Optimize directory lookup

[dmcgowan]

When resolving paths, previously an entire directory would be scanned then iterated through. With this change it is able to directly do the lookup while reading from the blocks. It performs binary search within each block since EROFS directories are sorted by name.

Also fixes a panic when closing a file that was opened but never read.

Benchmark: directory lookup

goos: linux
goarch: amd64
cpu: Intel(R) Xeon(R) CPU D-1528 @ 1.90GHz

Time (sec/op)

Case	main	this PR	Change
shallow	37.20µs	26.26µs	-29.41%
deep	72.82µs	52.05µs	-28.52%
bigdir-first	4094.9µs	43.71µs	-98.93%
bigdir-last	3912.6µs	41.61µs	-98.94%
bigdir-notfound	3911.6µs	39.34µs	-98.99%
geomean	701.4µs	39.63µs	-94.35%

Memory (B/op)

Case	main	this PR	Change
shallow	16.8Ki	8.4Ki	-50.30%
deep	33.4Ki	16.7Ki	-50.05%
bigdir-first	659.8Ki	8.4Ki	-98.73%
bigdir-last	659.9Ki	8.4Ki	-98.73%
bigdir-notfound	659.8Ki	8.3Ki	-98.74%
geomean	174.4Ki	9.6Ki	-94.50%

Allocations (allocs/op)

Case	main	this PR	Change
shallow	25	8	-68.00%
deep	43	16	-62.79%
bigdir-first	10,065	8	-99.92%
bigdir-last	10,065	8	-99.92%
bigdir-notfound	10,064	7	-99.93%
geomean	1,019	8.9	-99.12%

[Copilot]

Pull request overview

This PR optimizes EROFS path resolution by replacing full-directory scans with an on-the-fly directory entry lookup while reading directory blocks, improving lookup time and reducing allocations.

Changes:

• Added dir.lookup that performs per-block binary search (with linear-scan fallback) during path resolution instead of reading all entries.
• Standardized several path/operation errors via exported ErrNotDirectory, ErrIsDirectory, and ErrLoop, and fixed a Close() panic when a file is opened but never initialized via readInfo.
• Expanded tests/benchmarks to cover new error behaviors and measure lookup performance.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
erofs.go	Implements block-level directory lookup for path resolution and adds/uses standardized errors; includes Close() panic fix.
internal/erofstest/testcase.go	Adds test helpers and new assertions for correct errors on invalid operations (non-dir path components, ReadDir on file, ReadLink on regular file).
erofs_test.go	Adds a benchmark that exercises shallow/deep and large-directory lookup scenarios.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

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[hsiangkao]

readDir:
...
			} else {
				name = string(buf[dirents[0].NameOff:])
			}

needs to be fixed too.

[dmcgowan]

@hsiangkao updated, binary search across the blocks is reflect in the benchmarks now, much faster

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

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[hsiangkao]

Do we have some random tests in go-erofs for the tree hierarchy? I suggest introducing a random test with two-level directories at least with random numbers of dirs/files; and check the correction with looking up each individual path.

Otherwise it looks good to me.