perf: Optimize folder UpdateTime timestamp by using int64 instead of time.Time

[vedantdas-source]

Description

By default,Folder.UpdateTime was stored as a time.Time value, which occupies 24 bytes. By converting this field to a Unix nanosecond int64 representation which uses 8 bytes, we can safely reduce the structural footprint of every Folder object .

Changes Made

• Changed Folder.UpdateTime from time.Time to int64.
• Added a zero-value check !t.IsZero() in GCSFolder to ensure that uninitialized folder timestamps default to 0 instead of a negative value.
• Updated the mock bucket generation and FUSE assertions to compare the equivalent UnixNano() values.

Benchmarking results

Using time.Time

goos: linux
goarch: amd64
pkg: github.com/googlecloudplatform/gcsfuse/v3/internal/storage/gcs
cpu: Intel(R) Xeon(R) CPU @ 2.60GHz
BenchmarkGCSFolder-48           12818931                94.74 ns/op      96 B/op          2 allocs/op

Using int64

goos: linux
goarch: amd64
pkg: github.com/googlecloudplatform/gcsfuse/v3/internal/storage/gcs
cpu: Intel(R) Xeon(R) CPU @ 2.60GHz
BenchmarkGCSFolder-48           14288154                87.04 ns/op      72 B/op          2 allocs/op

All existing tests pass successfully.

Link to the issue in case of a bug fix.

Testing details

1. Manual - NA
2. Unit tests - Added a BenchmarkGCSFolder benchmark in folder_test.go to compare memory and CPU performance.
3. Integration tests - NA

Any backward incompatible change? If so, please explain.

[gemini-code-assist]

Code Review

This pull request refactors the Folder struct's UpdateTime field from time.Time to int64 (Unix nanoseconds) across the GCS and fake storage implementations, and adds a benchmark test for GCSFolder. The review feedback highlights a critical concurrency issue in RenameFolder where the mutex lock b.mu is not acquired, potentially causing race conditions. Additionally, the reviewer recommends replacing instances of time.Now() with the mock clock b.clock.Now() in the fake bucket implementation to maintain deterministic test behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.70%. Comparing base (b61c988) to head (c598bc3).

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4859       +/-   ##
===========================================
+ Coverage        0   83.70%   +83.70%     
===========================================
  Files           0      170      +170     
  Lines           0    20954    +20954     
===========================================
+ Hits            0    17539    +17539     
- Misses          0     2760     +2760     
- Partials        0      655      +655     

Flag	Coverage Δ
unittests	83.70% <100.00%> (?)

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[raj-prince]

LGTM.