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Augment tests for FileMode.Append #55513

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Jul 13, 2021
Merged

Augment tests for FileMode.Append #55513

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4 changes: 4 additions & 0 deletions src/libraries/System.IO.FileSystem/tests/FileStream/Position.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,10 @@ public void SetPositionAppendModify()

fs.Position = length + 1;
Assert.Equal(length + 1, fs.Position);

fs.Write(TestBuffer);
fs.Position = length + 1;
Assert.Equal(length + 1, fs.Position);
}
}

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3 changes: 3 additions & 0 deletions src/libraries/System.IO.FileSystem/tests/FileStream/Seek.cs
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Expand Up @@ -32,6 +32,9 @@ public void SeekAppendModifyThrows()
Assert.Equal(length, fs.Position);
Assert.Throws<IOException>(() => fs.Seek(-length, SeekOrigin.End));
Assert.Equal(length, fs.Position);

fs.Write(TestBuffer);
Assert.Equal(length, fs.Seek(length, SeekOrigin.Begin));
}
}
}
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6 changes: 6 additions & 0 deletions src/libraries/System.IO.FileSystem/tests/FileStream/SetLength.cs
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Expand Up @@ -23,6 +23,12 @@ public void SetLengthAppendModifyThrows()
Assert.Equal(length, fs.Length);
Assert.Throws<IOException>(() => fs.SetLength(0));
Assert.Equal(length, fs.Length);

fs.Write(TestBuffer);
Assert.Equal(length + TestBuffer.Length, fs.Length);

fs.SetLength(length);
Assert.Equal(length, fs.Length);
}
}

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34 changes: 31 additions & 3 deletions src/libraries/System.IO.FileSystem/tests/FileStream/ctor_str_fm.cs
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@@ -1,6 +1,7 @@
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Text;
using Xunit;

namespace System.IO.Tests
Expand Down Expand Up @@ -213,11 +214,23 @@ public void FileModeTruncateExisting(string streamSpecifier)
[Theory, MemberData(nameof(StreamSpecifiers))]
public virtual void FileModeAppend(string streamSpecifier)
{
using (FileStream fs = CreateFileStream(GetTestFilePath() + streamSpecifier, FileMode.Append))
string fileName = GetTestFilePath() + streamSpecifier;
using (FileStream fs = CreateFileStream(fileName, FileMode.Append))
{
Assert.False(fs.CanRead);
Assert.True(fs.CanWrite);

fs.Write(Encoding.ASCII.GetBytes("abcde"));
Assert.Equal(5, fs.Length);
Assert.Equal(5, fs.Position);
Assert.Equal(1, fs.Seek(1, SeekOrigin.Begin));
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I would expect that FileMode.Append means "I want to append data to EOF" rather than "I want to write data at requested offset". The official doc for FileMode.Append says:

Trying to seek to a position before the end of the file throws an IOException exception

I believe that current implementation of Append is buggy and we should fix it instead of adding a test that ensures that it's buggy. I am going to reopen #55465 for that

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@stephentoub stephentoub Aug 19, 2021 •
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I interpret that to mean before the end of the file at the time it was opened, and the current implementation is consistent with that and has been for 20 years. If you believe it's worth taking a breaking change, that's a fine discussion to have, but it's absolutely a breaking change.

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adding a test that ensures that it's buggy

The tests validate the expected behavior.


fs.Write(Encoding.ASCII.GetBytes("xyz"));
Assert.Equal(4, fs.Position);
Assert.Equal(5, fs.Length);
}

Assert.Equal("axyze", File.ReadAllText(fileName));
}

[Theory, MemberData(nameof(StreamSpecifiers))]
Expand All @@ -226,20 +239,35 @@ public virtual void FileModeAppendExisting(string streamSpecifier)
string fileName = GetTestFilePath() + streamSpecifier;
using (FileStream fs = CreateFileStream(fileName, FileMode.Create))
{
fs.WriteByte(0);
fs.WriteByte((byte)'s');
}

string initialContents = File.ReadAllText(fileName);
using (FileStream fs = CreateFileStream(fileName, FileMode.Append))
{
// Ensure that the file was re-opened and position set to end
Assert.Equal(Math.Max(1L, InitialLength), fs.Length);
Assert.Equal(fs.Length, fs.Position);

long position = fs.Position;
Assert.Equal(fs.Length, position);

Assert.False(fs.CanRead);
Assert.True(fs.CanSeek);
Assert.True(fs.CanWrite);

Assert.Throws<IOException>(() => fs.Seek(-1, SeekOrigin.Current));
Assert.Throws<IOException>(() => fs.Seek(0, SeekOrigin.Begin));
Assert.Throws<NotSupportedException>(() => fs.ReadByte());

fs.Write(Encoding.ASCII.GetBytes("abcde"));
Assert.Equal(position + 5, fs.Position);

Assert.Equal(position, fs.Seek(position, SeekOrigin.Begin));
Assert.Equal(position + 1, fs.Seek(1, SeekOrigin.Current));
fs.Write(Encoding.ASCII.GetBytes("xyz"));
}

Assert.Equal(initialContents + "axyze", File.ReadAllText(fileName));
}
}
}