tpl/partials: Make sure a cached partial is invoked only once

[bep]

This commit revises the locking strategy for partialCached. We have added a benchmark that may be a little artificial, but it should at least show that we're not losing any performance over this:

name              old time/op    new time/op    delta
IncludeCached-10    12.2ms ± 2%    11.3ms ± 1%   -7.36%  (p=0.029 n=4+4)

name              old alloc/op   new alloc/op   delta
IncludeCached-10    7.17MB ± 0%    5.09MB ± 0%  -29.00%  (p=0.029 n=4+4)

name              old allocs/op  new allocs/op  delta
IncludeCached-10      128k ± 1%       70k ± 0%  -45.42%  (p=0.029 n=4+4)

This commit also revises the template metrics hints logic a little, and add a test for it, which output is currently this:

 cumulative       average       maximum      cache  percent  cached  total
       duration      duration      duration  potential   cached   count  count  template
     ----------      --------      --------  ---------  -------  ------  -----  --------
      163.334µs     163.334µs     163.334µs          0        0       0      1  index.html
       23.749µs       5.937µs      19.916µs         25       50       2      4  partials/dynamic1.html
        9.625µs       4.812µs        6.75µs        100       50       1      2  partials/static1.html
        7.625µs       7.625µs       7.625µs        100        0       0      1  partials/static2.html

Some notes:

• The duration now includes the cached invocations (which should be very short)
• A cached template gets executed once before it gets cached, so the "percent cached" will never be 100.

Fixes #4086
Fixes #9506

[bep]

OK, I didn't get the new numbers from the "template hints" output to add up, so I adjusted the logic a little and added a test case.

@davidsneighbour @moorereason @jmooring some feedback on this would be appreciated, ref. my test output above:

 cumulative       average       maximum      cache  percent  cached  total
       duration      duration      duration  potential   cached   count  count  template
     ----------      --------      --------  ---------  -------  ------  -----  --------
      163.334µs     163.334µs     163.334µs          0        0       0      1  index.html
       23.749µs       5.937µs      19.916µs         25       50       2      4  partials/dynamic1.html
        9.625µs       4.812µs        6.75µs        100       50       1      2  partials/static1.html
        7.625µs       7.625µs       7.625µs        100        0       0      1  partials/static2.html

My statement about A cached template gets executed once before it gets cached, so the "percent cached" will never be 100 could be questioned ...

An example of that would be partials/static1.html above, which is included twice with partialCached. So the cached percentage is 50. In real situations this will probably round up to 100, so I guess this is correct. Just wanted to shout out about it.

[moorereason]

I agree with your comments @bep and the revisions (at least from looking at it on my phone). Would it be clearer if the "cache count" column was called "cache hits"?

[bep]

Test failing on Windows, happy times...

[bep]

@moorereason we can rename the column(s) later.

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