fix: end a build whose output report has nowhere to go

.changeset/output-report-without-a-file-system.md

@@ -0,0 +1,5 @@
+---
+"diagnostics-webpack-plugin": patch
+---
+
+End the build rather than hanging it where an `outputReport` has no file system to be written to.

src/utils.js

@@ -190,7 +190,13 @@ const jsonStringifyReplacerSortKeys = (_, value) => {
 function writeOutputFile(compiler, name, content) {
   return /** @type {Promise<void>} */ (
     new Promise((finish, bail) => {
-      if (!compiler.outputFileSystem) return;
+      // Nothing to write it to. Answering rather than returning: what awaits
+      // this is the compilation's own callback, so a promise left unsettled
+      // is a build that never ends.
+      if (!compiler.outputFileSystem) {
+        finish();
+        return;
+      }
 
       const { mkdir, writeFile } = compiler.outputFileSystem;
 

test/utils.test.js

@@ -7,6 +7,7 @@ import {
   parseFiles,
   parseFoldersToGlobs,
   toPosixPath,
+  writeOutputFile,
 } from "../src/utils.js";
 
 // `parseFoldersToGlobs` reads what it is given, so the fixtures have to exist.
@@ -85,6 +86,27 @@ describe("utils", () => {
     ]);
   });
 
+  it("writeOutputFile should answer where there is nothing to write to", async () => {
+    // What awaits this is the compilation's own callback, so a promise left
+    // unsettled is a build that never ends rather than one that reports.
+    /** @type {NodeJS.Timeout} */
+    let waiting;
+    const answered = await Promise.race([
+      writeOutputFile(
+        /** @type {EXPECTED_ANY} */ ({}),
+        join(directory, "report.json"),
+        "{}",
+      ).then(() => "answered"),
+      new Promise((done) => {
+        waiting = setTimeout(() => done("never answered"), 1000);
+      }),
+    ]);
+
+    clearTimeout(waiting);
+
+    assert.strictEqual(answered, "answered");
+  });
+
   it("parseFoldersToGlobs should cover a path that is not there yet both ways", async () => {
     const absent = join(directory, "not-written-yet");