THRIFT-6056: Limit recursion depth in Dart struct read/write

[Jens-G]

Summary

THRIFT-6056 — bound the recursion depth of Dart struct read/write.

On master the generated Dart read() / write() call readStructBegin /
writeStructBegin directly with no depth bound, and TProtocol has no
recursion counter, so a deeply nested or cyclic struct is processed without a
limit in the generated path. (This is unlike netstd/Delphi, whose generated
code already brackets every read/write with a recursion tracker — there the
equivalent change would double-count; here there is no pre-existing bound.)

This PR:

• adds a _recursionDepth counter with incrementRecursionDepth() /
  decrementRecursionDepth() to TProtocol, bounded at 64
  (TProtocolErrorType.DEPTH_LIMIT on excess);
• updates the Dart generator to wrap each generated struct read/write body in
  try { … } finally { decrementRecursionDepth(); } so the counter is always
  restored. The bound lives only in the generated path (one count per struct);
  readStructBegin / writeStructBegin are left untouched, so there is no
  double-counting.

Unknown-field skipping during read is handled separately by TProtocolUtil
(which carries its own depth parameter) and is left unchanged here.

Test

The previous test exercised increment/decrementRecursionDepth() in isolation,
which does not cover the real generated read/write path. It is replaced by
test/dart/recursion_depth_test — a generated-code round-trip over the
recursive IDL types in test/Recursive.thrift (CoRec ⇄ CoRec2, RecTree),
run across the binary, compact and JSON protocols:

case	expectation
chain at the limit (64)	round-trips
chain one over (65), write	DEPTH_LIMIT
crafted payload one over (65), read	DEPTH_LIMIT
wide shallow tree (192 siblings)	round-trips (decrement unwinds each sibling)
cyclic graph	DEPTH_LIMIT instead of unbounded recursion

Validation with Dart 3.12 (dart:stable):

• with this PR: 18/18 pass (6 cases × 3 protocols).
• against a master baseline (unpatched generator + master TProtocol):
  the 9 over-limit / cyclic cases fail — writes and reads are not bounded and
  the cyclic case recurses to StackOverflowError — confirming the gap this
  change closes.

Generation and the test run are wired through make -C test/dart recursion-test
(mirroring the existing thrift_test stub rules) and kept out of the default
cross chain; the suite above was validated directly with dart pub get && dart test under Dart 3.x, since it needs a null-safe Dart ≥ 2.12 and the cross
images currently pin 2.7.2.

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com