Convert custom StarlarkValues to string for json.encode (#65)

Custom `StarlarkValue`s can show up in `settings`, but are not supported
by `json.encode`. Convert them to strings before turning them into JSON
to fix issues such as:

```
Error in encode: in dict key "//command_line_option:compilation_mode": cannot encode CompilationMode as JSON
```

Fixes #63

examples/opt_filegroup_with_reset/BUILD.bazel

@@ -0,0 +1,43 @@
+load(":opt_filegroup.bzl", "opt_filegroup", "opt_filegroup_reset")
+load("@with_cfg.bzl", "original_settings")
+
+cc_binary(
+    name = "slow_to_run_app",
+    srcs = ["slow_to_run_app.cpp"],
+    # Don't unnecessarily build the app in fastbuild mode.
+    tags = ["manual"],
+)
+
+cc_binary(
+    name = "slow_to_build_app",
+    srcs = ["slow_to_build_app.cpp"],
+    tags = ["manual"],
+)
+
+opt_filegroup_reset(
+    name = "slow_to_build_app_reset",
+    exports = ":slow_to_build_app",
+)
+
+opt_filegroup(
+    name = "opt_test_data",
+    srcs = [
+        ":slow_to_build_app_reset",
+        ":slow_to_run_app",
+    ],
+)
+
+sh_test(
+    name = "slow_apps_integration_test",
+    srcs = ["slow_apps_integration_test.sh"],
+    data = [":opt_test_data"],
+    env = select({
+        "@platforms//os:windows": {"EXE_SUFFIX": ".exe"},
+        "//conditions:default": {"EXE_SUFFIX": ""},
+    }),
+    deps = ["@bazel_tools//tools/bash/runfiles"],
+)
+
+original_settings(
+    name = "opt_filegroup_original_settings",
+)

examples/opt_filegroup_with_reset/opt_filegroup.bzl

@@ -0,0 +1,3 @@
+load("@with_cfg.bzl", "with_cfg")
+
+opt_filegroup, opt_filegroup_reset = with_cfg(native.filegroup).set("compilation_mode", "opt").resettable(Label(":opt_filegroup_original_settings")).build()

examples/opt_filegroup_with_reset/slow_apps_integration_test.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+
+# --- begin runfiles.bash initialization v3 ---
+# Copy-pasted from the Bazel Bash runfiles library v3.
+set -uo pipefail; set +e; f=bazel_tools/tools/bash/runfiles/runfiles.bash
+source "${RUNFILES_DIR:-/dev/null}/$f" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "${RUNFILES_MANIFEST_FILE:-/dev/null}" | cut -f2- -d' ')" 2>/dev/null || \
+  source "$0.runfiles/$f" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "$0.runfiles_manifest" | cut -f2- -d' ')" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "$0.exe.runfiles_manifest" | cut -f2- -d' ')" 2>/dev/null || \
+  { echo>&2 "ERROR: cannot find $f"; exit 1; }; f=; set -e
+# --- end runfiles.bash initialization v3 ---
+
+slow_to_run_app=$(rlocation "with_cfg_examples/opt_filegroup_with_reset/slow_to_run_app${EXE_SUFFIX}")
+slow_to_build_app=$(rlocation "with_cfg_examples/opt_filegroup_with_reset/slow_to_build_app${EXE_SUFFIX}")
+
+if [ "$(uname)" == "Darwin" ]; then
+  slow_to_run_app_realpath=$(python3 -c "import os, sys; print(os.path.realpath(sys.argv[1]))" "$slow_to_run_app")
+  slow_to_build_app_realpath=$(python3 -c "import os, sys; print(os.path.realpath(sys.argv[1]))" "$slow_to_build_app")
+else
+  slow_to_run_app_realpath=$(realpath "$slow_to_run_app")
+  slow_to_build_app_realpath=$(realpath "$slow_to_build_app")
+fi
+
+if [[ "$slow_to_run_app_realpath" != *"-opt-ST-"*"/bin/opt_filegroup_with_reset/slow_to_run_app"* ]]; then
+  echo "ERROR: slow_to_run_app wasn't built in opt mode: $slow_to_run_app_realpath"
+  exit 1
+fi
+
+"$slow_to_run_app"
+
+if [[ "$slow_to_build_app_realpath" != *"-fastbuild/bin/opt_filegroup_with_reset/slow_to_build_app"* ]]; then
+  echo "ERROR: slow_to_build_app wasn't built in fastbuild mode: $slow_to_build_app_realpath"
+  exit 1
+fi
+
+"$slow_to_build_app"

examples/opt_filegroup_with_reset/slow_to_build_app.cpp

@@ -0,0 +1,6 @@
+#include <iostream>
+
+int main(int arg, char** argv) {
+  std::cout << "This app is slow to build with optimizations." << std::endl;
+  return 0;
+}

examples/opt_filegroup_with_reset/slow_to_run_app.cpp

@@ -0,0 +1,6 @@
+#include <iostream>
+
+int main(int arg, char** argv) {
+  std::cout << "This app is slow to run unless built in release mode." << std::endl;
+  return 0;
+}

with_cfg/private/transition.bzl

@@ -88,7 +88,7 @@ def _transition_base_impl(settings, attr, *, operations, original_settings_label
                 )
 
         else:
-            new_settings[str(original_settings_label)] = json.encode(settings)
+            new_settings[str(original_settings_label)] = _encode_settings(settings)
 
     return new_settings
 
@@ -99,3 +99,18 @@ def _get_settings_key(setting):
         return "//command_line_option:" + setting
     else:
         fail("Expected Label or string, got: {} ({})".format(setting, type(setting)))
+
+_STARLARK_TYPES = {
+    type(t): None
+    for t in [True, "", 0, 0.0, []]
+}
+
+def _encode_settings(settings):
+    # Certain setting values supplied by Bazel are special StarlarkValue types and thus unsupported
+    # by json.encode. We work around this by converting everything that isn't a native Starlark type
+    # to a string.
+    fixed_settings = {
+        k: v if type(v) in _STARLARK_TYPES else str(v)
+        for k, v in settings.items()
+    }
+    return json.encode(fixed_settings)