feat(mount): Implement negative cache

[ZaRdEr15]

All lookups must go through the network to master which is time consuming. This is especially a problem if it constantly asks for files that don't exist.

Implement a thread-safe negative cache that reduces network latency by avoiding repeated calls to the master server by replying early with fuse_entry_param with fields .ino=0 and .entry_timeout=sfsnegativecachetimeout.

Kernel may cache negative entries, but with internal negative cache implementation it is guaranteed if both options sfsnegativecachesize and sfsnegativecachetimeout are set.

When any option equals to 0, both internal and Linux kernel-level negative caching are disabled.

Refactor lookup() in sauna_client.cc to improve readability and code style.

[gemini-code-assist]

Summary of Changes

Hello @ZaRdEr15, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the file system's performance by implementing a negative caching mechanism. This feature aims to minimize redundant network requests to the master server for files that have previously been determined not to exist, thereby reducing latency and improving overall responsiveness, especially in environments with frequent lookups for non-existent paths.

Highlights

• Negative Cache Implementation: Introduced a new thread-safe negative cache to reduce network latency by storing information about non-existent files, preventing repeated master server lookups.
• Configurable Options: Added new mount options sfsnegativecachetimeout and sfsnegativecachesize to control the behavior and size of the negative cache. Setting these to 0 disables the cache.
• Integration with Lookup Operations: The lookup function now checks the negative cache before querying the master. If a file is found to be non-existent (ENOENT), it's added to the negative cache for a specified duration.
• New Files and Tests: Added negative_cache.h for the cache implementation, negative_cache_unittest.cc for unit tests, and test_negative_cache.sh for system-level validation of the feature.

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[gemini-code-assist]

Code Review

This pull request introduces a negative cache for file lookups, which is a great feature for improving performance on lookups for non-existent files. The implementation is well-structured, with a new NegativeCache class, corresponding unit tests, and integration into the existing lookup logic.

I've found one critical issue in the NegativeCache implementation that could lead to undefined behavior if the cache size is set to zero. I've also provided a few medium-severity suggestions to improve a misleading comment about the cache eviction policy and to add a unit test for the aforementioned edge case.

Overall, the changes are good and the feature is valuable. After addressing the critical issue, this PR should be in good shape to merge.

[dmga44]

Please consider my comments.

[rolysr]

Very nice addition @ZaRdEr15. Please consider checking my suggestions. When final version of changes is reached, I recommend to you to squash most of changes to more general commits, specially before merging, for a cleaner commit history.

[rolysr]

LGTM. Just a minor suggestion for test style. Very nice to have this feature 👍. Consider when merging, to squash these commits as all these changes should be consider as one atomic change in the history.

[uristdwarf]

Consider when merging, to squash these commits as all these changes should be consider as one atomic change in the history.

It may be better to keep the commits instead of squashing, as they are very well done and atomic themselves.

[dmga44]

LGTM.