Add directfile package that satisfies io.Reader and io.ReaderAt

Author: cmmarslender

pkg/directfile/alignedbuffer.go

@@ -0,0 +1,17 @@
+package directfile
+
+import (
+	"unsafe"
+)
+
+// In addition to reading the full block from disk, the buffer has to be aligned in memory as well
+// Create a buffer that is as long as we need plus one extra block, then get the part of the buffer
+// that starts memory aligned
+func alignedBuffer(size, align int) ([]byte, error) {
+	raw := make([]byte, size+align)
+	offset := int(uintptr(unsafe.Pointer(&raw[0])) & uintptr(align-1))
+	if offset != 0 {
+		offset = align - offset
+	}
+	return raw[offset : size+offset], nil
+}

pkg/directfile/directfile.go

@@ -0,0 +1,92 @@
+package directfile
+
+import (
+	"fmt"
+	"io"
+	"os"
+)
+
+// DirectFile is a wrapper around os.File that ensures aligned reads for O_DIRECT support,
+// even when trying to read small chunks
+type DirectFile struct {
+	file      *os.File
+	blockSize uint32
+
+	// Direct indicates whether the current platform actually supported O_DIRECT when opening
+	Direct bool
+}
+
+func openNodirect(path string) (*DirectFile, error) {
+	// O_DIRECT is not supported on this platform, fallback to normal open
+	fmt.Println("O_DIRECT not supported, using regular open")
+	file, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("error calling os.Open on file: %w", err)
+	}
+
+	return &DirectFile{file: file, Direct: false}, nil
+}
+
+// Read satisfies the io.Reader
+func (df *DirectFile) Read(p []byte) (n int, err error) {
+	blockSize := int(df.blockSize)
+
+	// Ensure buffer length is aligned with BlockSize for O_DIRECT
+	alignedSize := (len(p) / blockSize) * blockSize
+	if alignedSize == 0 {
+		return 0, fmt.Errorf("buffer size must be at least %d bytes for O_DIRECT", blockSize)
+	}
+
+	// Create an aligned buffer
+	buf, err := alignedBuffer(alignedSize, blockSize)
+	if err != nil {
+		return 0, fmt.Errorf("failed to create aligned buffer: %w", err)
+	}
+
+	// Perform the read
+	n, err = df.file.Read(buf)
+	if n > len(p) {
+		n = len(p) // Only copy as much as p can hold
+	}
+	copy(p, buf[:n])
+	return n, err
+}
+
+// ReadAt satisfies the io.ReaderAt interface
+func (df *DirectFile) ReadAt(p []byte, off int64) (n int, err error) {
+	blockSize := int(df.blockSize)
+	// Calculate aligned offset by rounding down to the nearest BlockSize boundary
+	// Integer division in go always discards remainder
+	alignedOffset := (int(off) / blockSize) * blockSize
+
+	// Difference between aligned offset and requested offset
+	// Need to read at least this many extra bytes, since we moved the starting point earlier this much
+	offsetDiff := int(off) - alignedOffset
+
+	// Calculate how much data to read to cover the requested segment, ensuring alignment
+	alignedReadSize := ((len(p) + offsetDiff + blockSize - 1) / blockSize) * blockSize
+
+	// Create an aligned buffer for the full read
+	buf, err := alignedBuffer(alignedReadSize, blockSize)
+	if err != nil {
+		return 0, fmt.Errorf("failed to create aligned buffer: %w", err)
+	}
+
+	// Perform the read at the aligned offset
+	n, err = df.file.ReadAt(buf, int64(alignedOffset))
+	if err != nil && err != io.EOF {
+		return 0, err
+	}
+
+	// Calculate how much of the read buffer to copy into p
+	copyLen := n - offsetDiff
+	if copyLen > len(p) {
+		copyLen = len(p)
+	} else if copyLen < 0 {
+		return 0, fmt.Errorf("read beyond end of file")
+	}
+
+	// Copy the relevant part of the buffer to p
+	copy(p, buf[offsetDiff:offsetDiff+copyLen])
+	return copyLen, nil
+}

pkg/directfile/file_linux.go

@@ -0,0 +1,27 @@
+//go:build linux
+
+package directfile
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+)
+
+// OpenFileWithODirect Opens a file without system cache (DIRECT)
+func OpenFileWithODirect(path string, blockSize uint32) (*DirectFile, error) {
+	// Try opening the file with O_DIRECT
+	fd, err := syscall.Open(path, syscall.O_DIRECT|syscall.O_RDONLY, 0)
+	if err != nil {
+		// Fallback to normal open if O_DIRECT is not supported
+		return openNodirect(path)
+	}
+
+	// Success: Convert file descriptor to os.File and return
+	file, err := os.NewFile(uintptr(fd), path), nil
+	if err != nil {
+		return nil, fmt.Errorf("error calling os.NewFile on file: %w", err)
+	}
+
+	return &DirectFile{file: file, blockSize: blockSize, Direct: true}, nil
+}

pkg/directfile/file_nodirect.go

@@ -0,0 +1,9 @@
+//go:build !linux
+
+package directfile
+
+// OpenFileWithODirect Opens a file without system cache (DIRECT)
+func OpenFileWithODirect(path string, blockSize uint32) (*DirectFile, error) {
+	// O_DIRECT is not supported on this platform, fallback to normal open
+	return openNodirect(path)
+}