Fix DNS resolver object churn for multiple sessions

[bdraco]

Problem

As reported in #10847, aiohttp currently creates a new DNSResolver object for each AsyncResolver instance, leading to excessive resolver object churn when using multiple sessions. This causes unnecessary resource usage and potential performance degradation, as the c-ares library documentation indicates that a single resolver can handle unlimited queries.

Solution

This PR introduces a shared resolver management system:

1. Created a new _DNSResolverManager singleton class that maintains a single shared aiodns.DNSResolver instance
2. Modified AsyncResolver to use the shared resolver for default arguments while still supporting custom resolver configurations
3. Implemented proper client tracking and cleanup through the get_resolver and release_resolver methods
4. Added comprehensive tests to ensure correct behavior

The key design point is requiring clients to be explicitly passed to get_resolver, ensuring that the manager can track all users of the shared resolver and properly clean up when they're done.

Implementation Details

• _DNSResolverManager is a singleton that ensures only one shared resolver exists
• Custom resolver configurations still create dedicated resolver instances
• Client tracking uses weakref.WeakSet to avoid reference cycles
• The shared resolver is automatically canceled and cleaned up when the last client is released
• All resolvers are properly cleaned up after tests

Benefits

• Drastically reduces the number of DNSResolver objects created for multiple sessions
• Decreases memory and resource usage
• Improves performance by avoiding unnecessary resolver creation and destruction
• Properly leverages c-ares's ability to handle unlimited queries with a single resolver

Related issue number

fixes #10847

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.76%. Comparing base (802152a) to head (dfbcb9f).
Report is 3 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #10897    +/-   ##
========================================
  Coverage   98.75%   98.76%            
========================================
  Files         129      129            
  Lines       38931    39085   +154     
  Branches     2164     2169     +5     
========================================
+ Hits        38447    38601   +154     
  Misses        336      336            
  Partials      148      148            

Flag	Coverage Δ
CI-GHA	98.64% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.35% <100.00%> (+<0.01%)	⬆️
OS-Windows	96.52% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.51% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.40% <100.00%> (+0.01%)	⬆️
Py-3.10.17	97.93% <100.00%> (+<0.01%)	⬆️
Py-3.11.12	98.00% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	97.48% <100.00%> (+0.01%)	⬆️
Py-3.12.10	98.42% <100.00%> (+<0.01%)	⬆️
Py-3.13.3	98.41% <100.00%> (-0.01%)	⬇️
Py-3.9.13	97.27% <100.00%> (+0.01%)	⬆️
Py-3.9.22	97.80% <100.00%> (+0.01%)	⬆️
Py-pypy7.3.16	88.86% <100.00%> (-4.63%)	⬇️
VM-macos	97.51% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.35% <100.00%> (+<0.01%)	⬆️
VM-windows	96.52% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #10897 will improve performances by 8.06%

_{Comparing single_async_resolver (dfbcb9f) with master (a4be2cb)}

Summary

⚡ 1 improvements
✅ 59 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_one_hundred_simple_get_requests_no_session[pyloop]	84.6 ms	78.3 ms	+8.06%

[bdraco]

oh right, we support multiple event loops still so we need to account for that

[bdraco]

Something else is actually leaking them outside of this

[bdraco]

I need to move the fixture to top level and make it auto use to find it.

I'm going to put it at top level, and than revert it after I fix all the test leaks

[bdraco]

We have far too many session leaks to fix them in this PR. I'll have to count before and after instead

[patchback]

Backport to 3.12: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 4624fed on top of patchback/backports/3.12/4624fed82608d9b659c4284814a570e0b332b8db/pr-10897

Backporting merged PR #10897 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.12/4624fed82608d9b659c4284814a570e0b332b8db/pr-10897 upstream/3.12

4. Now, cherry-pick PR Fix DNS resolver object churn for multiple sessions #10897 contents into that branch:

   $ git cherry-pick -x 4624fed82608d9b659c4284814a570e0b332b8db

   If it'll yell at you with something like fatal: Commit 4624fed82608d9b659c4284814a570e0b332b8db is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 4624fed82608d9b659c4284814a570e0b332b8db

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix DNS resolver object churn for multiple sessions #10897 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.12/4624fed82608d9b659c4284814a570e0b332b8db/pr-10897

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
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