fix: JSONB ASM direct-write skips level tracking causing false 'level too large' on flat lists, for issue #3616

[daguimu]

Problem

JSONB.toBytes(list) on a flat list of JavaBeans throws JSONException: level too large : 2049 after ~2049 elements, even though the structure has no actual nesting. Reported reproducer (issue #3616):

List<TestObject> list = new ArrayList<>();
for (int i = 0; i < 2100; i++) {
    list.add(new TestObject("a" + i, new BigDecimal("1232132")));
}
JSONB.toBytes(list);  // throws "level too large : 2049"

TestObject has a String field and a BigDecimal field — both standard, both unrelated to nesting. Stack trace:

JSONException: level too large : 2049
  at JSONWriter.overflowLevel
  at JSONWriterJSONB.startObject (line 81)
  at OWG_x_x_TestObject.writeJSONB (ASM-generated)
  at ObjectWriterImplList.writeJSONB

Root cause

ObjectWriterCreatorASM.genMethodWriteJSONB partitions the bean's field writers into "direct" and "non-direct" groups. The two paths emit the BC_OBJECT / BC_OBJECT_END framing differently:

• Non-direct path (group.start): calls JSONWriter.startObject() — writes BC_OBJECT and increments level.
• Non-direct path (group.end): calls JSONWriter.endObject() — writes BC_OBJECT_END and decrements level.
• Direct path (group.start): writes BC_OBJECT raw to the byte buffer — does not touch level.
• Direct path (group.end): writes BC_OBJECT_END raw to the byte buffer — does not touch level.

For TestObject, the field groups order out as [BigDecimal c (non-direct, start=true)] + [String d (direct, end=true)]. So startObject() increments level but the matching end is the raw-byte direct path, which leaves level incremented. Each bean in the list leaks 1 level. At iteration 2049 we hit context.maxLevel and overflowLevel() fires.

The same imbalance occurs on any field-group split where start and end fall on different paths.

Fix

Mirror the level bookkeeping on the direct-write path so it stays consistent with startObject() / endObject():

• New JSONWriter.incrementLevel() — bumps level and runs the same overflow check as startObject(). No byte output.
• New JSONWriter.decrementLevel() — decrements level. No byte output.

ObjectWriterCreatorASM calls incrementLevel() after writing BC_OBJECT raw and decrementLevel() after writing BC_OBJECT_END raw. The direct path keeps its raw-byte field-value writes, so the performance optimisation is preserved.

Tests

core/src/test/java/com/alibaba/fastjson2/issues_3600/Issue3616.java:

• testFlatListBigDecimalAndString — 2100-element list of (String, BigDecimal) beans; serialise to JSONB and round-trip back, asserting size and field values. Reproduces the issue exactly and verifies it is gone.
• testFlatListWithDateObject — 2100-element list of (String, Date) beans, exercising another non-direct field type to guard against future regressions.

Existing regression tests still pass:

• Issue3657 (intentional level too large for genuinely deep nesting) — 6 tests
• Issue7616 (related fix for direct-write byte-frame capacity) — 3 tests
• *Writer* test suite — 394 tests, all green

Impact

Surgical: 2 helper methods on JSONWriter (no behaviour change for existing callers) plus 6 lines of bytecode emission in the JSONB ASM generator. No JSON-text serialiser changes, no API surface broken.

Fixes #3616