FileStream position is not updated when sync write partially succeeds

[tmds]

For async operations, FileStream position is updated if a write is only partial. For example:

runtime/src/libraries/System.Private.CoreLib/src/Microsoft/Win32/SafeHandles/SafeFileHandle.ThreadPoolValueTaskSource.cs

Line 126 in 055e04b

_strategy.OnIncompleteOperation(_singleSegment.Length, 0);

For sync operations, if the write succeeds only partially the position on the FileStream is not updated:

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

Lines 244 to 245 in 055e04b

	RandomAccess.WriteAtOffset(_fileHandle, buffer, _filePosition);
	_filePosition += buffer.Length;

cc @adamsitnik

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[adamsitnik]

For sync operations, if the write succeeds only partially the position on the FileStream is not updated:

RandomAccess.WriteAtOffset is responsible for ensuring that it writes all bytes:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Unix.cs

Lines 146 to 149 in c0bea40

	// The write completed successfully but for fewer bytes than requested.
	// We need to try again for the remainder.
	buffer = buffer.Slice(bytesWritten);
	fileOffset += bytesWritten;

[tmds]

When this loop round on a partial write, and on the next iteration it throws at FileStreamHelpers.CheckFileCall, then FileStream position does not match that of the partial write.

Is that the intended behavior?

[adamsitnik]

Is that the intended behavior?

I am not 100% sure, from my experience we usually don't ensure a valid state after an exception is throw. At the same time, I would not mind passing the offset by ref to this method to get it updated on every iteration.

@jkotas @stephentoub what is our official guidance?

[tmds]

Because the async methods handle this, I'd expect the sync methods to handle it too.