fix(tasks): Make every background task cancellable, queued or running (#177)

Background tasks often couldn't be cancelled, or cancelling didn't stop them:
- A cancel that arrived before the backend registered the request was dropped (build while saving files, multi-repository fetch/pull between repositories). The host now remembers early cancels (bounded, 2 min), and request ids start at a random value per page.
- There was no queue and no common cancel path: backend-reported tasks (LSP progress, deployment transfers) couldn't be cancelled, and tool installs, VCS operations and update checks had no cancel button in Background Tasks.
- Only the direct child process was killed, and JDK / tool archives were extracted without checking for cancellation.

Tool installs and JDK downloads now share a downloads queue; cancelling a queued task removes it without starting any work. Every task cancels through one path that shows "Cancelling…" and then removes it; tasks that can't be interrupted show a disabled cancel button with the reason. IdeEvent::Task carries `cancellable` with a cancel_task command, cancellable LSP progress sends window/workDoneProgress/cancel, installers, curl and tar run in their own process group and are killed as a group, and extraction checks for cancellation between entries.

Claude-Session: https://claude.ai/code/session_01Dq6jQQn8ZhRfkkyj3PRutA

src/features/tools/__tests__/tools.test.tsx

@@ -70,6 +70,22 @@ describe("tools store", () => {
     expect(useTools.getState().tools.find((t) => t.name === "gopls")?.status).toBe("missing");
   });
 
+  it("installs one at a time; one cancelled while it waits never starts", async () => {
+    backend.stepMs = 10;
+    await refreshTools();
+    const install = vi.spyOn(backend, "install");
+    const first = installTool("gopls");
+    const queued = installTool("vtsls");
+    expect(useTasks.getState().tasks.find((t) => t.id === "tools:vtsls")).toMatchObject({ queued: true });
+    cancelInstall("vtsls");
+    expect(useTasks.getState().tasks.find((t) => t.id === "tools:vtsls")).toBeUndefined();
+    expect(await queued).toBeNull();
+    expect(useTools.getState().logs["vtsls"]?.error).toBe("Cancelled");
+    expect((await first)?.status).toBe("installed");
+    expect(install.mock.calls.map((c) => c[0])).toEqual(["gopls"]);
+    expect(notes.filter((n) => n.level === "error")).toHaveLength(0);
+  });
+
   it("uninstalls", async () => {
     await refreshTools();
     expect(await uninstallTool("rust-analyzer")).toBe(true);

src/features/tools/jdk.ts

@@ -17,10 +17,12 @@ import {
   requiredJdk,
   TEMURIN,
   type DownloadJdkArgs,
+  type JdkDownload,
 } from "@/platform/jdk";
 import { useProject } from "@/platform/project";
 import { notifications } from "@/workbench/contributions";
-import { upsertTask } from "@/workbench/tasks";
+import { queueTask, upsertTask } from "@/workbench/tasks";
+import { DOWNLOAD_QUEUE } from "./store";
 import { tx } from "@/platform/i18n";
 
 const JAVA_SETTINGS = "languages.java";
@@ -42,16 +44,19 @@ export function downloadJdkFor(args: DownloadJdkArgs): Promise<Sdk | null> {
   const title = `Downloading JDK ${args.version}`;
   const taskId = `jdk-download:${args.version}`;
   const key = `jdk-download-${args.version}`;
-  const cancel = () => job.cancel();
-  const job = downloadJdk(TEMURIN, args.version, null, (e) => {
-    const message = e.type === "line" ? e.text : e.message;
-    const fraction = e.type === "progress" ? e.fraction : null;
-    upsertTask({ id: taskId, title, message, fraction, done: false, cancel });
-  });
-  upsertTask({ id: taskId, title, message: "Eclipse Temurin", fraction: null, done: false, cancel });
   const p = (async () => {
     try {
-      const sdk = await job.promise;
+      // Waits for the downloads ahead of it; cancelling it there means it never starts.
+      const sdk = await queueTask(DOWNLOAD_QUEUE, { id: taskId, title }, () => {
+        const cancel = () => job.cancel();
+        upsertTask({ id: taskId, title, message: "Eclipse Temurin", fraction: null, done: false, cancel });
+        const job: JdkDownload = downloadJdk(TEMURIN, args.version, null, (e) => {
+          const message = e.type === "line" ? e.text : e.message;
+          const fraction = e.type === "progress" ? e.fraction : null;
+          upsertTask({ id: taskId, title, message, fraction, done: false, cancel });
+        });
+        return job.promise;
+      });
       if (args.languageServer) await chooseForLanguageServer(sdk.path);
       notifications().notify({
         group: "tools",

