fix(diagnostics_channel): make channel.hasSubscribers a getter property

src/node/diagnostics_channel.ts

@@ -34,6 +34,9 @@ import { ERR_INVALID_ARG_TYPE } from 'node-internal:internal_errors';
 
 import { validateObject } from 'node-internal:validators';
 
+const hasSubscribersGetter =
+  !!Cloudflare.compatibilityFlags['diagnostics_channel_has_subscribers_getter'];
+
 export const { Channel } = diagnosticsChannel;
 
 export function hasSubscribers(name: string | symbol): boolean {
@@ -109,6 +112,47 @@ export class TracingChannel {
     return this[kError];
   }
 
+  // `hasSubscribers` is installed on the prototype below, gated by the
+  // `diagnostics_channel_has_subscribers_getter` compatibility flag. When the
+  // flag is enabled it's a getter property (matching Node.js); when disabled
+  // it's a method (preserving the original workerd behavior).
+  static {
+    if (hasSubscribersGetter) {
+      Object.defineProperty(this.prototype, 'hasSubscribers', {
+        get(this: TracingChannel): boolean {
+          return [
+            this.start,
+            this.end,
+            this.asyncStart,
+            this.asyncEnd,
+            this.error,
+          ].some((c) => c != null && (c.hasSubscribers as unknown as boolean));
+        },
+        configurable: true,
+        enumerable: false,
+      });
+    } else {
+      Object.defineProperty(this.prototype, 'hasSubscribers', {
+        value: function hasSubscribers(this: TracingChannel): boolean {
+          return [
+            this.start,
+            this.end,
+            this.asyncStart,
+            this.asyncEnd,
+            this.error,
+          ].some(
+            (c) =>
+              c != null &&
+              (c.hasSubscribers as unknown as () => boolean).call(c)
+          );
+        },
+        writable: true,
+        configurable: true,
+        enumerable: false,
+      });
+    }
+  }
+
   subscribe(subscriptions: TracingChannelSubscriptions): void {
     if (subscriptions.start !== undefined)
       this[kStart]?.subscribe(subscriptions.start);

src/workerd/api/node/diagnostics-channel.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <workerd/api/node/async-hooks.h>
+#include <workerd/io/compatibility-date.capnp.h>
 #include <workerd/jsg/jsg.h>
 
 #include <kj/map.h>
@@ -31,8 +32,12 @@ class Channel: public jsg::Object {
       jsg::Optional<v8::Local<v8::Value>> maybeReceiver,
       jsg::Arguments<jsg::Value> args);
 
-  JSG_RESOURCE_TYPE(Channel) {
-    JSG_METHOD(hasSubscribers);
+  JSG_RESOURCE_TYPE(Channel, CompatibilityFlags::Reader flags) {
+    if (flags.getDiagnosticsChannelHasSubscribersGetter()) {
+      JSG_READONLY_PROTOTYPE_PROPERTY(hasSubscribers, hasSubscribers);
+    } else {
+      JSG_METHOD(hasSubscribers);
+    }
     JSG_METHOD(publish);
     JSG_METHOD(subscribe);
     JSG_METHOD(unsubscribe);

src/workerd/api/node/tests/BUILD.bazel

@@ -148,6 +148,12 @@ wd_test(
     data = ["diagnostics-channel-test.js"],
 )
 
+wd_test(
+    src = "diagnostics-channel-getter-test.wd-test",
+    args = ["--experimental"],
+    data = ["diagnostics-channel-getter-test.js"],
+)
+
 wd_test(
     src = "mimetype-test.wd-test",
     args = ["--experimental"],

