fix(shell-evaluator): do not let eval() hold extra context MONGOSH-3223 (#2677)

Calling `eval()` makes the entire outside scope available to
the evaluated code, and consequentially keeps it alive for garbage
collection purposes longer than it should or than it needs to.

Since we only use `eval()` to modify global state, putting the
call into a file where no other extra context exists is a decent
solution to this issue.

Author: addaleax

packages/shell-evaluator/src/contextless-eval.ts

@@ -0,0 +1,12 @@
+// This may seem like a pointless function, but eval() in JS
+// is tricky. If we were to call eval() from inside a function
+// in another file directly, the variables in that function would
+// affect and be available to the eval'ed source, which can
+// generally cause some weirdness in evaluating the inner code,
+// but also in particular establishes a garbage collection link
+// between the eval'ed source and those variables.
+// Putting this function in a separate file ensures that no
+// unintentional references to other variables can be captured.
+export function contextlessEval(code: string): void {
+  eval(code);
+}

packages/shell-evaluator/src/shell-evaluator.ts

@@ -1,3 +1,4 @@
+import { contextlessEval } from './contextless-eval';
 import type { ShellInstanceState } from '@mongosh/shell-api';
 import {
   toShellResult,
@@ -26,8 +27,8 @@ try {
 if (v8?.startupSnapshot?.isBuildingSnapshot?.()) {
   v8.startupSnapshot.addSerializeCallback(() => {
     // Ensure that any lazy loading performed by Babel is part of the snapshot
-    eval(new AsyncWriter().runtimeSupportCode());
-    eval(new AsyncWriter().process('1+1'));
+    contextlessEval(new AsyncWriter().runtimeSupportCode());
+    contextlessEval(new AsyncWriter().process('1+1'));
     hasAlreadyRunGlobalRuntimeSupportEval = true;
   });
 }
@@ -120,7 +121,8 @@ class ShellEvaluator<EvaluationResultType = ShellResult> {
       // in which the shell-api package lives and not from the context inside
       // the REPL (i.e. `db.test.find().toArray() instanceof Array` is `false`).
       if (!hasAlreadyRunGlobalRuntimeSupportEval) {
-        eval(supportCode);
+        contextlessEval(supportCode);
+        hasAlreadyRunGlobalRuntimeSupportEval = true;
       }
       this.markTime?.(
         TimingCategories.AsyncRewrite,