[experiments] Add support for testing disabling already enabled experiments (#30831)

bazel/experiments.bzl

@@ -28,3 +28,6 @@ EXPERIMENTS = {
         "tcp_read_chunks",
     ],
 }
+
+NEGATED_EXPERIMENTS = {
+}

bazel/grpc_build_system.bzl

@@ -29,7 +29,7 @@ Contains macros used throughout the repo.
 
 load("//bazel:cc_grpc_library.bzl", "cc_grpc_library")
 load("//bazel:copts.bzl", "GRPC_DEFAULT_COPTS")
-load("//bazel:experiments.bzl", "EXPERIMENTS")
+load("//bazel:experiments.bzl", "EXPERIMENTS", "NEGATED_EXPERIMENTS")
 load("@upb//bazel:upb_proto_library.bzl", "upb_proto_library", "upb_proto_reflection_library")
 load("@build_bazel_rules_apple//apple:ios.bzl", "ios_unit_test")
 load("@build_bazel_rules_apple//apple/testing/default_runner:ios_test_runner.bzl", "ios_test_runner")
@@ -336,6 +336,14 @@ def expand_tests(name, srcs, deps, tags, args, exclude_pollers, uses_event_engin
             experiments[experiment] = 1
     experiments = list(experiments.keys())
 
+    negated_experiments = {}
+    for tag in tags:
+        if tag not in NEGATED_EXPERIMENTS:
+            continue
+        for experiment in NEGATED_EXPERIMENTS[tag]:
+            negated_experiments[experiment] = 1
+    negated_experiments = list(negated_experiments.keys())
+
     experiment_config = list(poller_config)
     for experiment in experiments:
         for config in poller_config:
@@ -356,6 +364,25 @@ def expand_tests(name, srcs, deps, tags, args, exclude_pollers, uses_event_engin
                     tags = tags + [tag]
             config["tags"] = tags
             experiment_config.append(config)
+    for experiment in negated_experiments:
+        for config in poller_config:
+            config = dict(config)
+            config["name"] = config["name"] + "@experiment=no_" + experiment
+            config["args"] = config["args"] + ["--experiment=-" + experiment]
+            tags = config["tags"]
+            must_have_tags = [
+                # We don't run experiments on cmake builds
+                "bazel_only",
+                # Nor on windows
+                "no_windows",
+                # Nor on mac
+                "no_mac",
+            ]
+            for tag in must_have_tags:
+                if tag not in tags:
+                    tags = tags + [tag]
+            config["tags"] = tags
+            experiment_config.append(config)
 
     return experiment_config
 

tools/codegen/core/gen_experiments.py

@@ -187,9 +187,14 @@ with open('src/core/lib/experiments/experiments.cc', 'w') as C:
     print("}  // namespace grpc_core", file=C)
 
 tags_to_experiments = collections.defaultdict(list)
+tags_to_negated_experiments = collections.defaultdict(list)
 for attr in attrs:
-    for tag in attr['test_tags']:
-        tags_to_experiments[tag].append(attr['name'])
+    if attr['default']:
+        for tag in attr['test_tags']:
+            tags_to_negated_experiments[tag].append(attr['name'])
+    else:
+        for tag in attr['test_tags']:
+            tags_to_experiments[tag].append(attr['name'])
 
 with open('bazel/experiments.bzl', 'w') as B:
     put_copyright(B, "#")
@@ -211,3 +216,11 @@ with open('bazel/experiments.bzl', 'w') as B:
             print("        \"%s\"," % experiment, file=B)
         print("    ],", file=B)
     print("}", file=B)
+    print(file=B)
+    print("NEGATED_EXPERIMENTS = {", file=B)
+    for tag, experiments in sorted(tags_to_negated_experiments.items()):
+        print("    \"%s\": [" % tag, file=B)
+        for experiment in sorted(experiments):
+            print("        \"%s\"," % experiment, file=B)
+        print("    ],", file=B)
+    print("}", file=B)