chenBright opened a new pull request, #3470:
URL: https://github.com/apache/brpc/pull/3470
### What problem does this PR solve?
Issue Number: resolve #2888
Problem Summary:
bvar's global `VarMap` is guarded by a pthread mutex.
`Variable::describe_exposed()`
(and `describe_series_exposed()`, `dump_exposed()`, plus the multi-dimension
`MVariableBase` counterparts) used to invoke `var->describe()` while holding
that lock.
For `PassiveStatus`, `describe()` runs a user-provided callback; if the
callback yields
the bthread (e.g. by acquiring a `bthread::Mutex`), the pthread mutex is
never released
and the process deadlocks.
### What is changed and the side effects?
Changed:
Run user callbacks OUTSIDE the global map lock via a small indirection
handle:
- New `bvar/detail/exposed_ref.h`: `ExposedRef<T>` (a reference-counted
handle
guarding an exposed object). It uses `butil::Mutex` +
`butil::ConditionVariable`.
- `describe_exposed()` / `describe_series_exposed()` / `get_exposed()`:
under the
map lock they now only `seek` + `acquire()` (ref-count +1, serialized with
`hide()`'s
erase); the lock is released, `describe()` is called outside the lock,
then `release()`.
- `hide()` now also invalidates the handle and blocks (`hide_and_wait()`)
until
all in-flight readers finish, so a Variable cannot be destroyed while a
concurrent
`describe()` is still using it. Each `expose()` rebuilds a fresh handle
(the old one
is single-use once hidden).
- `MVariableBase::describe_exposed()` / `dump_exposed()` get the same
treatment;
`dump()` in particular is moved outside the lock because `Dumper` is a
user-overridable interface that may yield.
- Since callbacks no longer run under the lock, the recursive `VarMap` mutex
is no
longer needed and is reverted to a plain mutex (now consistent with
`MVarMap`).
Side effects:
- Performance effects:
- Breaking backward compatibility:
---
### Check List:
- Please make sure your changes are compilable.
- When providing us with a new feature, it is best to add related tests.
- Please follow [Contributor Covenant Code of
Conduct](https://github.com/apache/brpc/blob/master/CODE_OF_CONDUCT.md).
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