src/workerd/api/node/tests/diagnostics-channel-getter-test.js

@@ -0,0 +1,67 @@
+// Copyright (c) 2026 Cloudflare, Inc.
+// Licensed under the Apache 2.0 license found in the LICENSE file or at:
+//     https://opensource.org/licenses/Apache-2.0
+//
+// Tests the `hasSubscribers` getter behavior of `Channel` and `TracingChannel`
+// when the `diagnostics_channel_has_subscribers_getter` compat flag is enabled.
+// Legacy method behavior is covered by `diagnostics-channel-test`.
+
+import { ok, strictEqual } from 'node:assert';
+
+import { channel, tracingChannel } from 'node:diagnostics_channel';
+
+export const test_channel_hasSubscribers_is_a_getter = {
+  async test() {
+    const ch = channel('getter-test');
+
+    // It is a boolean, not a function (matches Node.js).
+    strictEqual(typeof ch.hasSubscribers, 'boolean');
+    strictEqual(ch.hasSubscribers, false);
+
+    const listener = () => {};
+    ch.subscribe(listener);
+    strictEqual(ch.hasSubscribers, true);
+
+    ch.unsubscribe(listener);
+    strictEqual(ch.hasSubscribers, false);
+
+    // Defined on the prototype as a read-only getter.
+    const desc = Object.getOwnPropertyDescriptor(
+      Object.getPrototypeOf(ch),
+      'hasSubscribers'
+    );
+    ok(desc);
+    strictEqual(typeof desc.get, 'function');
+    strictEqual(desc.set, undefined);
+    strictEqual(desc.value, undefined);
+  },
+};
+
+export const test_tracingChannel_hasSubscribers_is_a_getter = {
+  async test() {
+    const tc = tracingChannel('tracing-getter-test');
+
+    strictEqual(typeof tc.hasSubscribers, 'boolean');
+    strictEqual(tc.hasSubscribers, false);
+
+    const listener = () => {};
+
+    // Each sub-channel independently flips the aggregate getter.
+    for (const sub of ['start', 'end', 'asyncStart', 'asyncEnd', 'error']) {
+      tc[sub].subscribe(listener);
+      strictEqual(tc.hasSubscribers, true, `via ${sub}`);
+      tc[sub].unsubscribe(listener);
+      strictEqual(tc.hasSubscribers, false, `after unsubscribing ${sub}`);
+    }
+
+    // TracingChannel.hasSubscribers should also be a prototype getter.
+    const desc = Object.getOwnPropertyDescriptor(
+      Object.getPrototypeOf(tc),
+      'hasSubscribers'
+    );
+    ok(desc);
+    strictEqual(typeof desc.get, 'function');
+    strictEqual(desc.set, undefined);
+    strictEqual(desc.value, undefined);
+  },
+};

src/workerd/api/node/tests/diagnostics-channel-getter-test.wd-test

@@ -0,0 +1,18 @@
+using Workerd = import "/workerd/workerd.capnp";
+
+const unitTests :Workerd.Config = (
+  services = [
+    ( name = "diagnostics-channel-getter-test",
+      worker = (
+        modules = [
+          (name = "worker", esModule = embed "diagnostics-channel-getter-test.js")
+        ],
+        compatibilityFlags = [
+          "nodejs_compat",
+          "nodejs_compat_v2",
+          "diagnostics_channel_has_subscribers_getter",
+        ]
+      )
+    ),
+  ],
+);

