The broadcast channel allows multiple senders to send messages to
multiple receivers, where each receiver receives messages starting from
when it subscribes. After all senders are dropped, the receivers will
continue to receive all waiting messages in the buffer and then receive
a `Closed` error.
To mark that a channel has closed, it stores two closed flags, one on
the channel level and another in the buffer slot *after* the last used
slot (this may also be the earliest entry being kept for lagged
receivers, see #2425).
However, we don't need both closed flags, keeping the channel level
closed flag is sufficient.
Without the slot level closed flag, each receiver receives each message
until it is up to date and for that receiver the channel is empty. Then,
the actual return message is chosen depending on the channel level
closed flag; if the channel is NOT closed, then `Empty` is returned, if
the channel is closed then `Closed` is returned instead.
With the modified logic, there is no longer a need to append a closed
token to the internal buffer (by setting the slot level closed flag on
the next slot). This fixes the off by one error described in #4814,
which caused a receiver which was created after the channel was already
closed to get `Empty` from `try_recv` (or hang forever when calling
`recv`) instead of receiving `Closed`.
As a bonus, we save a single `bool` on each buffer slot.
Refs: #4814
Functions that may panic can be annotated with `#[track_caller]` so that
in the event of a panic, the function where the user called the
panicking function is shown instead of the file and line within Tokio
source.
This change adds track caller to all the non-unstable public APIs in
Tokio core where the documentation describes how the function may panic
due to incorrect context or inputs. Since each internal function needs
to be annotated down to the actual panic, it makes sense to start in
Tokio core functionality.
Tests are needed to ensure that all the annotations remain in place in
case internal refactoring occurs.
The test installs a panic hook to extract the file location from the
`PanicInfo` struct and clone it up to the outer scope to check that the
panic was indeed reported from within the test file. The downside to
this approach is that the panic hook is global while set and so we need
a lot of extra functionality to effectively serialize the tests so that
only a single panic can occur at a time.
The annotation of `block_on` was removed as it did not work. It appears
to be impossible to correctly chain track caller when the call stack to
the panic passes through clojures, as the track caller annotation can
only be applied to functions. Also, the panic message itself is very
descriptive.
Hayden Stainsby