Prevent reserved bytes underflow

[variadico]

Currently, there is a bug where js.storeReserved and js.memReserved can underflow, causing /varz metrics to be incorrect. Below is a history of the state changes.

1. Create stream with no MaxBytes: js.storeReserved → 0
2. Update stream with MaxBytes=1234: js.storeReserved → 0 (should have reserved 1234)
3. Delete stream: js.storeReserved → -1234

Bug:js.storeReserved=-1234 is an int that later gets converted to a uint64, which causes an underflow.

This change releases the old MaxBytes and reserves the new MaxBytes during the stream update step. This way, when a stream is deleted, these values don't become negative.

Resolves #2882 (see for additional info)

[matthiashanel]

Would you mind adding the following tests?

1. Reserve 2/3rd of the total account limit, then update to one extra byte.
2. Reserve 2/3rd of the total server limit, then update to one extra byte.

I believe we may have an accounting issue there as well.

These two scenarios apply to going down by a byte as well.

[variadico]

The newer commits add the following new tests. They caught some bugs, so the original handling of MaxBytes was also updated.

--- PASS: TestStorageReservedBytes (0.06s)
    --- PASS: TestStorageReservedBytes/file_reserve_66%_of_system_limit (0.01s)
    --- PASS: TestStorageReservedBytes/memory_reserve_66%_of_system_limit (0.00s)

    --- PASS: TestStorageReservedBytes/file_update_past_system_limit (0.00s)
    --- PASS: TestStorageReservedBytes/memory_update_past_system_limit (0.00s)

    --- PASS: TestStorageReservedBytes/file_update_to_system_limit (0.00s)
    --- PASS: TestStorageReservedBytes/memory_update_to_system_limit (0.00s)

    --- PASS: TestStorageReservedBytes/file_reserve_66%_of_account_limit (0.00s)
    --- PASS: TestStorageReservedBytes/memory_reserve_66%_of_account_limit (0.00s)

    --- PASS: TestStorageReservedBytes/file_update_to_account_limit (0.00s)
    --- PASS: TestStorageReservedBytes/memory_update_to_account_limit (0.00s)

[variadico]

Added more tests to verify that the leader's system limits don't override non-leader server's limits.

--- PASS: TestJetStreamSystemLimitsPlacement (2.11s)
    --- PASS: TestJetStreamSystemLimitsPlacement/file_create_large_stream_on_small_server (0.00s)
    --- PASS: TestJetStreamSystemLimitsPlacement/memory_create_large_stream_on_small_server (0.00s)

    --- PASS: TestJetStreamSystemLimitsPlacement/file_create_large_stream_on_medium_server (0.00s)
    --- PASS: TestJetStreamSystemLimitsPlacement/memory_create_large_stream_on_medium_server (0.00s)

    --- PASS: TestJetStreamSystemLimitsPlacement/file_create_large_stream_on_large_server (0.01s)
    --- PASS: TestJetStreamSystemLimitsPlacement/memory_create_large_stream_on_large_server (0.00s)

[matthiashanel]

LGTM

[variadico]

Added new tests where we create a supercluster and ensure we can't create a large MaxBytes stream on a smaller cluster.

--- PASS: TestJetStreamSuperClusterSystemLimitsPlacement (5.22s)
    --- PASS: TestJetStreamSuperClusterSystemLimitsPlacement/file_create_large_stream_on_small_cluster_b0 (0.01s)
    --- PASS: TestJetStreamSuperClusterSystemLimitsPlacement/memory_create_large_stream_on_small_cluster_b0 (0.00s)
    --- PASS: TestJetStreamSuperClusterSystemLimitsPlacement/file_create_large_stream_on_small_cluster_b1 (0.00s)
    --- PASS: TestJetStreamSuperClusterSystemLimitsPlacement/memory_create_large_stream_on_small_cluster_b1 (0.00s)
    --- PASS: TestJetStreamSuperClusterSystemLimitsPlacement/file_create_large_stream_on_small_cluster_b2 (0.00s)
    --- PASS: TestJetStreamSuperClusterSystemLimitsPlacement/memory_create_large_stream_on_small_cluster_b2 (0.00s)
    --- PASS: TestJetStreamSuperClusterSystemLimitsPlacement/file_create_large_stream_on_large_cluster_a0 (0.01s)
    --- PASS: TestJetStreamSuperClusterSystemLimitsPlacement/memory_create_large_stream_on_large_cluster_a0 (0.01s)
    --- PASS: TestJetStreamSuperClusterSystemLimitsPlacement/file_create_large_stream_on_large_cluster_a1 (0.01s)
    --- PASS: TestJetStreamSuperClusterSystemLimitsPlacement/memory_create_large_stream_on_large_cluster_a1 (0.00s)
    --- PASS: TestJetStreamSuperClusterSystemLimitsPlacement/file_create_large_stream_on_large_cluster_a2 (0.00s)
    --- PASS: TestJetStreamSuperClusterSystemLimitsPlacement/memory_create_large_stream_on_large_cluster_a2 (0.00s)

[kozlovic]

LGTM