whatcanGOwrong

go/pkg/mod/github.com/cilium/ebpf@v0.11.0/internal/io.go

@@ -0,0 +1,128 @@
+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
+}