Fix fastdev divergence bug

src/Makefile.am

@@ -75,9 +75,11 @@ MAIN_FILES = $(GENERATED_VERSION_SOURCES) main/main.cpp
 
 if BUILD_TESTS
 stellar_core_SOURCES = $(MAIN_FILES) $(SRC_CXX_FILES) $(SRC_TEST_CXX_FILES)
+CARGO_FEATURE_FUZZ_TARGETS = --features fuzz_targets
 CARGO_FEATURE_TESTUTILS = --features testutils
 else # !BUILD_TESTS
 stellar_core_SOURCES = $(MAIN_FILES) $(SRC_CXX_FILES)
+CARGO_FEATURE_FUZZ_TARGETS =
 CARGO_FEATURE_TESTUTILS =
 endif # !BUILD_TESTS
 
@@ -310,17 +312,24 @@ $(SOROBAN_BUILD_DIR)/%/target/git-state.txt: $(top_srcdir)/.git/modules/src/rust
 	printf '%s\n' "$$state" > $@.tmp; \
 	if cmp -s $@.tmp $@; then rm -f $@.tmp; else mv -f $@.tmp $@; fi
 
-# The fastdev rust build is a special non-production mode that builds all of
-# the rust dependencies of librust_stellar_core.a through a single cargo
-# invocation, which is actually the "normal" way cargo operates, but which also
-# has the negative side effect of resolving (merging) different point releases
-# and pre-release minor versions across transitive dependencies, which means we
+# The fastdev rust build is a special non-production mode that builds all of the
+# rust dependencies of librust_stellar_core.a through a single cargo invocation,
+# which is actually the "normal" way cargo operates, but which also has the
+# negative side effect of resolving (merging) different point releases and
+# pre-release minor versions across transitive dependencies, which means we
 # can't control the _exact_ transitive dependencies as well as we'd like.
 #
-# So we only use the fastdev rust build for certain special cases such as
-# testing with asan/tsan (they seem to only work well when built this way) and
-# use the non-unified build (with separate .a files for each separate soroban
-# version) for production builds.
+# So we only use the fastdev rust build for fast iteration while doing
+# development, or for special cases like testing with asan/tsan (they seem to
+# only work well when built this way), and use the non-unified build (with
+# separate .a files for each separate soroban version) for production builds.
+#
+# One important caveat: the $(CARGO_FEATURE_FUZZ_TARGETS) feature should _not_
+# be passed to fastdev mode. It pulls in a separate copy of the soroban host
+# with `testutils` enabled, which winds up up causing `testutils` to be enabled
+# _on the production soroban host_ as well, which causes the build to diverge if
+# run against mainnet. Leave $(CARGO_FEATURE_FUZZ_TARGETS) for the non-fastdev
+# build.
 
 if ENABLE_FASTDEV_UNSAFE_FOR_PRODUCTION
 
@@ -461,7 +470,7 @@ $(LIBRUST_STELLAR_CORE): $(RUST_HOST_DEPFILES) $(SRC_RUST_FILES) $(ALL_SOROBAN_L
 		--locked \
 		--target-dir $(abspath $(RUST_TARGET_DIR)) \
 		$(CARGO_FEATURE_TRACY) $(CARGO_FEATURE_NEXT) $(CARGO_FEATURE_TESTUTILS) \
-		$(CARGO_FEATURE_TCMALLOC) \
+		$(CARGO_FEATURE_TCMALLOC) $(CARGO_FEATURE_FUZZ_TARGETS) \
 		-- \
 		$(ALL_SOROBAN_EXTERN_ARGS) \
 		$(ALL_SOROBAN_DEPEND_ARGS)

src/rust/Cargo.toml

@@ -227,10 +227,20 @@ tcmalloc = []
 # staging work across intra-protocol releases.
 next = []
 
-
 # The testutils feature turns on stuff that should only be available in a core
 # BUILD_TESTS build, such as the code to support transaction re-execution on a
-# secondary host, AND enables the soroban-fuzz-targets dependency. This allows
-# fuzz smoke tests to run in normal test builds without requiring the full fuzz
-# _instrumentation_ infrastructure.
-testutils = ["dep:soroban-fuzz-targets"]
+# secondary host.
+
+testutils = []
+
+# Enables the soroban-fuzz-targets dependency. This may be enabled either in
+# full fuzz-instrumentation builds, or even just during normal test-enabled
+# builds in which the core unit tests will run soroban fuzz targets as smoke
+# tests.
+#
+# However, this should _not_ be enabled in a fastdev build, because the
+# soroban-fuzz-targets crate will pull in its own copy of the soroban host _with
+# testutils enabled_ (for recording), and since fastdev is a unified build, this
+# will turn on testutils in the accompanying _production soroban host_, which
+# will cause it to diverge if run on mainnet.
+fuzz_targets = ["dep:soroban-fuzz-targets"]

src/rust/src/soroban_fuzz.rs

@@ -21,18 +21,18 @@
 use crate::rust_bridge::FuzzResultCode;
 
 // Stub implementation when testutils feature is disabled
-#[cfg(not(feature = "testutils"))]
+#[cfg(not(feature = "fuzz_targets"))]
 pub fn run_soroban_fuzz_target(_name: &str, _data: &[u8]) -> FuzzResultCode {
     FuzzResultCode::FUZZ_DISABLED
 }
 
 // When testutils feature is enabled, import and use the actual implementations
-#[cfg(feature = "testutils")]
+#[cfg(feature = "fuzz_targets")]
 use soroban_fuzz_targets::{self as fuzz, FuzzResult};
 
 /// Run a Soroban fuzz target with the given input bytes.
 /// Panics on internal errors (which is what the fuzzer wants to find!)
-#[cfg(feature = "testutils")]
+#[cfg(feature = "fuzz_targets")]
 pub fn run_soroban_fuzz_target(name: &str, data: &[u8]) -> FuzzResultCode {
     let result = match name {
         "soroban_expr" => fuzz::expr::run_fuzz_target(data),