avoid request stalls during proactive rebuilds

docs/wasm-runtime-architecture.md

@@ -136,6 +136,15 @@ refreshes the epoch deadline before its first guest call. Otherwise a component
 whose compilation outlasted the guest CPU budget would immediately trap in the
 following allocator/relocation call even though no guest loop consumed it.
 
+The proactive compiler and request workers coordinate through a per-unit file
+lock. Unit compilation writes and validates a process-unique temporary wasm
+file, then publishes it with an atomic rename. While another process holds the
+lock for a stale unit, a request may therefore keep using the last complete
+artifact instead of waiting across a transitive rebuild. Once the lock is
+released, normal freshness checks require the new artifact; a failed rebuild
+removes availability and surfaces the compiler error rather than serving the
+old unit indefinitely.
+
 ---
 
 ## 4. The workspace runtime
@@ -346,7 +355,10 @@ header free-functions are `inline`. The wasm backend exposes only declarations
   recycling.
   `scripts/test_cold_component_deadline.sh` separately compiles a deliberately
   cold component that exceeds the development epoch window and proves the
-  parent request still renders it.
+  parent request still renders it. The dependency-invalidation gate also holds
+  parent and child compile locks across a transitive source edit, proves a
+  warmed request returns the last atomically published result without waiting,
+  and then proves the new dependency result appears after rebuild completion.
 
 - **WebSocket end-to-end**: a headless client performs a raw WS handshake to
   `:HTTP_PORT` with path `/site/tests/websockets.ws.uce` (self-resolving

scripts/compile_wasm_unit

@@ -15,6 +15,7 @@ ABI_VERSION=${UCE_UNIT_ABI_VERSION:-6}
 ROOT=$(pwd)
 OBJ_FN="$DEST_DIR/$PP_FN.wasm.o"
 ABI_TMP="$DEST_DIR/$PP_FN.uce-abi.txt"
+WASM_TMP="$DEST_DIR/$WASM_FN.tmp.$$"
 PCH_ENABLED=${UCE_WASM_UNIT_PCH:-1}
 PCH_DIR=${UCE_WASM_PCH_DIR:-/tmp/uce/wasm-w2/pch}
 COMMON_FLAGS=(
@@ -44,6 +45,7 @@ PCH_KEY=$(printf '%s\n%s\n%s\n%s\n' "$ABI_VERSION" "$TOOLCHAIN_ID" "$HEADER_HASH
 PCH_FN="$PCH_DIR/uce_lib-wasm-unit-$PCH_KEY.pch"
 
 mkdir -p "$DEST_DIR" >/dev/null 2>&1
+trap 'rm -f "$OBJ_FN" "$ABI_TMP" "$WASM_TMP"' EXIT
 
 build_pch_if_needed() {
 	if [ "$PCH_ENABLED" = "0" ]; then
@@ -80,7 +82,7 @@ fi
 "$SDK/bin/wasm-ld" -shared --experimental-pic \
 	--unresolved-symbols=import-dynamic \
 	--Bsymbolic \
-	"$OBJ_FN" -o "$DEST_DIR/$WASM_FN" \
+	"$OBJ_FN" -o "$WASM_TMP" \
 	--export-if-defined=__uce_set_current_request \
 	--export-if-defined=__uce_render \
 	--export-if-defined=__uce_component \
@@ -90,8 +92,9 @@ fi
 	--export-if-defined=__uce_once \
 	--export-if-defined=__uce_init
 
-"$SDK/bin/llvm-objcopy" --add-section=uce.abi="$ABI_TMP" "$DEST_DIR/$WASM_FN"
+"$SDK/bin/llvm-objcopy" --add-section=uce.abi="$ABI_TMP" "$WASM_TMP"
 
-python3 scripts/check_unit_wasm.py "$DEST_DIR/$WASM_FN" --abi-version "$ABI_VERSION" --llvm-nm "$SDK/bin/llvm-nm"
+python3 scripts/check_unit_wasm.py "$WASM_TMP" --abi-version "$ABI_VERSION" --llvm-nm "$SDK/bin/llvm-nm"
+mv "$WASM_TMP" "$DEST_DIR/$WASM_FN"
 
 rm -f "$OBJ_FN" "$ABI_TMP"

scripts/test_dependency_invalidation.sh

@@ -48,13 +48,37 @@ assert_marker parent dependency-marker-a
 sed -i 's/dependency-marker-a/dependency-marker-b/' "$source_dir/child.uce"
 assert_marker parent dependency-marker-b
 
