bug#54825: 29.0.50; Relax mutual dependency between erc and erc-backend

[J.P.]

Tags: patch

Someone on Libera.Chat recently reported that a recipe similar to the
following results in ERC-occupied portions of the mode-line breaking
down.

  (setq erc-email-userid  "tester/foonet")

  (with-eval-after-load 'erc
    (erc-update-modules)) ; [1]

  (erc
   :server "127.0.0.1"
   :port 6670
   :nick "tester"
   :password "changeme")

Such behavior may be unsurprising to folks familiar with ERC (its root
cause being the interdependency between erc.el and erc-backend.el). But
others may find it unsettling to have a user-facing library only
partially loaded when its namesake feature is provided, not to mention
top-level forms evaluated multiple times. While this situation is
documented in code comments and its possibility warned about in the
Emacs manual, attempting to alleviate some of the attendant awkwardness
seems a worthy endeavor (IMO).

Attached are some traces showing the loading sequence before and after
the proposed changes (which also do away with the surprise produced by
the snippet shown above). As always, superior solutions are welcome.
Thanks.


[1] Please ignore the fact that calling `erc-update-modules' here is
    superfluous. It could just as well be anything that depends on
    library functions provided by ERC.


In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.31, cairo 
version 1.17.4)
 of 2022-04-05 built on localhost
Repository revision: e2fb5ecaea67497224455fdbfe4850a5a74c9d00
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: Fedora Linux 35 (Workstation Edition)

Configured using:
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Load-path shadows:
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Memory information:
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0001-Rework-mutual-dependency-between-erc-and-erc-backend.patch
Description: Text Data

after.log
Description: Text document

before.log
Description: Text document