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Add a facility for unlocked director iteration #4142

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Add a facility for unlocked director iteration #4142

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Commits on Nov 9, 2024

  1. cache_vcl: Add a facility for unlocked director iteration 

    Pondering varnishcache#4140 made me realize that our central per-vcl director list
constitutes a classic case of iterating a mutable list. That besides the already
known fact that running potentially expensive iterator functions while holding
the vcl_mtx is a very bad idea.

So this patch is a suggestion on how to get out of this situation. It does not
go all the way (it still leaves unsolved a similar problem of iterating over all
backends of _all vcls_), but shows an attempt on how to tackle the "iterate over
one VCL's backends".

We add a fittingly named 'vdire' facility which manages the director list and,
in particular, director retirement. The main change is that, while iterators are
active on the director list, vcl_mtx is _not_ held and any request of a director
to retire only ends up in a resignation, which manifests by this director being
put on a second list.

When all iterators have completed, the resignation list is worked and the actual
retirement carried out.
    @nigoroll
    nigoroll committed Nov 9, 2024
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  2. vdire: Wait for directors to retire before staring a new iteration 

    This is to prevent a potential pileup of resigned directors with a constant
stream of iterators coming on
    @nigoroll
    nigoroll committed Nov 9, 2024
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