+# A proactive rebuild owns these same per-unit locks. While it publishes fresh
+# artifacts, requests must use the last complete artifacts instead of waiting
+# across the transitive graph. Atomic publication keeps those artifacts safe.
+parent_wasm="$cache_dir/parent.uce.wasm"
+child_wasm="$cache_dir/child.uce.wasm"
+(
+	exec 8>"$parent_wasm.lock"
+	exec 9>"$child_wasm.lock"
+	flock 8
+	flock 9
+	sleep 3
+) &
+rebuild_lock_pid=$!
+sleep 0.2
+sed -i 's/dependency-marker-b/dependency-marker-d/' "$source_dir/child.uce"
+started_at=$(date +%s%N)
+assert_marker parent dependency-marker-b
+elapsed_ms=$(( ($(date +%s%N) - started_at) / 1000000 ))
+if (( elapsed_ms >= 2000 )); then
+	echo "request waited ${elapsed_ms}ms for an active transitive rebuild" >&2
+	exit 1
+fi
+wait "$rebuild_lock_pid"
+assert_marker parent dependency-marker-d
+
 # Warm every configured worker, then replace the artifact while retaining its
 # whole-second mtime. The worker cache must notice the nanosecond/ctime change.
-for _ in {1..16}; do assert_marker parent dependency-marker-b; done
+for _ in {1..16}; do assert_marker parent dependency-marker-d; done
-sed 's/dependency-marker-b/dependency-marker-c/' "$source_dir/child.uce" >"$source_dir/alternate-child.uce"
+sed 's/dependency-marker-d/dependency-marker-c/' "$source_dir/child.uce" >"$source_dir/alternate-child.uce"
 sed 's/child.uce/alternate-child.uce/' "$source_dir/parent.uce" >"$source_dir/alternate.uce"
 assert_marker alternate dependency-marker-c
-parent_wasm="$cache_dir/parent.uce.wasm"
 alternate_wasm="$cache_dir/alternate.uce.wasm"
 parent_mtime=$(stat -c %Y "$parent_wasm")
 cp "$alternate_wasm" "$parent_wasm"

src/lib/compiler.cpp

@@ -296,7 +296,7 @@ String compiler_registry_lock_file_name(Request* context)
 	return(compiler_registry_file_name(context) + ".lock");
 }
 
-int compiler_open_lock_file(String file_name, String purpose)
+int compiler_open_lock_file(String file_name, String purpose, bool nonblocking = false)
 {
 	(void)purpose;
 	auto lock_dir = dirname(file_name);
@@ -309,8 +309,13 @@ int compiler_open_lock_file(String file_name, String purpose)
 		return(fdlock);
 	}
 	fcntl(fdlock, F_SETFD, FD_CLOEXEC);
-	if(flock(fdlock, LOCK_EX) != 0)
+	if(flock(fdlock, LOCK_EX | (nonblocking ? LOCK_NB : 0)) != 0)
 	{
+		if(nonblocking && (errno == EWOULDBLOCK || errno == EAGAIN))
+		{
+			close(fdlock);
+			return(-2);
+		}
 		close(fdlock);
 		printf("(!) Could not lock file %s\n", file_name.c_str());
 		return(-1);
@@ -845,7 +850,19 @@ SharedUnit* compiler_get_shared_unit_internal(Request* context, String file_name
 	SharedUnit* su = new SharedUnit();
 	setup_unit_paths(context, su, file_name);
 
-	int fdlock = compiler_open_lock_file(su->wasm_name + ".lock", "shared-unit:" + file_name);
+	int fdlock = compiler_open_lock_file(su->wasm_name + ".lock", "shared-unit:" + file_name, !force_recompile);
+	if(fdlock == -2 && file_exists(su->wasm_name))
+	{
+		auto state = inspect_shared_unit_filesystem(context, su);
+		su->api_declarations = split(file_get_contents(su->api_file_name), "\n");
+		su->last_compiled = state.compiled_time;
+		su->compile_status = "rebuilding";
+		compiler_record_observed_filesystem_state(su, state);
+		context->server->units[file_name] = su;
+		return(su);
+	}
+	if(fdlock == -2)
+		fdlock = compiler_open_lock_file(su->wasm_name + ".lock", "shared-unit:" + file_name);
 	if(fdlock == -1)
 	{
 		su->compiler_messages = "could not open compile lock";
@@ -947,6 +964,17 @@ bool compiler_unit_compile_pending(Request* context, String file_name)
 	return(true);
 }
 
+bool compiler_unit_compile_in_progress(Request* context, String file_name)
+{
+	SharedUnit su;
+	setup_unit_paths(context, &su, compiler_normalize_unit_path(context, file_name));
+	int fdlock = compiler_open_lock_file(su.wasm_name + ".lock", "compile-probe", true);
+	if(fdlock == -2)
+		return(true);
+	compiler_close_lock_file(fdlock);
+	return(false);
+}
+
 void unit_render(String file_name)
 {
 	unit_render(file_name, *context);

src/lib/compiler.h

@@ -17,6 +17,7 @@ void compile_shared_unit(Request* context, SharedUnit* su);
 SharedUnit* get_shared_unit(Request* context, String file_name);
 String compiler_error_page_unit(Request* context, String config_key);
 bool compiler_unit_compile_pending(Request* context, String file_name);
+bool compiler_unit_compile_in_progress(Request* context, String file_name);
 String compiler_site_directory(Request* context);
 StringList compiler_scan_site_units(Request* context);
 StringList compiler_list_known_units(Request* context);

src/wasm/backend.cpp

@@ -100,7 +100,7 @@ static bool wasm_artifact_exists(Request* context, const String& entry_unit)
 	// metadata mismatches, which can leave stale wasm with old imports.
 	bool source_missing = false;
 	if(compiler_unit_needs_recompile(context, entry_unit, &source_missing))
-		return(false);
+		return(compiler_unit_compile_in_progress(context, entry_unit));
 	if(source_missing)
 		return(false);
 	return(true);

src/wasm/worker.cpp

@@ -1606,7 +1606,7 @@ private:
 			return(1);
 		}
 
-		if(!file_exists_host(worker.unit_wasm_path(resolved)) || compiler_unit_needs_recompile(context, resolved, 0))
+		if(!file_exists_host(worker.unit_wasm_path(resolved)) || (compiler_unit_needs_recompile(context, resolved, 0) && !compiler_unit_compile_in_progress(context, resolved)))
 			get_shared_unit(context, resolved);
 
 		size_t unit_index = 0;