Makefile: separate meson rerun from the rest of the ninja invocation

The rules to build Makefile.mtest are suffering from the "tunnel vision"
problem that is common with recursive makefiles.  Makefile.mtest depends
on build.ninja, but Make does not know when build.ninja needs to be
rebuilt before creating Makefile.mtest.

To fix this, separate the ninja invocation into the "regenerate build
files" phase and the QEMU build phase.  Sentinel files such as
meson-private/coredata.dat or build.ninja are used to figure out the
phases that haven't run yet; however, because those files' timestamps
are not guaranteed to be touched, the usual makefile stamp-file trick
is used on top.

Reported-by: Havard Skinnemoen <hskinnemoen@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Makefile

@@ -92,6 +92,8 @@ endif
 ifeq ($(NINJA),)
 .PHONY: config-host.mak
 x := $(shell rm -rf meson-private meson-info meson-logs)
+else
+export NINJA
 endif
 ifeq ($(wildcard build.ninja),)
 .PHONY: config-host.mak
@@ -100,31 +102,44 @@ endif
 
 # 1. ensure config-host.mak is up-to-date
 config-host.mak: $(SRC_PATH)/configure $(SRC_PATH)/pc-bios $(SRC_PATH)/VERSION
-	@echo $@ is out-of-date, running configure
+	@echo config-host.mak is out-of-date, running configure
 	@if test -f meson-private/coredata.dat; then \
 	  ./config.status --skip-meson; \
 	else \
-	  ./config.status; \
+	  ./config.status && touch build.ninja.stamp; \
 	fi
 
-# 2. ensure generated build files are up-to-date
+# 2. meson.stamp exists if meson has run at least once (so ninja reconfigure
+# works), but otherwise never needs to be updated
+meson-private/coredata.dat: meson.stamp
+meson.stamp: config-host.mak
+	@touch meson.stamp
+
+# 3. ensure generated build files are up-to-date
 
 ifneq ($(NINJA),)
-# A separate rule is needed for Makefile dependencies to avoid -n
-export NINJA
 Makefile.ninja: build.ninja
-	$(quiet-@){ echo 'ninja-targets = \'; $(NINJA) -t targets all | sed 's/:.*//; $$!s/$$/ \\/'; } > $@
+	$(quiet-@){ \
+	  echo 'ninja-targets = \'; \
+	  $(NINJA) -t targets all | sed 's/:.*//; $$!s/$$/ \\/'; \
+	  echo 'build-files = \'; \
+	  $(NINJA) -t query build.ninja | sed -n '1,/^  input:/d; /^  outputs:/q; s/$$/ \\/p'; \
+	} > $@.tmp && mv $@.tmp $@
 -include Makefile.ninja
+
+# A separate rule is needed for Makefile dependencies to avoid -n
+build.ninja: build.ninja.stamp
+build.ninja.stamp: meson.stamp $(build-files)
+	$(NINJA) $(if $V,-v,) build.ninja && touch $@
 endif
 
 ifneq ($(MESON),)
-# The dependency on config-host.mak ensures that meson has run
-Makefile.mtest: build.ninja scripts/mtest2make.py config-host.mak
+Makefile.mtest: build.ninja scripts/mtest2make.py
 	$(MESON) introspect --targets --tests --benchmarks | $(PYTHON) scripts/mtest2make.py > $@
 -include Makefile.mtest
 endif
 
-# 3. Rules to bridge to other makefiles
+# 4. Rules to bridge to other makefiles
 
 ifneq ($(NINJA),)
 NINJAFLAGS = $(if $V,-v,) \
@@ -135,7 +150,10 @@ ninja-cmd-goals = $(or $(MAKECMDGOALS), all)
 ninja-cmd-goals += $(foreach t, $(.tests), $(.test.deps.$t))
 
 makefile-targets := build.ninja ctags TAGS cscope dist clean uninstall
-ninja-targets := $(filter-out $(makefile-targets), $(ninja-targets))
+# "ninja -t targets" also lists all prerequisites.  If build system
+# files are marked as PHONY, however, Make will always try to execute
+# "ninja build.ninja".
+ninja-targets := $(filter-out $(build-files) $(makefile-targets), $(ninja-targets))
 .PHONY: $(ninja-targets) run-ninja
 $(ninja-targets): run-ninja
 
@@ -214,7 +232,7 @@ distclean: clean
 	rm -f qemu-plugins-ld.symbols qemu-plugins-ld64.symbols
 	rm -f *-config-target.h *-config-devices.mak *-config-devices.h
 	rm -rf meson-private meson-logs meson-info compile_commands.json
-	rm -f Makefile.ninja Makefile.mtest
+	rm -f Makefile.ninja Makefile.mtest build.ninja.stamp meson.stamp
 	rm -f config.log
 	rm -f linux-headers/asm
 	rm -Rf .sdk