Struct TagPool 

pub struct TagPool { /* private fields */ }

r[impl jetstream.subscription.cancel] Three regions of the tag space, not one pool.

A subscription holds its tag for its whole life. Drawing subscriptions and unary calls from one pool therefore means enough live subscriptions make an ordinary call impossible — at max_concurrent_requests = 1, one open room blocks every post for its duration.

Cancellation is worse than starved, it deadlocks: a cancellation needs a fresh tag, the subscription’s own tag is not released until its terminator arrives, and the terminator cannot arrive until the cancellation is sent. The pool is saturated by exactly the subscriptions the cancellation exists to end.

r[jetstream.subscription.cancel] requires control capacity reserved outside the pool and forbids subscriptions exhausting the capacity available to ordinary calls. Both are satisfied here by giving each its own region: ordinary calls keep 1..=size, and the meaning max_concurrent_requests always had, while subscriptions and cancellations are drawn from above it.

The control region is sized equal to the streaming region, which is what makes the deadlock unreachable rather than merely unlikely: at most span subscriptions can exist, so at most span cancellations can ever be in flight at once.

The backend is whichever of channel, notify or semaphor JETSTREAM_TAG_POOL_BACKEND selected; each region is one instance of it, and the offsets live here so no backend has to know about any of this.

Implementations

impl TagPool

pub fn new(size: u16) -> Self

pub async fn acquire_tag(&self) -> u16

A tag for a unary call. Bounded by max_concurrent_requests, and unaffected by how many subscriptions are open.

pub async fn acquire_streaming_tag(&self) -> u16

r[impl jetstream.subscription.cancel] A tag for a subscription, from its own region, so no number of live subscriptions can make an ordinary call impossible.

pub async fn acquire_control_tag(&self) -> u16

r[impl jetstream.subscription.cancel] A tag for a cancellation, from capacity reserved outside both other regions.

pub async fn release_tag(&self, tag: u16)

pub fn is_streaming_tag(&self, tag: u16) -> bool

Which region a tag came from. Used by the multiplexer to tell a subscription’s tag from an ordinary call’s without consulting the in-flight map.