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--- misc/Python-2.6.1/Makefile.pre.in.fix-parallel-make 2010-07-22 15:01:39.567996932 -0400
+++ misc/build/Python-2.6.1/Makefile.pre.in 2010-07-22 15:47:02.437998509 -0400
@@ -207,6 +207,7 @@ SIGNAL_OBJS= @SIGNAL_OBJS@
##########################################################################
# Grammar
+GRAMMAR_STAMP= $(srcdir)/grammar-stamp
GRAMMAR_H= $(srcdir)/Include/graminit.h
GRAMMAR_C= $(srcdir)/Python/graminit.c
GRAMMAR_INPUT= $(srcdir)/Grammar/Grammar
@@ -530,10 +531,24 @@ Modules/getpath.o: $(srcdir)/Modules/get
Modules/python.o: $(srcdir)/Modules/python.c
$(MAINCC) -c $(PY_CFLAGS) -o $@ $(srcdir)/Modules/python.c
+# GNU "make" interprets rules with two dependents as two copies of the rule.
+#
+# In a parallel build this can lead to pgen being run twice, once for each of
+# GRAMMAR_H and GRAMMAR_C, leading to race conditions in which the compiler
+# reads a partially-overwritten copy of one of these files, leading to syntax
+# errors (or linker errors if the fragment happens to be syntactically valid C)
+#
+# See http://www.gnu.org/software/hello/manual/automake/Multiple-Outputs.html
+# for more information
+#
+# Introduce ".grammar-stamp" as a contrived single output from PGEN to avoid
+# this:
+$(GRAMMAR_H) $(GRAMMAR_C): $(GRAMMAR_STAMP)
-$(GRAMMAR_H) $(GRAMMAR_C): $(PGEN) $(GRAMMAR_INPUT)
+$(GRAMMAR_STAMP): $(PGEN) $(GRAMMAR_INPUT)
-@$(INSTALL) -d Include
-$(PGEN) $(GRAMMAR_INPUT) $(GRAMMAR_H) $(GRAMMAR_C)
+ touch $(GRAMMAR_STAMP)
$(PGEN): $(PGENOBJS)
$(CC) $(OPT) $(LDFLAGS) $(PGENOBJS) $(LIBS) -o $(PGEN)
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