[Libguestfs] [PATCH] build: Fix haskell dependencies and parallel build

[Matthew Booth]

Without this change all the tests in the haskell bindings are rebuilt
every time. The primary motivation for this change is to fix this.

The fix for the above also allows parallel builds to be re-enabled.
---
 haskell/Makefile.am | 56 +++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 41 insertions(+), 15 deletions(-)

diff --git a/haskell/Makefile.am b/haskell/Makefile.am
index 531e404..5328ca0 100644
--- a/haskell/Makefile.am
+++ b/haskell/Makefile.am
@@ -46,25 +46,51 @@ $(all_targets): $(top_builddir)/src/libguestfs.la
 
 all: $(all_targets)
 
-Guestfs.o: $(srcdir)/Guestfs.hs
-	$(GHC)  $(GHCFLAGS) -c $< -o $@
+built_tests = Bindtests Guestfs010Load Guestfs030Config Guestfs050LVCreate
 
-Bindtests: $(srcdir)/Bindtests.hs Guestfs.o
-	$(GHC)  $(GHCFLAGS) -main-is $(shell basename $@) --make -o $@ $< -lguestfs
+# Building with ghc --make doesn't work properly here because it
+# always rebuilds Guestfs.o despite it being up to date. So if you:
+#
+# * build Guestfs010Load, then build it again, the second time it will
+# not be rebuilt.
+#
+# * build Guestfs010Load, then build Guestfs030Config, then build
+# Guestfs010Load again, it will be rebuilt every time.
+#
+# In the second case, building Guestfs030Config rebuilt Guestfs.o. As
+# this is a dependency of Guestfs010Load, Guestfs010Load is now
+# unnecessarily out of date.
+#
+# Because the default target builds all of the above, they will all be
+# rebuilt every time.
+#
+# An obvious choice would be to remove the Guestfs.o dependency, but
+# this would potentially result in corruption during a parallel build
+# as multiple ghc processes rebuild Guestfs.o simultaneously. I had
+# hoped that the solution below would work. It correctly builds
+# Guestfs010Load and Guestfs030Config, but Guestfs050LVCreate requires
+# additional link options which I haven't been able to work out.
+
+#Guestfs.o: $(srcdir)/Guestfs.hs
+#	$(GHC)  $(GHCFLAGS) -c $< -o $@
+#
+#$(built_tests:%=%.o): %.o: %.hs Guestfs.o
+#	$(GHC)  $(GHCFLAGS) -main-is $(basename $<) -c $< -o $@
+#
+#$(built_tests): %: %.o Guestfs.o
+#	$(GHC)  $(GHCFLAGS) -main-is $@ -o $@ $< Guestfs.o -lguestfs
 
-Guestfs010Load: $(srcdir)/Guestfs010Load.hs Guestfs.o
-	$(GHC)  $(GHCFLAGS) -main-is $(shell basename $@) --make -o $@ $< -lguestfs
+# The solution below isn't ideal. It uses --make and avoids the
+# parallel make problem by putting object files in separate
+# directories per target.
 
-Guestfs030Config: $(srcdir)/Guestfs030Config.hs Guestfs.o
-	$(GHC)  $(GHCFLAGS) -main-is $(shell basename $@) --make -o $@ $< -lguestfs
+$(built_tests): %: %.hs Guestfs.hs
+	$(GHC)  $(GHCFLAGS) --make -main-is $@ -odir .$@ -o $@ $< Guestfs.hs -lguestfs
 
-Guestfs050LVCreate: $(srcdir)/Guestfs050LVCreate.hs Guestfs.o
-	$(GHC)  $(GHCFLAGS) -main-is $(shell basename $@) --make -o $@ $< -lguestfs
 
-CLEANFILES = $(all_targets) *~ *.hi *.o
+CLEANFILES = $(all_targets) *~ *.hi *.o test.img
 
-endif
+clean-local:
+	-rm -rf $(built_tests:%=.%)
 
-# Building in parallel causes linker corruption errors since it
-# appears that multiple runs of ghc conflict with each other.
-.NOTPARALLEL:
+endif
-- 
1.8.1