feat(bazel): setup karma web test wrapper with "local" debug target

bazel/BUILD.bazel

@@ -11,6 +11,7 @@ filegroup(
         "//bazel/esbuild:files",
         "//bazel/http-server:files",
         "//bazel/integration:files",
+        "//bazel/karma:files",
         "//bazel/remote-execution:files",
         "//bazel/spec-bundling:files",
     ],

bazel/karma/BUILD.bazel

@@ -0,0 +1,9 @@
+package(default_visibility = ["//visibility:public"])
+
+exports_files(["karma-local-config.js"])
+
+# Make source files available for distribution via pkg_npm
+filegroup(
+    name = "files",
+    srcs = glob(["*"]),
+)

bazel/karma/index.bzl

@@ -0,0 +1,52 @@
+load(
+    "@npm//@bazel/concatjs:index.bzl",
+    _karma_web_test = "karma_web_test",
+    _karma_web_test_suite = "karma_web_test_suite",
+)
+
+def _karma_local_browsers_target(name, **web_test_args):
+    """Macro for defining a standalone karma web test target that starts Karma
+      without a browser, allowing for manual debugging."""
+
+    # Custom standalone web test that can be run to test against any browser
+    # that is manually connected to.
+    _karma_web_test(
+        name = "%s_bin" % name,
+        config_file = "//bazel/karma:karma-local-config.js",
+        tags = ["manual"],
+        **web_test_args
+    )
+
+    # Workaround for: https://github.com/bazelbuild/rules_nodejs/issues/1429
+    native.sh_test(
+        name = name,
+        srcs = ["%s_bin" % name],
+        tags = ["manual", "local", "ibazel_notify_changes"],
+        testonly = True,
+    )
+
+def karma_web_test_suite(name, **kwargs):
+    """Wrapper for the default `karma_web_test_suite` with additional default browsers,
+      and a local target to ease debugging."""
+
+    # Set up default browsers if no explicit `browsers` have been specified.
+    if not hasattr(kwargs, "browsers"):
+        kwargs["tags"] = ["native"] + kwargs.get("tags", [])
+        kwargs["browsers"] = [
+            "//bazel/browsers/chromium:chromium",
+            "//bazel/browsers/firefox:firefox",
+        ]
+
+    # Filter out options which are specific to "karma_web_test" targets. We cannot
+    # pass options like "browsers" to the local web test target.
+    web_test_args = {}
+    for opt_name in kwargs.keys():
+        if not opt_name in ["wrapped_test_tags", "browsers", "wrapped_test_tags", "tags"]:
+            web_test_args[opt_name] = kwargs[opt_name]
+
+    # Custom standalone web test that can be run to test against any browser
+    # that is manually connected to.
+    _karma_local_browsers_target(name = "%s_local" % name)
+
+    # Default test suite with all configured browsers.
+    _karma_web_test_suite(name = name, **kwargs)

bazel/karma/karma-local-config.js

@@ -0,0 +1,46 @@
+/**
+ * Karma configuration that is used by Bazel `karma_web_test` local target which intends
+ * to not launch any browser, enabling manual browser debugging.
+ */
+
+module.exports = (config) => {
+  const overwrites = {};
+
+  // By default "@bazel/concatjs" configures Chrome as browser. Since we don't want
+  // to launch any browser at all, we overwrite the "browsers" option. Since the
+  // default config tries to extend the browsers array with "Chrome", we need to
+  // always return a new empty array.
+  Object.defineProperty(overwrites, 'browsers', {
+    get: () => [],
+    set: () => {},
+    enumerable: true,
+  });
+
+  // Ensures that tests start executing once browsers have been manually connected. We need
+  // to use "defineProperty" because the default "@bazel/concatjs" config overwrites the option.
+  Object.defineProperty(overwrites, 'autoWatch', {
+    get: () => true,
+    set: () => {},
+    enumerable: true,
+  });
+
+  // When not running with ibazel, do not set up the `@bazel/concatjs` watcher. This one
+  // relies on ibazel to write to the `stdin` interface. When running without ibazel, the
+  // watcher will kill concatjs since there is no data written to the `stdin` interface.
+  if (process.env['IBAZEL_NOTIFY_CHANGES'] !== 'y') {
+    // We pre-define a plugins array that captures registration of Karma plugins
+    // and unsets the watcher definitions so that no watcher can be configured.
+    overwrites.plugins = new KarmaPluginArrayWithoutWatchers();
+  }
+
+  config.set(overwrites);
+};
+
+class KarmaPluginArrayWithoutWatchers extends Array {
+  // The Bazel Karma rules only register new plugins using `.push`.
+  push(...plugins) {
+    plugins.filter((p) => typeof p === 'object').forEach((p) => delete p.watcher);
+
+    super.push(...plugins);
+  }
+}