package internal
import (
"bufio"
"bytes"
"compress/gzip"
"errors"
"fmt"
"io"
"os"
"path/filepath"
"sync"
)
// NewBufferedSectionReader wraps an io.ReaderAt in an appropriately-sized
// buffered reader. It is a convenience function for reading subsections of
// ELF sections while minimizing the amount of read() syscalls made.
//
// Syscall overhead is non-negligible in continuous integration context
// where ELFs might be accessed over virtual filesystems with poor random
// access performance. Buffering reads makes sense because (sub)sections
// end up being read completely anyway.
//
// Use instead of the r.Seek() + io.LimitReader() pattern.
func NewBufferedSectionReader(ra io.ReaderAt, off, n int64) *bufio.Reader {
// Clamp the size of the buffer to one page to avoid slurping large parts
// of a file into memory. bufio.NewReader uses a hardcoded default buffer
// of 4096. Allow arches with larger pages to allocate more, but don't
// allocate a fixed 4k buffer if we only need to read a small segment.
buf := n
if ps := int64(os.Getpagesize()); n > ps {
buf = ps
}
return bufio.NewReaderSize(io.NewSectionReader(ra, off, n), int(buf))
}
// DiscardZeroes makes sure that all written bytes are zero
// before discarding them.
type DiscardZeroes struct{}
func (DiscardZeroes) Write(p []byte) (int, error) {
for _, b := range p {
if b != 0 {
return 0, errors.New("encountered non-zero byte")
}
}
return len(p), nil
}
// ReadAllCompressed decompresses a gzipped file into memory.
func ReadAllCompressed(file string) ([]byte, error) {
fh, err := os.Open(file)
if err != nil {
return nil, err
}
defer fh.Close()
gz, err := gzip.NewReader(fh)
if err != nil {
return nil, err
}
defer gz.Close()
return io.ReadAll(gz)
}
// ReadUint64FromFile reads a uint64 from a file.
//
// format specifies the contents of the file in fmt.Scanf syntax.
func ReadUint64FromFile(format string, path ...string) (uint64, error) {
filename := filepath.Join(path...)
data, err := os.ReadFile(filename)
if err != nil {
return 0, fmt.Errorf("reading file %q: %w", filename, err)
}
var value uint64
n, err := fmt.Fscanf(bytes.NewReader(data), format, &value)
if err != nil {
return 0, fmt.Errorf("parsing file %q: %w", filename, err)
}
if n != 1 {
return 0, fmt.Errorf("parsing file %q: expected 1 item, got %d", filename, n)
}
return value, nil
}
type uint64FromFileKey struct {
format, path string
}
var uint64FromFileCache = struct {
sync.RWMutex
values map[uint64FromFileKey]uint64
}{
values: map[uint64FromFileKey]uint64{},
}
// ReadUint64FromFileOnce is like readUint64FromFile but memoizes the result.
func ReadUint64FromFileOnce(format string, path ...string) (uint64, error) {
filename := filepath.Join(path...)
key := uint64FromFileKey{format, filename}
uint64FromFileCache.RLock()
if value, ok := uint64FromFileCache.values[key]; ok {
uint64FromFileCache.RUnlock()
return value, nil
}
uint64FromFileCache.RUnlock()
value, err := ReadUint64FromFile(format, filename)
if err != nil {
return 0, err
}
uint64FromFileCache.Lock()
defer uint64FromFileCache.Unlock()
if value, ok := uint64FromFileCache.values[key]; ok {
// Someone else got here before us, use what is cached.
return value, nil
}
uint64FromFileCache.values[key] = value
return value, nil
}