[Bf-blender-cvs] [89565e06310] lanpr-under-gp: Fix unreported Eisenbug leading to a crash when reading a blend file.

[Bastien Montagne]

Commit: 89565e06310356118858858e7720f204e845f980
Author: Bastien Montagne
Date:   Tue Sep 29 16:13:42 2020 +0200
Branches: lanpr-under-gp
https://developer.blender.org/rB89565e06310356118858858e7720f204e845f980

Fix unreported Eisenbug leading to a crash when reading a blend file.

This took more than a day to fully investigate and understand, one of
the reasons being that the probability of the issue to show up was
extremely low, and subjected to very specific random factors.

Root of the issue is that, in some very rare cases, a newly read ID
might get the exact same memory address as the one it had when the blend
file was saved.

In that case, `BKE_workspace_active_set` would return immediately, since
the pointer to the active workspace would remain unchanged. But that
lead to having an unset NULL active layout pointer, which would crash
when attempting to get e.g. the active screen.

For the record, I ran into this when running a specific build (master
with one flag added to the `LIB_ID_CREATE` ones, with value `1 << 3`),
using a specific set of options (`--background --factory-startup -noaudio`),
and passing the .blend file from T80090 as argument.

===================================================================

M	source/blender/blenkernel/intern/workspace.c

===================================================================

diff --git a/source/blender/blenkernel/intern/workspace.c b/source/blender/blenkernel/intern/workspace.c
index e5e6a5d67c8..324c8db0fe9 100644
--- a/source/blender/blenkernel/intern/workspace.c
+++ b/source/blender/blenkernel/intern/workspace.c
@@ -429,9 +429,13 @@ WorkSpace *BKE_workspace_active_get(WorkSpaceInstanceHook *hook)
 }
 void BKE_workspace_active_set(WorkSpaceInstanceHook *hook, WorkSpace *workspace)
 {
-  if (hook->active == workspace) {
-    return;
-  }
+  /* DO NOT check for `hook->active == workspace` here. Caller code is supposed to do it if
+   * that optimization is possible and needed.
+   * This code can be called from places where we might have this equality, but still want to
+   * ensure/update the active layout below.
+   * Known case where this is buggy and will crash later due to NULL active layout: reading
+   * a blend file, when the new read workspace ID happens to have the exact same memory address
+   * as when it was saved in the blend file (extremely unlikely, but possible). */
 
   hook->active = workspace;
   if (workspace) {