Make try_exists return Ok(true) for Windows Unix Sockets

[ChrisDenton]

This is a follow up to #109106 but for fs::try_exists, which doesn't need to get the metadata of a file (which can fail even if a file exists).

fs::try_exists currently fails on Windows if encountering a Unix Domain Socket (UDS). This PR fixes it by checking for an error code that's returned when there's a failure to use a reparse point.

Reparse points

A reparse point is a way to invoke a filesystem filter on a file instead of the file being opened normally. This is used to implement symbolic links (by redirecting to a different path) but also to implement other types of special files such as Unix domain sockets. If the reparse point is not a link type then opening it with CreateFileW may fail with ERROR_CANT_ACCESS_FILE because the filesystem filter does not implement that operation. This differs from resolving links which may fail with errors such as ERROR_FILE_NOT_FOUND or ERROR_CANT_RESOLVE_FILENAME.

So ERROR_CANT_ACCESS_FILE means that the file exists but that we can't open it normally. Still, the file does exist on the filesystem so try_exists should report that as Ok(true).

r? libs

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit 2f5dea0 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2f5dea0 with merge ff5664d...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing ff5664d to master...

[rust-timer]

Finished benchmarking commit (ff5664d): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

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)	1.2%	[0.8%, 1.9%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.8%	[-3.8%, -3.8%]	1
All ❌✅ (primary)	1.2%	[0.8%, 1.9%]	4

Cycles

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

Binary size

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

Bootstrap: 622.812s -> 624.492s (0.27%)
Artifact size: 305.53 MiB -> 305.52 MiB (-0.00%)

[roblabla]

This test fails on versions of windows before Windows 10 1803 (Redstone 4, April 2018 update), as those versions of windows don't have AF_UNIX support.

[ChrisDenton]

@roblabla Hm, I can add a check for if we're running in CI? I could also unconditionally skip the test if AF_UNIX fails but I'm more reluctant to do that as it may mask a problem with the test.