gate: add named_gate

[bhalevy]

Allow specifying a name for the gate when created using a named exception factory,
to help debugging gate_closed_exception.

[bhalevy]

Intended use case: scylladb/scylladb#23329

[xemul]

Same problem for semaphores is solved with the help of "named" exception factory (59a6a47). It's worth making gates and semaphores symmetrical in this sense

[bhalevy]

In v2 (380d057):

• rebased
• used exception factory to distinguish between gate and named_gate.
  • added a templated basic_gate(T value) constructor as a helper to construct named gate as named_gate("foo")

[xemul]

LGTM, but

1. "gate: add named_gate" patch comment should be updated (patch title should probably be updated too)
2. CI shows compilation failures

[bhalevy]

In v3 (7c31e6f):

• rebased
• extracted gate: templatize class basic_gate
• renamed patch and edited git commit message
• also this PR's title and description
• fixed build issues in debug mode

[bhalevy]

FWIW, the strong named_gate type causes a lot of churn since it is not derived from gate :-/

[bhalevy]

@xemul please see alternative design here: bhalevy@4907d7d
This one causes much less churn as named_gate is derived from gate in this model and other than using named_gate for defining members/variables so they can constructed with a name, they are indistinguishable from regular, unnamed, gates.

[xemul]

FWIW, the strong named_gate type causes a lot of churn since it is not derived from gate :-/

If this whole churn is isolated in gate.hh, I'd still go this way, it looks more solid and self-contained to me

[bhalevy]

FWIW, the strong named_gate type causes a lot of churn since it is not derived from gate :-/

If this whole churn is isolated in gate.hh, I'd still go this way, it looks more solid and self-contained to me

No, it requires to change current user code that use gate::holder to be modified to named_gate::holder, since named_gate is strongly distinguishable from gate.
This it not the case with semaphores since there, sempahore_units is an independent template, allowing get_units to operate on any specialized semaphore with no change to user code.
But in contrast, gate::holder is defined as a nested type.