refactor(sandbox): name the E2B ledger meta-field count (#4764)

Admission derived the live-entry count as `HLEN - 3`, where 3 was the number
of `meta:*` fields written 35 lines earlier in initialize(). Nothing tied the
two together, so adding a fourth meta field would shift the capacity ceiling
by one.

Name the offset `META_FIELD_COUNT` next to initialize(), and add a guard test
asserting a freshly initialized ledger holds exactly those three fields, plus
one pinning that a hard_limit of N admits exactly N reservations.

References #4575

Co-authored-by: icn5381 <255778606+icn5381@users.noreply.github.com>

backend/packages/harness/deerflow/community/e2b_sandbox/capacity/redis.py

@@ -18,6 +18,13 @@ local function now_ms()
     return tonumber(current[1]) * 1000 + math.floor(tonumber(current[2]) / 1000)
 end
 
+-- Number of 'meta:*' fields initialize() writes. Live entries ('r:'/'s:') are
+-- counted as HLEN minus this, so the two must move together: adding a meta
+-- field without updating this constant makes the ledger over-count usage and
+-- reject one reservation early. test_ledger_meta_field_count_matches_constant
+-- pins it against initialize()'s actual output.
+local META_FIELD_COUNT = 3
+
 local function initialize(hard_limit)
     redis.call('HSET', KEYS[1],
         'meta:state', 'initializing',
@@ -57,7 +64,7 @@ if operation == 'reserve' then
     if redis.call('HEXISTS', KEYS[1], field) == 1 then
         return 'GRANTED'
     end
-    if redis.call('HLEN', KEYS[1]) - 3 >= tonumber(hard_limit) then
+    if redis.call('HLEN', KEYS[1]) - META_FIELD_COUNT >= tonumber(hard_limit) then
         return 'FULL'
     end
     redis.call('HSET', KEYS[1], field, tostring(now_ms()))

backend/tests/test_e2b_capacity_store_redis.py

@@ -176,3 +176,33 @@ def test_mismatched_hard_limits_fail_closed(make_store) -> None:
         gateway_b.revision()
     with pytest.raises(CapacityBackendError, match="configuration mismatch"):
         gateway_b.reserve("reservation")
+
+
+def test_ledger_meta_field_count_matches_constant(make_store) -> None:
+    # Admission derives the live-entry count as HLEN - META_FIELD_COUNT, so the
+    # Lua constant must equal the number of 'meta:*' fields initialize() writes.
+    # Adding a meta field without updating the constant also trips the existing
+    # concurrency tests, but those report an unexpected reservation outcome; this
+    # one names the cause.
+    store = make_store(3)
+    _initialize(store)
+    fields = store._redis.hkeys(store.key)
+
+    assert _counts(store) == (0, 0)
+    assert sorted(fields) == ["meta:hard_limit", "meta:revision", "meta:state"]
+    assert len(fields) == 3
+
+
+def test_reserve_admits_exactly_hard_limit_entries(make_store) -> None:
+    # Locks the invariant the constant exists to protect: an off-by-one in the
+    # HLEN offset shifts the ceiling, which this catches regardless of how the
+    # offset happens to be spelled.
+    store = make_store(3)
+    _initialize(store)
+
+    granted = [store.reserve(f"reservation-{index}") for index in range(3)]
+
+    assert granted == [ReserveStatus.GRANTED] * 3
+    assert _counts(store) == (0, 3)
+    assert store.reserve("reservation-overflow") is ReserveStatus.FULL
+    assert store.reserve("reservation-0") is ReserveStatus.GRANTED