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fix(web_core): reject writes through a primitive value consistently #2499

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Merged
gspencergoog merged 4 commits into from
Sep 4, 2026
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fix(web_core): reject writes through a primitive value consistently #2499

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4802696
fix(dart): reject writes through a primitive instead of dropping them
diegolopezrm Sep 3, 2026
2034591
fix(web_core): reject writes through a primitive root, keeping the root
diegolopezrm Sep 3, 2026
b024f77
Merge remote-tracking branch 'upstream/main' into fix-datamodel-primi…
diegolopezrm Sep 3, 2026
1da404c
Merge branch 'main' into fix-datamodel-primitive-writes
diegolopezrm Sep 4, 2026
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9 changes: 9 additions & 0 deletions dart/a2ui_core/lib/src/core/data_model.dart
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Original file line number Diff line number Diff line change
Expand Up @@ -132,6 +132,15 @@ class DataModel {
current.add(null);
}
current[index] = value;
} else {
// Neither a map nor a list: the value that would contain this
// segment is a primitive, so there is nowhere to write. Silently
// dropping the write would leave the client holding state the
// server believes it applied.
throw A2uiDataError(
"Cannot set path '$path': the containing value is a primitive.",
path: path,
);
}
}

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14 changes: 14 additions & 0 deletions dart/a2ui_core/test/data_model_test.dart
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Expand Up @@ -155,6 +155,20 @@ void main() {
expect(model.get('/'), isEmpty);
});

test('rejects writes through a primitive value', () {

@gspencergoog gspencergoog Sep 4, 2026

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This is a Dart test, but the rest of the code changes are web_core changes. Did you mean to separate it like that?

@diegolopezrm diegolopezrm Sep 4, 2026

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Fair question — it did not start that way, and the split is a leftover from a rebase.

The PR originally fixed the same bug in both: Dart's DataModel.set walked to the container for the last segment, found neither a map nor a list, and returned normally having written nothing, so the agent got no error and the value simply was not there. web_core had two variants of the same hole — a raw TypeError on a primitive root, and a root holding false, 0 or '' being silently discarded, because the guard was if (!this.data).

Then #2439 landed with the same Dart fix, at the same branch, raising the same A2uiDataError. I dropped my version of that hunk and kept its wording, which left what you are looking at: the web_core fix, plus the Dart tests, which now cover #2439's fix rather than my own. They assert the error type rather than the message, so they passed against that wording unmodified — and the case they pin, a write when the root itself is a primitive, has no test on main.

So the Dart test is deliberate but not original intent. Happy to split it into its own PR if you would rather review them separately; it is two tests and it moves cleanly.

@gspencergoog gspencergoog Sep 4, 2026

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No, as long as it is adding to the Dart test corpus, and not a mistake, then this is fine to keep here.

final model = DataModel();
model.set('/a/b', 's');
expect(() => model.set('/a/b/c', 1), throwsA(isA<A2uiDataError>()));
expect(model.get('/a/b'), 's');
});

test('rejects writes when the root itself is a primitive', () {
final model = DataModel();
model.set('/', 1);
expect(() => model.set('/a', 2), throwsA(isA<A2uiDataError>()));
expect(model.get('/'), 1);
});

test('rejects excessively large list indices to prevent OOM', () {
final model = DataModel();
expect(
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17 changes: 17 additions & 0 deletions renderers/web_core/src/v0_9/state/data-model.test.ts
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Expand Up @@ -17,6 +17,7 @@
import * as assert from 'node:assert';
import {describe, it, beforeEach} from 'node:test';
import {DataModel} from './data-model.js';
import {A2uiDataError} from '../errors.js';

describe('DataModel', () => {
let model: DataModel;
Expand Down Expand Up @@ -446,6 +447,22 @@ describe('DataModel', () => {
assert.strictEqual(model.get('/items/invalid'), undefined);
});

it('rejects writes when the root itself is a primitive', () => {
const primitiveRoot = new DataModel({});
primitiveRoot.set('/', 1);
assert.throws(() => primitiveRoot.set('/a', 2), A2uiDataError);
assert.strictEqual(primitiveRoot.get('/'), 1);
});

it('keeps a falsy primitive root instead of replacing it', () => {
for (const root of [false, 0, '']) {
const falsyRoot = new DataModel({});
falsyRoot.set('/', root);
assert.throws(() => falsyRoot.set('/a', 2), A2uiDataError);
assert.strictEqual(falsyRoot.get('/'), root);
}
});

it('rejects leading-zero array indices (RFC 6901)', () => {
assert.throws(() => {
model.set('/items/01', 'value');
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10 changes: 9 additions & 1 deletion renderers/web_core/src/v0_9/state/data-model.ts
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Expand Up @@ -116,8 +116,16 @@ export class DataModel {
const segments = this.parsePath(path);
const lastSegment = segments.pop()!;

if (!this.data) {
// Only an absent root is replaced with a container. A primitive root is a
// value the caller put there, and writing a path through it is the same
// error as writing through a primitive at any other depth.
if (this.data === undefined || this.data === null) {
this.data = {};
} else if (typeof this.data !== 'object') {
throw new A2uiDataError(
`Cannot set path '${path}': the data model root is a primitive value.`,
path,
);
}
let current: any = this.data;
for (let i = 0; i < segments.length; i++) {
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