From 170d363743c5097c7a94443599d1f7ae3d18f037 Mon Sep 17 00:00:00 2001
From: Frank Mittelbach <frank.mittelbach@latex-project.org>
Date: Thu, 2 Jan 2025 22:51:32 +0100
Subject: [PATCH] fix for #1604

---
 base/changes.txt                       |  5 +++++
 base/ltclass.dtx                       | 18 +++++++++++++++---
 base/testfiles-lthooks/lthooks-034.lvt |  3 +++
 base/testfiles-lthooks/lthooks-034.tlg |  2 ++
 4 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/base/changes.txt b/base/changes.txt
index 663354dfe..f92cbcd88 100644
--- a/base/changes.txt
+++ b/base/changes.txt
@@ -6,6 +6,11 @@ to completeness or accuracy and it contains some references to files that are
 not part of the distribution.
 ================================================================================
 
+2025-01-02  Frank Mittelbach  <Frank.Mittelbach@latex-project.org>
+
+	* ltclass.dtx (subsection{Hooks}):
+	Do not make \AtBeginDocument preamble only (gh/1604)
+
 2024-12-27  Frank Mittelbach  <Frank.Mittelbach@latex-project.org>
 
 	* ltsockets.dtx:
diff --git a/base/ltclass.dtx b/base/ltclass.dtx
index 8fa1acf39..3e7aff03c 100644
--- a/base/ltclass.dtx
+++ b/base/ltclass.dtx
@@ -33,7 +33,7 @@
 %<*driver>
 % \fi
 \ProvidesFile{ltclass.dtx}
-             [2024/08/28 v1.5m LaTeX Kernel (Class & Package Interface)]
+             [2025/01/02 v1.5n LaTeX Kernel (Class & Package Interface)]
 % \iffalse
 \documentclass{ltxdoc}
 \GetFileInfo{ltclass.dtx}
@@ -2808,8 +2808,20 @@
 %<*2ekernel>
 %    \end{macrocode}
 %
-%    \begin{macrocode}
-\@onlypreamble\AtBeginDocument
+%    In its initial implementation (not using the hook system)
+%    \cs{AtBeginDocument} was made \cs{@preambleonly} because using it
+%    later had no effect whatsoever, thus was most certainly an
+%    unintended programming error. With the reimplementation, using
+%    the \texttt{begindocument} hook internally, this has changed
+%    because adding to a onetime hook after it has already been used
+%    simply executes the additional code immediately. We therefore no
+%    longer generate an error if it is used inside the document so
+%    that \verb=\AddToHook{begindocument}= and \cs{AtBeginDocument}
+%    are truly equivalent (as claimed in the hook documentation).
+% \changes{v1.5n}{2025/01/02}{Do not make \cs{AtBeginDocument}
+%    preamble only (gh/1604)}
+%    \begin{macrocode}
+%\@onlypreamble\AtBeginDocument
 %    \end{macrocode}
 % \end{macro}
 % \end{macro}
diff --git a/base/testfiles-lthooks/lthooks-034.lvt b/base/testfiles-lthooks/lthooks-034.lvt
index 2f3d44a04..c7b2637a4 100644
--- a/base/testfiles-lthooks/lthooks-034.lvt
+++ b/base/testfiles-lthooks/lthooks-034.lvt
@@ -20,4 +20,7 @@
 \typeout{====== E}
 \UseHook{foo}
 
+\AtBeginDocument{\typeout{--> A (test for gh/1604)}}
+\AddToHook{begindocument}{\typeout{--> B (test for gh/1604)}}
+
 \END
diff --git a/base/testfiles-lthooks/lthooks-034.tlg b/base/testfiles-lthooks/lthooks-034.tlg
index cd959fdb8..2d66c4247 100644
--- a/base/testfiles-lthooks/lthooks-034.tlg
+++ b/base/testfiles-lthooks/lthooks-034.tlg
@@ -36,3 +36,5 @@ l. ...\ShowHook{foo}
 ====== E
 pkgb
 pkga
+--> A (test for gh/1604)
+--> B (test for gh/1604)