src/features/tools/store.ts

@@ -4,11 +4,13 @@ import type { RequestId, ToolInfo } from "@/bindings";
 import { executeAction } from "@/platform/actions";
 import { isCancelled, nextRequestId } from "@/platform/ipc";
 import { notifications } from "@/workbench/contributions";
-import { upsertTask } from "@/workbench/tasks";
+import { cancelTask, queueTask, upsertTask, useTasks } from "@/workbench/tasks";
 import { toolsBackend } from "./backend";
 import { tx } from "@/platform/i18n";
 
 export const SETTINGS_PAGE_ID = "languages.servers";
+/** Tool installs and JDK downloads take turns (one network download / package manager at a time). */
+export const DOWNLOAD_QUEUE = "downloads";
 const MAX_LOG_LINES = 400;
 
 export interface InstallJob {
@@ -100,24 +102,25 @@ export function installTool(name: string, opts: { notificationKey?: string } = {
   const taskId = `tools:${name}`;
   const key = opts.notificationKey ?? `tools-install-${name}`;
   useTools.setState((s) => ({ jobs: { ...s.jobs, [name]: { request, lines: [], fraction: null, message: null } } }));
-  upsertTask({ id: taskId, title: tx`Installing ${display}`, message: null, fraction: null, done: false });
+  const title = tx`Installing ${display}`;
+  const cancel = () => toolsBackend().cancel(request);
+  const task = (message: string | null, fraction: number | null) =>
+    upsertTask({ id: taskId, title, message, fraction, done: false, cancel });
 
   const p = (async () => {
     try {
-      const info = await toolsBackend().install(name, request, (e) => {
-        if (e.type === "line") {
-          patchJob(name, (j) => ({ ...j, lines: [...j.lines, e.text].slice(-MAX_LOG_LINES), message: e.text }));
-          upsertTask({ id: taskId, title: tx`Installing ${display}`, message: e.text, fraction: null, done: false });
-        } else {
-          patchJob(name, (j) => ({ ...j, fraction: e.fraction, message: e.message }));
-          upsertTask({
-            id: taskId,
-            title: tx`Installing ${display}`,
-            message: e.message,
-            fraction: e.fraction,
-            done: false,
-          });
-        }
+      // Downloads and package managers run one at a time; the rest wait, cancellable.
+      const info = await queueTask(DOWNLOAD_QUEUE, { id: taskId, title }, () => {
+        task(null, null);
+        return toolsBackend().install(name, request, (e) => {
+          if (e.type === "line") {
+            patchJob(name, (j) => ({ ...j, lines: [...j.lines, e.text].slice(-MAX_LOG_LINES), message: e.text }));
+            task(e.text, null);
+          } else {
+            patchJob(name, (j) => ({ ...j, fraction: e.fraction, message: e.message }));
+            task(e.message, e.fraction);
+          }
+        });
       });
       upsertTool(info);
       endJob(name, null);
@@ -151,7 +154,7 @@ export function installTool(name: string, opts: { notificationKey?: string } = {
       });
       return null;
     } finally {
-      upsertTask({ id: taskId, title: tx`Installing ${display}`, message: null, fraction: 1, done: true });
+      upsertTask({ id: taskId, title, message: null, fraction: 1, done: true });
       running.delete(name);
     }
   })();
@@ -159,7 +162,10 @@ export function installTool(name: string, opts: { notificationKey?: string } = {
   return p;
 }
 
+/** Cancels the install of `name`, queued or running (as the task's Cancel does). */
 export function cancelInstall(name: string) {
+  const taskId = `tools:${name}`;
+  if (useTasks.getState().tasks.some((t) => t.id === taskId)) return cancelTask(taskId);
   const job = useTools.getState().jobs[name];
   if (job) toolsBackend().cancel(job.request);
 }