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gh-129005: _pyio.BufferedIO remove copy on readall #129454
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    Slicing buf and appending chunk would always result in a copy. Commonly in a readall there is no already read data in buf, and the amount of data read may be large, so the copy is expensive.
              
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LGTM
              
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…hon#129454)" This reverts commit e1c4ba9.
    
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Slicing buf and appending chunk would always result in a copy. Commonly in a readall() there is no already read data in buf, and the amount of data read may be large, so the copy is expensive.
    
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…hon#129454)" (python#129500) This reverts commit e1c4ba9.
  
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Slicing buf and appending chunk would always result in a copy / another memory allocation of the length of chunk. Commonly in a readall there is no already read data in buf, and the amount of data read may be large, so the copy is expensive.