you are typing patterns, you can use C-! to turn off updating. When you are ready turn it on again to resume logging.

Once you exit your Helm session you can access the debug buffer with helm-debug-open-last-log. It is possible to save logs to dated files when helm-debug-root-directory is set to a valid directory.

Note: Be aware that Helm log buffers grow really fast, so use helm-debug only when needed.