Summary

Delete the unreachable rematch logic in SessionAwareCache._resolve_release_state, inline the trivial remainder into release_session, and drop the two unit tests that exercised only the dead path.

Why this is dead code

The rematch block was introduced in #21875 together with an early-return TODO that disabled it in the same commit:

# TODO: re-match logic disabled -- match_prefix has side effects
# (splits) that disturb tree accounting. Directly using
# slot.last_node + cache_protected_len is safe after split analysis.
return protected_len, lock_node

if (...):  # unreachable from here on
    ...
    rematch = self.inner.match_prefix(...)

So the ~40-line block under the early return has never executed in production. It was kept as a placeholder for "future rematch if split-safety assumption breaks."

Why rematch isn't needed

RadixCache._split_node mutates TreeNode objects in place:

• child node stays alive; only its key / value get shortened to the post-split suffix.
• new_node is inserted above it with the pre-split prefix; lock_ref propagates.

So slot.last_node = child stays a valid reference across splits, and slot.cache_protected_len (locked token count) is unchanged. The (last_node, cache_protected_len) pair remains correct for dec_lock_ref without any rematch.

Running match_prefix in release_session would actively cause further splits -- exactly the side effect the TODO comment flags.

What the PR does

1. Delete _resolve_release_state entirely.
2. Inline the trivial assignment at the release_session call site; rewrite the docstring to capture the split-safety reasoning.
3. Remove the now-unused req parameter from release_session (single production caller and one unit-test caller updated).
4. Delete the two unit tests (test_release_session_recomputes_current_tree_owned_prefix, test_release_session_never_grows_tree_owned_prefix) -- they set up _FakeInnerCache match_results and asserted rematch outputs that were never produced.

Test plan

• Syntax check all touched files
• Remaining 5 unit tests in test_streaming_session_unit.py pass
• Full streaming session integration suite passes on H200 with strict busy mem check (SGLANG_ENABLE_STRICT_MEM_CHECK_DURING_BUSY=2):
  • TestStreamingSession + TestStreamingSessionMixedChunk: 10/10
  • TestStreamingSessionRetract + TestStreamingSessionRetractMixedChunk: 10/10
  • TestStreamingSessionEagle + TestStreamingSessionEagleV2: 10/10 (1 expected skip for spec-v2 overshoot)
  • TestStreamingSessionAbortLeakRepro: 1/1
  • Zero leak assertions across all runs.