[FileStream] Flush(flushToDisk: true) should always flush to disk

[adamsitnik]

fixes #77373

5.0 implementation for reference:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/Strategies/Net5CompatFileStreamStrategy.cs

Lines 268 to 276 in fe21795

	internal override void Flush(bool flushToDisk)
	{
	FlushInternalBuffer();
	if (flushToDisk && CanWrite)
	{
	FileStreamHelpers.FlushToDisk(_fileHandle);
	}
	}

I've tried really hard to write a reliable test for this bug, but I've failed to do that in reasonable period of time.

Based on the docs:

File system metadata is always cached. Therefore, to store any metadata changes to disk, the file must either be flushed or be opened with FILE_FLAG_WRITE_THROUGH.

I've tried to find some way of writing and reading the metadata and using Flush(true) to flush the metadata, but I was unable to find a simple and reliable solution for doing that (the metadata that we expose is refreshed without the need to flush to disk, other metdata is hard to get and either requires some dirty COM code or 3rd party or .NET Framework-only dependency to do it).

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Issue Details

---

fixes #77373

6.0 implementation for reference:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/Strategies/Net5CompatFileStreamStrategy.cs

Lines 268 to 276 in fe21795

	internal override void Flush(bool flushToDisk)
	{
	FlushInternalBuffer();
	if (flushToDisk && CanWrite)
	{
	FileStreamHelpers.FlushToDisk(_fileHandle);
	}
	}

I've tried really hard to write a reliable test for this bug, but I've failed to do that in reasonable period of time.

Based on the docs:

File system metadata is always cached. Therefore, to store any metadata changes to disk, the file must either be flushed or be opened with FILE_FLAG_WRITE_THROUGH.

I've tried to find some way of writing and reading the metadata and using Flush(true) to flush the metadata, but I was unable to find a simple and reliable solution for doing that (the metadata that we expose is refreshed without the need to flush to disk, other metdata is hard to get and either requires some dirty COM code or 3rd party or .NET Framework-only dependency to do it).

Author:	adamsitnik
Assignees:	-
Labels:	area-System.IO
Milestone:	8.0.0

[stephentoub]

Why shouldn't / can't we still FSync / FlushFileBuffers in this case?

[adamsitnik]

Why shouldn't / can't we still FSync / FlushFileBuffers in this case?

I would expect it to throw for pipes. I am going to check it and get back to you.

[adamsitnik]

@stephentoub it turned out to be possible, moreover we even already have tests for that:

runtime/src/libraries/System.IO.FileSystem/tests/FileStream/Flush.cs

Lines 146 to 162 in 57bfe47

	[Theory]
	[InlineData(null)]
	[InlineData(false)]
	[InlineData(true)]
	[SkipOnPlatform(TestPlatforms.Browser, "IO.Pipes not supported")]
	public void FlushCanBeUsedOnPipes_Success(bool? flushToDisk)
	{
	using (var pipeStream = new AnonymousPipeServerStream(PipeDirection.In))
	using (var clientHandle = pipeStream.ClientSafePipeHandle)
	{
	SafeFileHandle handle = new SafeFileHandle((IntPtr)int.Parse(pipeStream.GetClientHandleAsString()), false);
	using (FileStream fs = new FileStream(handle, FileAccess.Write, 1, false))
	{
	Flush(fs, flushToDisk);
	}
	}
	}

I've updated the code.

[stephentoub]

Thanks for checking.