Restart a crashed language server per --restart never|always|N

The process is supervised: on exit after a successful initialize it is
spawned again with exponential backoff (500ms to 30s), open documents are
re-sent as didOpen before the new session is announced, and requests caught
by the crash wait for it and retry. A crash more than a minute after the
previous one starts a fresh streak; reaching the streak limit (default 3)
ends restarting for good, after which every request fails with an LspError
saying so. Restarts and give-ups are logged and forwarded to the MCP client.

Writes to the child's stdin go through a PassThrough so a broken pipe ends
the session instead of surfacing as an unhandled rejection.

test/Editor.test.ts

@@ -1,7 +1,7 @@
 import { NodeServices } from "@effect/platform-node";
 import { assert, describe, it } from "@effect/vitest";
 import { Effect, FileSystem, Layer } from "effect";
-import { RequestType } from "vscode-languageserver-protocol";
+import { NotificationType, RequestType } from "vscode-languageserver-protocol";
 import { Diagnostics } from "../src/lsp/Diagnostics.ts";
 import { Documents } from "../src/lsp/Documents.ts";
 import * as Editor from "../src/lsp/Editor.ts";
@@ -12,6 +12,7 @@ const State = new RequestType<null, { open: number; closes: number }, void>(
   "fixture/state",
 );
 const Progress = new RequestType<number, string, void>("fixture/progress");
+const Crash = new NotificationType<null>("fixture/crash");
 
 const TestLayer = Layer.mergeAll(
   Documents.layer({ capacity: 2, languages: {} }),
@@ -168,4 +169,39 @@ describe("Editor", () => {
       );
     },
   );
+
+  it.layer(
+    Layer.mergeAll(
+      Documents.layer({ capacity: 2, languages: {} }),
+      Diagnostics.layer,
+    ).pipe(
+      Layer.provideMerge(
+        workspace({ "a.ts": "const alpha = 1\n" }, { restart: 1 }),
+      ),
+      Layer.provideMerge(NodeServices.layer),
+    ),
+    { excludeTestServices: true },
+  )("when the server crashes", (it) => {
+    it.effect(
+      "starts it again with the same documents open, until the policy gives up",
+      () =>
+        Effect.gen(function* () {
+          const lsp = yield* LanguageServer;
+          const hover = Editor.hover({
+            file: "a.ts",
+            line: 1,
+            symbol: "alpha",
+          });
+          yield* hover;
+
+          yield* lsp.notify(Crash, null);
+          assert.strictEqual((yield* hover)?.contents, "word **alpha**");
+          assert.strictEqual((yield* lsp.request(State, null)).open, 1);
+
+          yield* lsp.notify(Crash, null);
+          const error = yield* Effect.flip(hover);
+          assert.match(error.message, /1 crash in a row/);
+        }),
+    );
+  });
 });

test/fixtures/server.ts

@@ -2,7 +2,8 @@
  * A miniature language server used by the tests. Words are symbols: hover
  * describes the word under the cursor, definition is its first occurrence,
  * references and rename cover every occurrence, and each line containing
- * "TODO" yields a warning. `fixture/state` reports open/close counts.
+ * "TODO" yields a warning. `fixture/state` reports open/close counts and
+ * `fixture/crash` kills the process.
  */
 import {
   createConnection,
@@ -136,6 +137,7 @@ connection.onDocumentSymbol(({ textDocument }) => {
   );
 });
 
+connection.onNotification("fixture/crash", () => process.exit(1));
 connection.onRequest("fixture/state", () => ({
   open: documents.all().length,
   closes,

test/fixtures/workspace.ts

@@ -1,11 +1,14 @@
 import { fileURLToPath } from "node:url";
 import { Effect, FileSystem, Layer, Path } from "effect";
-import { LanguageServer } from "../../src/lsp/LanguageServer.ts";
+import { LanguageServer, type Options } from "../../src/lsp/LanguageServer.ts";
 
 const server = fileURLToPath(new URL("./server.ts", import.meta.url));
 
 /** A temporary workspace with the given files, served by the fixture language server. */
-export const workspace = (files: Record<string, string>) =>
+export const workspace = (
+  files: Record<string, string>,
+  options?: Pick<Options, "restart">,
+) =>
   Layer.unwrap(
     Effect.gen(function* () {
       const fs = yield* FileSystem.FileSystem;
@@ -18,6 +21,7 @@ export const workspace = (files: Record<string, string>) =>
         command: process.execPath,
         args: [server],
         root,
+        ...options,
       });
     }),
   );