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avoid ReadFile syscall if we know that there is nothing to read from file #56387

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merged 2 commits into from
Jul 27, 2021
Merged

avoid ReadFile syscall if we know that there is nothing to read from file #56387

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avoid ReadFile syscall if we know that there is nothing to read from …
adamsitnik Jul 27, 2021
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adamsitnik Jul 27, 2021
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6 changes: 6 additions & 0 deletions ...braries/System.Private.CoreLib/src/System/IO/Strategies/AsyncWindowsFileStreamStrategy.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -50,6 +50,12 @@ private unsafe ValueTask<int> ReadAsyncInternal(Memory<byte> destination, Cancel
// touch the file pointer location at all. We will adjust it
// ourselves, but only in memory. This isn't threadsafe.
_filePosition += destination.Length;

// We know for sure that there is nothing to read, so we just return here and avoid a sys-call.
if (destination.IsEmpty && LengthCachingSupported)
{
return ValueTask.FromResult(0);
}
}

(SafeFileHandle.OverlappedValueTaskSource? vts, int errorCode) = RandomAccess.QueueAsyncReadFile(_fileHandle, destination, positionBefore, cancellationToken);
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2 changes: 1 addition & 1 deletion src/libraries/System.Private.CoreLib/src/System/IO/Strategies/OSFileStreamStrategy.cs
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Expand Up @@ -115,7 +115,7 @@ protected void UpdateLengthOnChangePosition()
}
}

private bool LengthCachingSupported => OperatingSystem.IsWindows() && _share <= FileShare.Read && !_exposedHandle;
protected bool LengthCachingSupported => OperatingSystem.IsWindows() && _share <= FileShare.Read && !_exposedHandle;

/// <summary>Gets or sets the position within the current stream</summary>
public sealed override long Position
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