feat(buffered reader default ON): Reuse read handle to optimize reader creation

[meet2mky]

Description

This pull request introduces a mechanism to reuse GCS read handles within the BufferedReader. By reusing the read handle, we can optimize the creation of subsequent GCS readers for the same GCS object, which is particularly beneficial for zonal buckets as it can bypass authorization and metadata checks.

Changes

• A new ReadHandle struct is introduced to store the read handle and its expiration time (5 mins)
• BufferedReader now uses an atomic.Pointer to the ReadHandle struct for thread-safe access and updates
• The read handle is updated by the downloadTask after a successful read from a zonal bucket after which it will be refreshed if it's expired.
• Unit tests have been added to verify the behaviour of the read handle logic, including initial state, updates on expiration, and no updates when the handle is still valid.

Performance

Sequential Reads on RAPID

Master: upto 4.2 GB/sec
PR: upto 4.5 GB/sec

New Reader Creation

Master (AVG: 22ms):

PR (AVG: 15ms):

Link to the issue in case of a bug fix.

Testing details

1. Manual - Yes
2. Unit tests - Yes
3. Integration tests - Part of pre-submit.

Any backward incompatible change? If so, please explain.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.26%. Comparing base (5e24904) to head (a14299c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4086       +/-   ##
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+ Coverage        0   83.26%   +83.26%     
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  Files           0      153      +153     
  Lines           0    18612    +18612     
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+ Hits            0    15498    +15498     
- Misses          0     2544     +2544     
- Partials        0      570      +570     

Flag	Coverage Δ
unittests	83.26% <100.00%> (?)

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[raj-prince]

Could you please confirm, if this expiry time is client specific? Or server maintains a shared handle and refresh every 5 mins?

[meet2mky]

Expiry time is based on per buffered Reader instance. I couldn't find any resources apart from it's mention in the source go/gcsfuse-zonal-buckets-read-design-doc