Source file src/pkg/cmd/internal/buildid/rewrite.go

     1	// Copyright 2017 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 buildid
     6	
     7	import (
     8		"bytes"
     9		"crypto/sha256"
    10		"fmt"
    11		"io"
    12	)
    13	
    14	// FindAndHash reads all of r and returns the offsets of occurrences of id.
    15	// While reading, findAndHash also computes and returns
    16	// a hash of the content of r, but with occurrences of id replaced by zeros.
    17	// FindAndHash reads bufSize bytes from r at a time.
    18	// If bufSize == 0, FindAndHash uses a reasonable default.
    19	func FindAndHash(r io.Reader, id string, bufSize int) (matches []int64, hash [32]byte, err error) {
    20		if bufSize == 0 {
    21			bufSize = 31 * 1024 // bufSize+little will likely fit in 32 kB
    22		}
    23		if len(id) > bufSize {
    24			return nil, [32]byte{}, fmt.Errorf("buildid.FindAndHash: buffer too small")
    25		}
    26		zeros := make([]byte, len(id))
    27		idBytes := []byte(id)
    28	
    29		// The strategy is to read the file through buf, looking for id,
    30		// but we need to worry about what happens if id is broken up
    31		// and returned in parts by two different reads.
    32		// We allocate a tiny buffer (at least len(id)) and a big buffer (bufSize bytes)
    33		// next to each other in memory and then copy the tail of
    34		// one read into the tiny buffer before reading new data into the big buffer.
    35		// The search for id is over the entire tiny+big buffer.
    36		tiny := (len(id) + 127) &^ 127 // round up to 128-aligned
    37		buf := make([]byte, tiny+bufSize)
    38		h := sha256.New()
    39		start := tiny
    40		for offset := int64(0); ; {
    41			// The file offset maintained by the loop corresponds to &buf[tiny].
    42			// buf[start:tiny] is left over from previous iteration.
    43			// After reading n bytes into buf[tiny:], we process buf[start:tiny+n].
    44			n, err := io.ReadFull(r, buf[tiny:])
    45			if err != io.ErrUnexpectedEOF && err != io.EOF && err != nil {
    46				return nil, [32]byte{}, err
    47			}
    48	
    49			// Process any matches.
    50			for {
    51				i := bytes.Index(buf[start:tiny+n], idBytes)
    52				if i < 0 {
    53					break
    54				}
    55				matches = append(matches, offset+int64(start+i-tiny))
    56				h.Write(buf[start : start+i])
    57				h.Write(zeros)
    58				start += i + len(id)
    59			}
    60			if n < bufSize {
    61				// Did not fill buffer, must be at end of file.
    62				h.Write(buf[start : tiny+n])
    63				break
    64			}
    65	
    66			// Process all but final tiny bytes of buf (bufSize = len(buf)-tiny).
    67			// Note that start > len(buf)-tiny is possible, if the search above
    68			// found an id ending in the final tiny fringe. That's OK.
    69			if start < len(buf)-tiny {
    70				h.Write(buf[start : len(buf)-tiny])
    71				start = len(buf) - tiny
    72			}
    73	
    74			// Slide ending tiny-sized fringe to beginning of buffer.
    75			copy(buf[0:], buf[bufSize:])
    76			start -= bufSize
    77			offset += int64(bufSize)
    78		}
    79		h.Sum(hash[:0])
    80		return matches, hash, nil
    81	}
    82	
    83	func Rewrite(w io.WriterAt, pos []int64, id string) error {
    84		b := []byte(id)
    85		for _, p := range pos {
    86			if _, err := w.WriteAt(b, p); err != nil {
    87				return err
    88			}
    89		}
    90		return nil
    91	}
    92	

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