Apps.set_installed_apps() re-executes AppConfig.ready() methods

[Aymeric Augustin]

The current implementation of set_installed_apps() calls populate(), which is going to execute ready() methods of applications.

We should:

• either add a flag to the app registry to track whether ready() has been executed yet,
• or document that ready() must be idempotent.

[Claude Paroz]

A possible fix: ​https://gist.github.com/claudep/82122f524b08385859ab

[Aymeric Augustin]

This is interesting but very tied to set_installed_apps. I was thinking of a flag on the AppConfig instance. It would be simple and easy to reset if needed.

Also I think there's a use case for reexecuting ready() whenever INSTALLED_APPS changes: apps that set up things based on INSTALLED_APPS.

[Claude Paroz]

Then the documentation approach:

diff --git a/docs/ref/applications.txt b/docs/ref/applications.txt
index 829e273..f68ea38 100644
--- a/docs/ref/applications.txt
+++ b/docs/ref/applications.txt
@@ -238,6 +238,15 @@ Methods
         separate from the production settings, ``manage.py test`` would still
         execute some queries against your **production** database!
 
+    .. note::
+
+        In the usual initialization process, the ``ready`` method is only called
+        once by Django. But in some corner cases, particularly in tests which
+        are fiddling with installed applications, ``ready`` might be called more
+        than once. In that case, either write idempotents methods, or put a flag
+        on your ``AppConfig`` classes to prevent re-running code which should
+        be executed exactly one time.
+
 .. _namespace package:
 
 Namespace packages as apps (Python 3.3+)

I purposefully mentioned AppConfig classes, not instances, because I think that populate is recreating new AppConfig instances.

[Aymeric Augustin]

Excellent, can you commit that?

It keeps the code simple and at worst we can adjust the behavior if it proves to be a problem in practice.

[Claude Paroz <claude@…>]

In b8fc167b32926e09d6d73eddc555343360f9c087:

Fixed #22688 -- Documented ready() may be called more than once

[Claude Paroz <claude@…>]

In dbdbda87fc38816c45584ea7a97876078cbffb3a:

[1.7.x] Fixed #22688 -- Documented ready() may be called more than once

Backport of b8fc167b32 from master.