type-context.h: Extent cancel_mutex lock to prevent theoretical race #245

1. 
   ryanofsky commented at 8:19 pm on February 24, 2026: collaborator

   This is a followup to #240 that fixes a theoretical race condition in that PR pointed out by janb https://github.com/bitcoin/bitcoin/pull/34422#discussion_r2849037119. Details are in the commit message. There is also an additional commit fixing up some documentation added in that PR
2. type-context.h: Extent cancel_mutex lock to prevent theoretical race

   As pointed out by janb84 in
   https://github.com/bitcoin/bitcoin/pull/34422#discussion_r2849037119 it makes
   sense for the on_cancel callback to lock cancel_mutex while it is assigning
   request_canceled = true.
   
   The lock and assigment were introduced in #240 and in an earlier version of
   that PR, request_canceled was a std::atomic and the assignment happened before
   the lock was acquired instead of after, so it was ok for the lock to be unnamed
   and immediately released after being acquired.
   
   But in the final verion of #240 request_canceled is an ordinary non-atomic
   bool, and it should be assigned true with the lock held to prevent a
   theoretical race condition where capn'proto event loop cancels the request
   before the execution thread runs, and the execution thread sees the old
   request_canceled = false value and then unsafely accesses deleted parameters.
   The request being canceled so quickly and parameters being accessed so slowly,
   and stale request_canceled value being read even after the execution thread has
   the cancel_mutex lock should be very unlikely to occur in practice, but could
   happen in theory and is good to fix.

   e0f1cd7621
3. doc: Comment cleanups after #240 ef96a5b2be
4. 
   DrahtBot commented at 8:19 pm on February 24, 2026: none

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5. 
   Sjors commented at 3:09 pm on February 25, 2026: member

   ACK ef96a5b2be2f35ef09c19a5ff4a113144c596309
6. ryanofsky merged this on Feb 25, 2026
7. ryanofsky closed this on Feb 25, 2026

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