wasm: reduce memory usage after crash

If a Process crashes, we continue to hold references to the WebAssembly
heap. We will only reclaim the memory if all the DocumentLinter-s
referring to that Process are modified by the user (triggering a
re-lint, triggering crash recovery).

Reduce memory usage by promptly reclaiming memory on crash.

Author: strager

wasm/quick-lint-js.js

@@ -455,9 +455,6 @@ class Process {
       }
       let func = wasmInstance.exports[name];
       return (...args) => {
-        if (process._crashedException !== null) {
-          throw process._crashedException;
-        }
         try {
           exports.maybeInjectFault(process, name);
           try {
@@ -467,7 +464,7 @@ class Process {
           }
         } catch (e) {
           if (e instanceof ProcessCrashed) {
-            process._crashedException = e;
+            process._taint(e);
           }
           throw e;
         }
@@ -492,6 +489,30 @@ class Process {
     return this._crashedException !== null;
   }
 
+  _taint(exception) {
+    this._crashedException = exception;
+
+    function tainted() {
+      throw this._crashedException;
+    }
+
+    // Reduce memory usage.
+    this._wasmInstance = null;
+    this._heap = null;
+
+    // Make future calls crash and also reduce memory usage.
+    this._malloc = tainted;
+    this._free = tainted;
+    this._vscodeCreateDocument = tainted;
+    this._vscodeDestroyDocument = tainted;
+    this._vscodeLint = tainted;
+    this._vscodeReplaceText = tainted;
+    this._webDemoCreateDocument = tainted;
+    this._webDemoDestroyDocument = tainted;
+    this._webDemoLint = tainted;
+    this._webDemoSetText = tainted;
+  }
+
   toString() {
     return `Process(id=${this._idForDebugging})`;
   }

wasm/test-document.js

@@ -655,6 +655,33 @@ describe("DocumentLinter", () => {
       "redeclaration of variable: y",
     ]);
   });
+
+  it("untouched documents with crashed process do not leak memory", async () => {
+    let numberOfDocuments = 200;
+    let sourceLength = 100_000;
+    let source = new Array(sourceLength + 1).join(" ");
+
+    let documentProcessManager = new DocumentProcessManager();
+    let linters = [];
+    for (let i = 0; i < numberOfDocuments; ++i) {
+      let document = new MockDocument(source);
+      crashProcessOnNextLint();
+      let linter = disposeAfterTest(
+        new DocumentLinter(document, documentProcessManager)
+      );
+      linters.push(linter);
+      await linter.editorChangedVisibilityAsync();
+    }
+
+    function crashProcessOnNextLint() {
+      qljs.maybeInjectFault = (_process, functionName) => {
+        if (functionName === "qljs_vscode_lint") {
+          qljs.maybeInjectFault = originalMaybeInjectFault;
+          throw new ProcessCrashed("(injected fault)");
+        }
+      };
+    }
+  }, /*timeout=*/ 30_000);
 });
 
 describe("ExhaustiveRNG", () => {