src/workerd/api/node/tests/diagnostics-channel-test.js

@@ -178,3 +178,54 @@ export const DiagChannelUnbindDuringRunStores = {
     strictEqual(transformCallCount, 1);
   },
 };
+
+// Legacy-method behavior: when the `diagnostics_channel_has_subscribers_getter`
+// compat flag is NOT enabled, `Channel.hasSubscribers` and
+// `TracingChannel.hasSubscribers` are registered as methods. The associated
+// `.wd-test` opts out of the flag explicitly so that this behavior is exercised
+// under both the default and `@all-compat-flags` test variants.
+export const test_channel_hasSubscribers_is_a_method = {
+  async test() {
+    const ch = channel('method-test');
+
+    // It is a method, not a boolean property.
+    strictEqual(typeof ch.hasSubscribers, 'function');
+    strictEqual(ch.hasSubscribers(), false);
+
+    const listener = () => {};
+    ch.subscribe(listener);
+    strictEqual(ch.hasSubscribers(), true);
+
+    ch.unsubscribe(listener);
+    strictEqual(ch.hasSubscribers(), false);
+
+    // Defined on the prototype as a method (value descriptor, no getter).
+    const desc = Object.getOwnPropertyDescriptor(
+      Object.getPrototypeOf(ch),
+      'hasSubscribers'
+    );
+    ok(desc);
+    strictEqual(typeof desc.value, 'function');
+    strictEqual(desc.get, undefined);
+    strictEqual(desc.set, undefined);
+  },
+};
+
+export const test_tracingChannel_hasSubscribers_is_a_method = {
+  async test() {
+    const tc = tracingChannel('tracing-method-test');
+
+    strictEqual(typeof tc.hasSubscribers, 'function');
+    strictEqual(tc.hasSubscribers(), false);
+
+    const listener = () => {};
+
+    // Each sub-channel independently flips the aggregate method result.
+    for (const sub of ['start', 'end', 'asyncStart', 'asyncEnd', 'error']) {
+      tc[sub].subscribe(listener);
+      strictEqual(tc.hasSubscribers(), true, `via ${sub}`);
+      tc[sub].unsubscribe(listener);
+      strictEqual(tc.hasSubscribers(), false, `after unsubscribing ${sub}`);
+    }
+  },
+};

src/workerd/api/node/tests/diagnostics-channel-test.wd-test

@@ -7,7 +7,15 @@ const unitTests :Workerd.Config = (
         modules = [
           (name = "worker", esModule = embed "diagnostics-channel-test.js")
         ],
-        compatibilityFlags = ["nodejs_compat", "nodejs_compat_v2"]
+        compatibilityFlags = [
+          "nodejs_compat",
+          "nodejs_compat_v2",
+          # Explicitly opt out of the `hasSubscribers` getter so this test file
+          # always exercises the legacy method behavior, even under
+          # `@all-compat-flags`. The getter behavior is covered by
+          # `diagnostics-channel-getter-test`.
+          "no_diagnostics_channel_has_subscribers_getter",
+        ]
       )
     ),
   ],

src/workerd/io/compatibility-date.capnp

@@ -1504,8 +1504,20 @@ struct CompatibilityFlags @0x8f8c1b68151b6cef {
   # `ctx.version` is much more well defined. The behaviour of this flag will change in the future.
 
   noResizableArrayBufferInBlob @173 :Bool
-    $compatEnableFlag("no_resizable_array_buffer_in_blob")
-    $compatDisableFlag("resizable_array_buffer_in_blob");
+      $compatEnableFlag("no_resizable_array_buffer_in_blob")
+      $compatDisableFlag("resizable_array_buffer_in_blob");
   # When enabled, creating a Blob with a resizable ArrayBuffer will throw a TypeError, matching
   # expected spec behavior.
+
+  diagnosticsChannelHasSubscribersGetter @174 :Bool
+      $compatEnableFlag("diagnostics_channel_has_subscribers_getter")
+      $compatDisableFlag("no_diagnostics_channel_has_subscribers_getter")
+      $compatEnableDate("2026-05-19");
+  # Node.js' `diagnostics_channel.Channel.hasSubscribers` and
+  # `TracingChannel.hasSubscribers` are boolean getter properties, not methods.
+  # Originally, workerd registered `Channel.hasSubscribers` as a method (so users
+  # had to call `ch.hasSubscribers()` with parentheses). When this flag is
+  # enabled, `hasSubscribers` becomes a read-only getter that evaluates directly
+  # to a boolean, matching Node.js behavior.
+  # See: https://nodejs.org/dist/latest-v20.x/docs/api/diagnostics_channel.html#channelhassubscribers
 }