add debug logs for workerd (#9640)

* debug logs for workerd

* test

* pr feedback

* fix tests

Author: emily-shen

.changeset/hot-lamps-laugh.md

@@ -0,0 +1,7 @@
+---
+"miniflare": minor
+---
+
+Add ability to dump workerd config into a file for debugging.
+
+You can enable this by setting `MINIFLARE_WORKERD_CONFIG_DEBUG` to a file path where you want the config to be written.

packages/miniflare/README.md

@@ -911,3 +911,5 @@ For example:
 ```shell
 $ export MINIFLARE_WORKERD_PATH="<WORKERD_REPO_DIR>/bazel-bin/src/workerd/server/workerd"
 ```
+
+For debugging purposes, you can also set `MINIFLARE_WORKERD_CONFIG_DEBUG=<file_path>` which will dump the workerd config to the specified file path.

packages/miniflare/src/runtime/config/index.ts

@@ -1,3 +1,4 @@
+import { writeFileSync } from "fs";
 import { Data, List, Message, Struct } from "capnp-es";
 import { Config as CapnpConfig } from "./generated";
 import { Config, kVoid } from "./workerd";
@@ -47,6 +48,10 @@ function encodeCapnpStruct(obj: any, struct: Struct) {
 }
 
 export function serializeConfig(config: Config): Buffer {
+	const debugPath = process.env.MINIFLARE_WORKERD_CONFIG_DEBUG;
+	if (debugPath) {
+		writeFileSync(debugPath, JSON.stringify(config, null, 2));
+	}
 	const message = new Message();
 	const struct = message.initRoot(CapnpConfig);
 	encodeCapnpStruct(config, struct);

packages/miniflare/test/index.spec.ts

@@ -3327,3 +3327,62 @@ test("Miniflare: custom Node outbound service", async (t) => {
 		`Response from custom Node outbound service. The value of "custom-header" is "foo".`
 	);
 });
+
+test("Miniflare: MINIFLARE_WORKERD_CONFIG_DEBUG controls workerd config file creation", async (t) => {
+	const originalEnv = process.env.MINIFLARE_WORKERD_CONFIG_DEBUG;
+	const configFilePath = "workerd-config.json";
+
+	// Clean up any existing config file
+	if (existsSync(configFilePath)) {
+		await fs.unlink(configFilePath);
+	}
+
+	t.teardown(async () => {
+		if (originalEnv === undefined) {
+			delete process.env.MINIFLARE_WORKERD_CONFIG_DEBUG;
+		} else {
+			process.env.MINIFLARE_WORKERD_CONFIG_DEBUG = originalEnv;
+		}
+		if (existsSync(configFilePath)) {
+			await fs.unlink(configFilePath);
+		}
+	});
+
+	// ensure the config file is not created without the flag
+	delete process.env.MINIFLARE_WORKERD_CONFIG_DEBUG;
+	let mf = new Miniflare({
+		modules: true,
+		script: `export default {
+			fetch() {
+				return new Response("Hello World");
+			}
+		}`,
+	});
+	// Trigger workerd config serialization by dispatching a request
+	let response = await mf.dispatchFetch("http://localhost");
+	// seems like miniflare doesn't like it if you don't read the response
+	await response.text();
+	t.false(
+		existsSync(configFilePath),
+		"config file should not be created when MINIFLARE_WORKERD_CONFIG_DEBUG is not set"
+	);
+	await mf.dispose();
+
+	// ensure the config file is created with the flag
+	process.env.MINIFLARE_WORKERD_CONFIG_DEBUG = configFilePath;
+	mf = new Miniflare({
+		modules: true,
+		script: `export default {
+			fetch() {
+				return new Response("Hello World");
+			}
+		}`,
+	});
+	response = await mf.dispatchFetch("http://localhost");
+	await response.text();
+	t.true(
+		existsSync(configFilePath),
+		"workerd-config.json should be created when MINIFLARE_WORKERD_CONFIG_DEBUG=true"
+	);
+	await mf.dispose();
+});

packages/miniflare/turbo.json

@@ -8,7 +8,7 @@
 		},
 		"test:ci": {
 			"dependsOn": ["build"],
-			"env": ["MINIFLARE_WORKERD_PATH"]
+			"env": ["MINIFLARE_WORKERD_PATH", "MINIFLARE_WORKERD_CONFIG_DEBUG"]
 		}
 	}
 }

packages/miniflare/types/env.d.ts

@@ -3,6 +3,7 @@ declare namespace NodeJS {
 		NODE_ENV?: string;
 		NODE_EXTRA_CA_CERTS?: string;
 		MINIFLARE_WORKERD_PATH?: string;
+		MINIFLARE_WORKERD_CONFIG_DEBUG?: string;
 		MINIFLARE_ASSERT_BODIES_CONSUMED?: "true";
 	}
 }