Set a trusted build aside while a getter runs outside a visit session

A getter or proxy trap that runs during a trusted build outside any
visit session now runs with the build's state set aside, as inside a
session. A conversion the getter starts, such as a recursive union
that opens and ends its own session, no longer clears the mark that
keeps the rest of the outer build from trusting its earlier match.

packages/runtime/src/scr_json.c

@@ -10932,9 +10932,9 @@ static void scr_dyn_visit_exit(void) {
 }
 
 /* Getters and proxy traps run user code, which may start conversions of
- * its own and catch their failures. It runs with the session set aside,
- * so its conversions use their own tables and leave the session's
- * entries alone. */
+ * its own and catch their failures. It runs with the session and any
+ * trusted build set aside, so its conversions use their own tables and
+ * leave the session's entries and the trusted build's state alone. */
 typedef struct {
   size_t depth;
   ScrDynVisitSet matching, built, viewed;
@@ -10977,12 +10977,10 @@ static void scr_dyn_visit_resume(const ScrDynVisitState *saved) {
   scr_dyn_session_ran_code = true;
 }
 
-/* Calls a getter for a member read, outside any visit session. */
+/* Calls a getter for a member read, outside any visit session or trusted
+ * build. */
 static ScrDyn *scr_dyn_visit_call_getter(ScrDyn *getter) {
-  if (scr_dyn_visit_depth == 0) {
-    if (scr_dyn_trusted) scr_dyn_session_ran_code = true;
-    return scr_dyn_call(getter, NULL, 0, "getter");
-  }
+  if (scr_dyn_visit_depth == 0 && !scr_dyn_trusted) return scr_dyn_call(getter, NULL, 0, "getter");
   ScrDynVisitState saved;
   scr_dyn_visit_suspend(&saved);
   ScrDyn *result = scr_dyn_call(getter, NULL, 0, "getter");
@@ -10990,12 +10988,10 @@ static ScrDyn *scr_dyn_visit_call_getter(ScrDyn *getter) {
   return result;
 }
 
-/* A proxy's get trap for a member read, outside any visit session. */
+/* A proxy's get trap for a member read, outside any visit session or
+ * trusted build. */
 static ScrDyn *scr_dyn_visit_proxy_get(const ScrDyn *d, ScrStr *name) {
-  if (scr_dyn_visit_depth == 0) {
-    if (scr_dyn_trusted) scr_dyn_session_ran_code = true;
-    return scr_dyn_proxy_get(d, name);
-  }
+  if (scr_dyn_visit_depth == 0 && !scr_dyn_trusted) return scr_dyn_proxy_get(d, name);
   ScrDynVisitState saved;
   scr_dyn_visit_suspend(&saved);
   ScrDyn *result = scr_dyn_proxy_get(d, name);

tests/harness/dyncheck.test.ts

@@ -686,6 +686,46 @@ console.log(JSON.stringify(plain));
     ]);
   });
 
+  test("nested unions: a getter that converts a recursive union keeps a non-recursive build untrusted", async () => {
+    // The getter converts a recursive union, which opens and ends a visit
+    // session of its own, before it answers. The outer build still matches
+    // the getter's second answer in full, and the failure names that union.
+    const r = await compileAndRun(
+      "flat-discriminated-getter-session",
+      `type Tree = { kind: "leaf"; n: number } | { kind: "node"; left: Tree; right: Tree };
+type Inner = { tag: "x"; n: number } | { tag: "y"; s: string };
+type Outer = { kind: "a"; inner: Inner } | { kind: "b"; other: number };
+const v = JSON.parse('{"kind":"a"}') as unknown;
+let calls = 0;
+let leaves = 0;
+Object.defineProperty(v, "inner", {
+  enumerable: true,
+  get() {
+    calls++;
+    const t = JSON.parse('{"kind":"node","left":{"kind":"leaf","n":1},"right":{"kind":"leaf","n":2}}') as Tree;
+    if (t.kind === "node") leaves += 2;
+    return calls === 1 ? JSON.parse('{"tag":"x","n":1}') : JSON.parse('{"tag":"x","n":"bad"}');
+  },
+});
+try {
+  const o = v as Outer;
+  console.log("ok", JSON.stringify(o));
+} catch (err) {
+  console.log((err as Error).message);
+}
+console.log("getter calls", calls, "leaves", leaves);
+const plain = JSON.parse('{"kind":"a","inner":{"tag":"y","s":"z"}}') as Outer;
+console.log(JSON.stringify(plain));
+`,
+    );
+    expect(r.exitCode).toBe(0);
+    expect(r.stdout.trimEnd().split("\n")).toEqual([
+      "expected object | object at $.inner, got object",
+      "getter calls 2 leaves 4",
+      '{"kind":"a","inner":{"tag":"y","s":"z"}}',
+    ]);
+  });
+
   test("array where a record is expected names the mismatch at the root", async () => {
     const r = await compileAndRun(
       "root-kind",