[make] fix Makefile bug in building models

Summary:
Nuking the specs then building the models was not a great idea: the models do
not look at specs but only at some dummy marker files, eg
infer/lib/specs/c_models, so they don't necessarily realize that they need to
be rebuilt when the specs have been nuked!

One easy workaround would be to also delete the marker files, but then we would
*always* rebuild the models when building infer. Not good.

The solution here is to nuke the specs and marker files only when the clang
dependencies change, then rebuild all the models.

Reviewed By: jberdine, jeremydubreil

Differential Revision: D4781424

fbshipit-source-id: 2d2606e

.gitignore

@@ -98,6 +98,7 @@ buck-out/
 /infer/models/cpp/out/
 /infer/models/objc/out/
 /infer/lib/specs/c_models
+/infer/lib/specs/clang_models
 /infer/lib/specs/cpp_models
 /infer/lib/specs/objc_models
 /infer/lib/specs/clean_models

Makefile

@@ -120,9 +120,6 @@ infer: src_build
 ifeq ($(BUILD_JAVA_ANALYZERS),yes)
 	$(QUIET)$(MAKE) -C $(ANNOTATIONS_DIR)
 endif
-#	Delete existing specs so that they are not used during the analysis of models. Infer may
-#	segfault in some cases otherwise.
-	$(QUIET)$(MAKE) -C $(MODELS_DIR) clean_specs
 	$(QUIET)$(MAKE) -C $(MODELS_DIR) all
 
 .PHONY: clang_setup

infer/models/Makefile

@@ -13,6 +13,8 @@ CPP_MODELS_DIR = cpp
 JAVA_MODELS_DIR = java
 OBJC_MODELS_DIR = objc
 
+CLANG_MODELS_FILE = $(SPECS_LIB_DIR)/clang_models
+
 CLANG_SUBDIRS = $(C_MODELS_DIR) $(CPP_MODELS_DIR)
 ifneq (no, $(XCODE_SELECT))
 CLANG_SUBDIRS += $(OBJC_MODELS_DIR)
@@ -29,12 +31,29 @@ endif
 .PHONY: java clang clean clean_specs $(CLANG_SUBDIRS)
 
 clean_specs:
-	$(REMOVE) $(SPECS_LIB_DIR)/*.specs
+	$(REMOVE) $(SPECS_LIB_DIR)/*.specs $(C_MODELS_FILE) $(CPP_MODELS_FILE) $(OBJC_MODELS_FILE)
 
 $(CLANG_SUBDIRS):
 	$(QUIET)$(MAKE) -C $@ install
 
-clang: $(CLANG_SUBDIRS)
+# The clang deps have changed, so the models need to be rebuilt. If infer itself has changed, we
+# need to nuke the previous specs files in case the serialization has changed, otherwise we might
+# encounter a segfault reading them.
+$(CLANG_MODELS_FILE): $(CLANG_DEPS_NO_MODELS)
+	$(QUIET)$(MAKE) clean_specs
+#	Since [CLANG_DEPS_NO_MODELS] is newer than [CLANG_MODELS_FILE], it is also newer than the
+#	previously built clang models, so they will all be rebuilt by the subdirs' Makefiles.
+	$(QUIET)$(MAKE) $(CLANG_SUBDIRS)
+#	Maintain the invariant that [CLANG_MODELS_FILE] is newer than the clang models.
+	$(QUIET)touch $@
+
+clang: $(CLANG_MODELS_FILE)
+#	Run the sub-makes unconditionally since they know more about when they need to be
+#	rebuilt. For instance, they need to be rebuilt when their source files change, which is not
+#	taken into account by the dependencies of the $(CLANG_MODELS_FILE) target. If we rebuilt
+#	$(CLANG_MODELS_FILE) already, then the command below will be a no-op (so we don't rebuild
+#	the models twice).
+	$(QUIET)$(MAKE) $(CLANG_SUBDIRS)
 
 java:
 	$(QUIET)$(MAKE) -C $@ install