From 98639f97ee0fd3e0f12be6c94ba094e522ca9967 Mon Sep 17 00:00:00 2001
From: Nathan Daly <nhdaly@gmail.com>
Date: Wed, 4 Sep 2019 13:24:57 -0400
Subject: [PATCH] Fix `assert_havelock(::ReentrantLock)` to assert that the
 _current-task_ has the lock.

Before this commit, new threads would incorrectly believe that they held
a lock on a Condition when they actually didn't, and would allow illegal
operations, e.g. notify:

```julia
julia> c = Threads.Condition()
Base.GenericCondition{ReentrantLock}(Base.InvasiveLinkedList{Task}(nothing, nothing), ReentrantLock(nothing, Base.GenericCondition{Base.Threads.SpinLock}(Base.InvasiveLinkedList{Task}(nothing, nothing), Base.Threads.SpinLock(Base.Threads.Atomic{Int64}(0))), 0))

julia> lock(c)

julia> fetch(Threads.@spawn Base.assert_havelock(c))  # This should be an ERROR (the new thread doesn't have the lock)

julia> fetch(Threads.@spawn notify(c))                # This should be an ERROR (the new thread doesn't have the lock)
0

julia> fetch(Threads.@spawn wait(c))                  # This error should be caught earlier (in assert_havelock).
ERROR: TaskFailedException:
unlock from wrong thread
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] unlockall(::ReentrantLock) at ./lock.jl:121
 [3] wait(::Base.GenericCondition{ReentrantLock}) at ./condition.jl:105
 [4] (::var"##19#20")() at ./threadingconstructs.jl:113
```

(The same holds for `@async` as `@spawn`.)

After this change, the assertion works correctly:
```
julia> c = Threads.Condition();

julia> lock(c)

julia> fetch(Threads.@spawn Base.assert_havelock(c))  # This correctly ERRORs
ERROR: TaskFailedException:
concurrency violation detected
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] concurrency_violation() at ./condition.jl:8
 [3] assert_havelock at ./condition.jl:28 [inlined]
 [4] assert_havelock at ./REPL[22]:1 [inlined]
 [5] assert_havelock(::Base.GenericCondition{ReentrantLock}) at ./condition.jl:73
 [6] (::var"##21#22")() at ./threadingconstructs.jl:113
```
---
 base/lock.jl | 1 +
 1 file changed, 1 insertion(+)

diff --git a/base/lock.jl b/base/lock.jl
index 60066220b744e6..ad728280c43e9d 100644
--- a/base/lock.jl
+++ b/base/lock.jl
@@ -16,6 +16,7 @@ mutable struct ReentrantLock <: AbstractLock
     ReentrantLock() = new(nothing, GenericCondition{Threads.SpinLock}(), 0)
 end
 
+assert_havelock(l::ReentrantLock) = assert_havelock(l, l.locked_by)
 
 """
     islocked(lock) -> Status (Boolean)