fs: Fix #50619 (again) and add a regression test

[tavianator]

Bug #50619 was fixed by adding an end_of_stream flag in #50630.
Unfortunately, that fix only applied to the readdir_r() path. When I
switched Linux to use readdir() in #92778, I inadvertently reintroduced
the bug on that platform. Other platforms that had always used
readdir() were presumably never fixed.

This patch enables end_of_stream for all platforms, and adds a
Linux-specific regression test that should hopefully prevent the bug
from being reintroduced again.

[rustbot]

r? @Mark-Simulacrum

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[tavianator]

A comment on the original fix said:

Adding a test for this would be fairly difficult, as the original reproducer relies on a fairly specific state of the system that would be pretty hard to emulate in way that wouldn’t spuriously fail all the time.

Which I took as a challenge :)

The test I added creates a zombie child process and uses /proc/$ZOMBIE_PID/net, same as the original reproducer. It can spuriously pass in a few different ways:

• The test is cfg(target_os = "Linux") so it won't be built/run on other platforms
• If creating the child process (echo) fails, we skip the test
• If killing the child process fails for any reason other than it already exited, we skip the test
• If we can't open /proc/$ZOMBIE_PID/net (e.g. procfs is not mounted or is inaccessible), we skip the test
• If reading /proc/$ZOMBIE_PID/net succeeds, the test will spuriously pass
  • In some experiments this happened less than 1% of the time for me, as we are racing against the kernel moving the process into Z state
  • If this is undesired, we could poll /proc/$ZOMBIE_PID/stat until it says Z. But I think it's not worth the added complexity.

I could not make it spuriously fail even running it 4,096 times in parallel.

[Mark-Simulacrum]

@bors r+ rollup=iffy

Thanks!

[bors]

📌 Commit 9fb7c5a has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit 9fb7c5a with merge 48b3c46...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 48b3c46 to master...

[rust-timer]

Finished benchmarking commit (48b3c46): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.4%	[-1.4%, -1.4%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.8%	[0.8%, 0.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.