io.go

Documentation: internal/saferio

     1  // Copyright 2022 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package saferio provides I/O functions that avoid allocating large
     6  // amounts of memory unnecessarily. This is intended for packages that
     7  // read data from an [io.Reader] where the size is part of the input
     8  // data but the input may be corrupt, or may be provided by an
     9  // untrustworthy attacker.
    10  package saferio
    11  
    12  import (
    13  	"io"
    14  	"unsafe"
    15  )
    16  
    17  // chunk is an arbitrary limit on how much memory we are willing
    18  // to allocate without concern.
    19  const chunk = 10 << 20 // 10M
    20  
    21  // ReadData reads n bytes from the input stream, but avoids allocating
    22  // all n bytes if n is large. This avoids crashing the program by
    23  // allocating all n bytes in cases where n is incorrect.
    24  //
    25  // The error is io.EOF only if no bytes were read.
    26  // If an io.EOF happens after reading some but not all the bytes,
    27  // ReadData returns io.ErrUnexpectedEOF.
    28  func ReadData(r io.Reader, n uint64) ([]byte, error) {
    29  	if int64(n) < 0 || n != uint64(int(n)) {
    30  		// n is too large to fit in int, so we can't allocate
    31  		// a buffer large enough. Treat this as a read failure.
    32  		return nil, io.ErrUnexpectedEOF
    33  	}
    34  
    35  	if n < chunk {
    36  		buf := make([]byte, n)
    37  		_, err := io.ReadFull(r, buf)
    38  		if err != nil {
    39  			return nil, err
    40  		}
    41  		return buf, nil
    42  	}
    43  
    44  	var buf []byte
    45  	buf1 := make([]byte, chunk)
    46  	for n > 0 {
    47  		next := n
    48  		if next > chunk {
    49  			next = chunk
    50  		}
    51  		_, err := io.ReadFull(r, buf1[:next])
    52  		if err != nil {
    53  			if len(buf) > 0 && err == io.EOF {
    54  				err = io.ErrUnexpectedEOF
    55  			}
    56  			return nil, err
    57  		}
    58  		buf = append(buf, buf1[:next]...)
    59  		n -= next
    60  	}
    61  	return buf, nil
    62  }
    63  
    64  // ReadDataAt reads n bytes from the input stream at off, but avoids
    65  // allocating all n bytes if n is large. This avoids crashing the program
    66  // by allocating all n bytes in cases where n is incorrect.
    67  func ReadDataAt(r io.ReaderAt, n uint64, off int64) ([]byte, error) {
    68  	if int64(n) < 0 || n != uint64(int(n)) {
    69  		// n is too large to fit in int, so we can't allocate
    70  		// a buffer large enough. Treat this as a read failure.
    71  		return nil, io.ErrUnexpectedEOF
    72  	}
    73  
    74  	if n < chunk {
    75  		buf := make([]byte, n)
    76  		_, err := r.ReadAt(buf, off)
    77  		if err != nil {
    78  			// io.SectionReader can return EOF for n == 0,
    79  			// but for our purposes that is a success.
    80  			if err != io.EOF || n > 0 {
    81  				return nil, err
    82  			}
    83  		}
    84  		return buf, nil
    85  	}
    86  
    87  	var buf []byte
    88  	buf1 := make([]byte, chunk)
    89  	for n > 0 {
    90  		next := n
    91  		if next > chunk {
    92  			next = chunk
    93  		}
    94  		_, err := r.ReadAt(buf1[:next], off)
    95  		if err != nil {
    96  			return nil, err
    97  		}
    98  		buf = append(buf, buf1[:next]...)
    99  		n -= next
   100  		off += int64(next)
   101  	}
   102  	return buf, nil
   103  }
   104  
   105  // SliceCapWithSize returns the capacity to use when allocating a slice.
   106  // After the slice is allocated with the capacity, it should be
   107  // built using append. This will avoid allocating too much memory
   108  // if the capacity is large and incorrect.
   109  //
   110  // A negative result means that the value is always too big.
   111  func SliceCapWithSize(size, c uint64) int {
   112  	if int64(c) < 0 || c != uint64(int(c)) {
   113  		return -1
   114  	}
   115  	if size > 0 && c > (1<<64-1)/size {
   116  		return -1
   117  	}
   118  	if c*size > chunk {
   119  		c = chunk / size
   120  		if c == 0 {
   121  			c = 1
   122  		}
   123  	}
   124  	return int(c)
   125  }
   126  
   127  // SliceCap is like SliceCapWithSize but using generics.
   128  func SliceCap[E any](c uint64) int {
   129  	var v E
   130  	size := uint64(unsafe.Sizeof(v))
   131  	return SliceCapWithSize(size, c)
   132  }
   133
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