Issue #27229: Fix in-tree cross-build rule, by Xavier de Gaye

Makefile.pre.in

@@ -793,13 +793,16 @@ $(GRAMMAR_H): $(GRAMMAR_INPUT) $(PGEN)
 	if test "$(cross_compiling)" != "yes"; then \
 		$(PGEN) $(GRAMMAR_INPUT) $(GRAMMAR_H) $(GRAMMAR_C); \
 	else \
-		cp $(srcdir)/Include/graminit.h $(GRAMMAR_H); \
+		# Avoid copying the file onto itself for an in-tree build \
+		cp $(srcdir)/Include/graminit.h $(GRAMMAR_H).tmp; \
+		mv $(GRAMMAR_H).tmp $(GRAMMAR_H); \
 	fi
 $(GRAMMAR_C): $(GRAMMAR_H)
 	if test "$(cross_compiling)" != "yes"; then \
 		touch $(GRAMMAR_C); \
 	else \
-		cp $(srcdir)/Python/graminit.c $(GRAMMAR_C); \
+		cp $(srcdir)/Python/graminit.c $(GRAMMAR_C).tmp; \
+		mv $(GRAMMAR_C).tmp $(GRAMMAR_C); \
 	fi
 
 $(PGEN): $(PGENOBJS)

Misc/NEWS

@@ -581,6 +581,9 @@ Build
   source code anyway, and is still regenerated when doing a native build.
   Patch by Xavier de Gaye.
 
+- Issue #27229: Fix the cross-compiling pgen rule for in-tree builds.  Patch
+  by Xavier de Gaye.
+
 - Issue #21668: Link audioop, _datetime, _ctypes_test modules to libm,
   except on Mac OS X. Patch written by Xavier de Gaye.