Try stamp files rather than group rules for sequencing (#4478)

I tried to use a clever feature of GNU make (the `&:` rule type) and it
does not seem to cause the sequencing it implies in the manual. We need
some sequencing to avoid running multiple cargos in parallel since they
in turn run rustup and rustup can't handle concurrent execution.

So let us try this with just "more stamp files", the old fashioned way.

.gitignore

@@ -21,6 +21,9 @@
 *.trs
 *.db
 
+# build stamps
+*.stamp
+
 # sourcetrail
 compile_commands.json
 *.srctrlbm

src/Makefile.am

@@ -88,6 +88,7 @@ RUST_TARGET_DIR=$(top_builddir)/target
 RUST_CXXBRIDGE=$(RUST_BIN_DIR)/cxxbridge
 RUST_PROFILE=release
 RUST_DEP_TREE_STAMP=$(RUST_BUILD_DIR)/src/dep-trees/equal-trees.stamp
+SOROBAN_LIBS_STAMP=$(RUST_BUILD_DIR)/soroban/soroban-libs.stamp
 RUST_HOST_DEPFILES=rust/Cargo.toml $(top_srcdir)/Cargo.toml $(top_srcdir)/Cargo.lock $(RUST_DEP_TREE_STAMP)
 LIBRUST_STELLAR_CORE=$(RUST_TARGET_DIR)/$(RUST_PROFILE)/librust_stellar_core.a
 stellar_core_LDADD += $(LIBRUST_STELLAR_CORE) -ldl
@@ -132,13 +133,12 @@ rust/RustBridge.cpp: rust/src/lib.rs $(SRC_RUST_FILES) Makefile $(RUST_CXXBRIDGE
 # check in to this repo as a secondary check on the lockfiles in the soroban
 # submodules. You should still carefully inspect them if they change!
 
-ALL_RUST_EXPECT_DEPTREES=$(foreach proto,$(ALL_SOROBAN_PROTOCOLS),rust/src/dep-trees/$(proto)-expect.txt)
-ALL_RUST_ACTUAL_DEPTREES=$(foreach proto,$(ALL_SOROBAN_PROTOCOLS),$(RUST_BUILD_DIR)/src/dep-trees/$(proto)-actual.txt)
-
-$(RUST_DEP_TREE_STAMP): $(ALL_RUST_EXPECT_DEPTREES) $(ALL_RUST_ACTUAL_DEPTREES) Makefile
+$(RUST_DEP_TREE_STAMP): $(wildcard rust/soroban/*/Cargo.*) Makefile
 	rm -f $@
 	for proto in $(ALL_SOROBAN_PROTOCOLS); \
 	do \
+		cargo tree --manifest-path rust/soroban/$${proto}/Cargo.toml --locked --package soroban-env-host --edges no-dev --target all \
+		| sed -e "s@$(abspath $(top_srcdir))/@@g" > $(RUST_BUILD_DIR)/src/dep-trees/$${proto}-actual.txt ; \
 		if ! diff -u rust/src/dep-trees/$${proto}-expect.txt $(RUST_BUILD_DIR)/src/dep-trees/$${proto}-actual.txt; \
 		then \
 			echo "dep trees differ, please update $${proto}-expect.txt or roll back submodule"; \
@@ -147,15 +147,6 @@ $(RUST_DEP_TREE_STAMP): $(ALL_RUST_EXPECT_DEPTREES) $(ALL_RUST_ACTUAL_DEPTREES)
 	done
 	touch $@
 
-# We sequentialize these calculations to avoid running two rustups at once.
-# We also use a group-target here (&:) to avoid running the rule once-for-each-target.
-$(ALL_RUST_ACTUAL_DEPTREES) &: $(wildcard rust/soroban/*/Cargo.*) Makefile
-	for proto in $(ALL_SOROBAN_PROTOCOLS); \
-	do \
-		cargo tree --manifest-path rust/soroban/$$proto/Cargo.toml --locked --package soroban-env-host --edges no-dev --target all \
-		| sed -e "s@$(abspath $(top_srcdir))/@@g" > $(RUST_BUILD_DIR)/src/dep-trees/$$proto-actual.txt ;\
-	done
-
 # This next build command looks a little weird but it's necessary. We have to
 # provide an auxiliary metadata string (using RUSTFLAGS=-Cmetadata=$*)
 # essentially manually marking-as-different the separate dependency trees
@@ -189,9 +180,8 @@ $(ALL_RUST_ACTUAL_DEPTREES) &: $(wildcard rust/soroban/*/Cargo.*) Makefile
 #
 # We also have to sequentialize the builds of the sorobans after the
 # dep-tree-stamp files and before the stellar-core build, because rustup
-# gets invoked and it is not concurrency-safe. We also use a group-target
-# here (&:) to avoid running the rule once-for-each-target.
-$(ALL_SOROBAN_LIBS) &: $(wildcard rust/soroban/p*/Cargo.lock) Makefile $(RUST_DEP_TREE_STAMP)
+# gets invoked and it is not concurrency-safe.
+$(SOROBAN_LIBS_STAMP): $(wildcard rust/soroban/p*/Cargo.lock) Makefile $(RUST_DEP_TREE_STAMP) $(SRC_RUST_FILES)
 	for proto in $(ALL_SOROBAN_PROTOCOLS) ; \
 	do \
 		FEATURE_FLAGS="" ; \
@@ -215,6 +205,7 @@ $(ALL_SOROBAN_LIBS) &: $(wildcard rust/soroban/p*/Cargo.lock) Makefile $(RUST_DE
 			$$FEATURE_FLAGS \
 		|| exit 1; \
 	done
+	touch $@
 
 # This one is also a bit challenging, though it's _less_ weird. What we're doing
 # here is supplying multiple separate .rlibs -- one for each soroban linked into
@@ -229,7 +220,7 @@ $(ALL_SOROBAN_LIBS) &: $(wildcard rust/soroban/p*/Cargo.lock) Makefile $(RUST_DE
 # them in as separate `--extern` arguments to a slightly-more-manual `cargo
 # rustc` invocation, along with `-L dependency=...` flags to tell cargo where to
 # find indirect deps of those .rlibs.
-$(LIBRUST_STELLAR_CORE): $(RUST_HOST_DEPFILES) $(SRC_RUST_FILES) Makefile $(ALL_SOROBAN_LIBS)
+$(LIBRUST_STELLAR_CORE): $(RUST_HOST_DEPFILES) $(SRC_RUST_FILES) Makefile $(SOROBAN_LIBS_STAMP)
 	CARGO_NET_GIT_FETCH_WITH_CLI=true cargo rustc \
 		--package stellar-core \
 		--$(RUST_PROFILE) \