gvisor.dev/gvisor@v0.0.0-20240520182842-f9d4d51c7e0f/pkg/secio/secio.go (about)

     1  // Copyright 2018 The gVisor Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  // Package secio provides support for sectioned I/O.
    16  package secio
    17  
    18  import (
    19  	"errors"
    20  	"io"
    21  )
    22  
    23  // ErrReachedLimit is returned when SectionReader.Read or SectionWriter.Write
    24  // reaches its limit.
    25  var ErrReachedLimit = errors.New("reached limit")
    26  
    27  // SectionReader implements io.Reader on a section of an underlying io.ReaderAt.
    28  // It is similar to io.SectionReader, but:
    29  //
    30  //   - Reading beyond the limit returns ErrReachedLimit, not io.EOF.
    31  //
    32  //   - Limit overflow is handled correctly.
    33  type SectionReader struct {
    34  	r     io.ReaderAt
    35  	off   int64
    36  	limit int64
    37  }
    38  
    39  // Read implements io.Reader.Read.
    40  func (r *SectionReader) Read(dst []byte) (int, error) {
    41  	if r.limit >= 0 {
    42  		if max := r.limit - r.off; max < int64(len(dst)) {
    43  			dst = dst[:max]
    44  		}
    45  	}
    46  	n, err := r.r.ReadAt(dst, r.off)
    47  	r.off += int64(n)
    48  	if err == nil && r.off == r.limit {
    49  		err = ErrReachedLimit
    50  	}
    51  	return n, err
    52  }
    53  
    54  // NewOffsetReader returns an io.Reader that reads from r starting at offset
    55  // off.
    56  func NewOffsetReader(r io.ReaderAt, off int64) *SectionReader {
    57  	return &SectionReader{r, off, -1}
    58  }
    59  
    60  // NewSectionReader returns an io.Reader that reads from r starting at offset
    61  // off and stops with ErrReachedLimit after n bytes.
    62  func NewSectionReader(r io.ReaderAt, off int64, n int64) *SectionReader {
    63  	// If off + n overflows, it will be < 0 such that no limit applies, but
    64  	// this is the correct behavior as long as r prohibits reading at offsets
    65  	// beyond MaxInt64.
    66  	return &SectionReader{r, off, off + n}
    67  }
    68  
    69  // SectionWriter implements io.Writer on a section of an underlying
    70  // io.WriterAt. Writing beyond the limit returns ErrReachedLimit.
    71  type SectionWriter struct {
    72  	w     io.WriterAt
    73  	off   int64
    74  	limit int64
    75  }
    76  
    77  // Write implements io.Writer.Write.
    78  func (w *SectionWriter) Write(src []byte) (int, error) {
    79  	if w.limit >= 0 {
    80  		if max := w.limit - w.off; max < int64(len(src)) {
    81  			src = src[:max]
    82  		}
    83  	}
    84  	n, err := w.w.WriteAt(src, w.off)
    85  	w.off += int64(n)
    86  	if err == nil && w.off == w.limit {
    87  		err = ErrReachedLimit
    88  	}
    89  	return n, err
    90  }
    91  
    92  // NewOffsetWriter returns an io.Writer that writes to w starting at offset
    93  // off.
    94  func NewOffsetWriter(w io.WriterAt, off int64) *SectionWriter {
    95  	return &SectionWriter{w, off, -1}
    96  }
    97  
    98  // NewSectionWriter returns an io.Writer that writes to w starting at offset
    99  // off and stops with ErrReachedLimit after n bytes.
   100  func NewSectionWriter(w io.WriterAt, off int64, n int64) *SectionWriter {
   101  	// If off + n overflows, it will be < 0 such that no limit applies, but
   102  	// this is the correct behavior as long as w prohibits writing at offsets
   103  	// beyond MaxInt64.
   104  	return &SectionWriter{w, off, off + n}
   105  }