Improve handling when setting an unseekable stream as a tar entry's DataStream

[carlossanlop]

Original report here: dotnet/sdk-container-builds#192 (comment)

If the user creates a new regular file tar entry, and sets the DataStream to an unseekable stream, we should either:

• Throw immediately if the stream is unseekable in the TarEntry.cs#L251 setter, or...
• Find a way to retrieve the unseekable stream's length and position without calling Length, right before writing the data to the archive: TarHeader.Write.cs#L510-L511

cc @jozkee

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Issue Details

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Original report here: dotnet/sdk-container-builds#192 (comment)

If the user creates a new regular file tar entry, and sets the DataStream to an unseekable stream, we should either:

• Throw immediately if the stream is unseekable in the TarEntry.cs#L251 setter, or...
• Find a way to retrieve the unseekable stream's length and position without calling Length, right before writing the data to the archive: TarHeader.Write.cs#L510-L511

cc @jozkee

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