At file package that can wrap files with some helpers (#54)

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

* Add Close()

* Add logging around requested bytes vs how many we're actually reading

* Check for direct mode and skip all this logic if not direct

* Add open with FADV_DONTNEED

* Fix unix return

* Fix linux

* Make go happy about filenames

* sdfkdjghdfg

* Remove all the directio stuff and rename to file

pkg/file/file_linux.go

@@ -0,0 +1,30 @@
+//go:build linux
+
+package file
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// OpenFileFADVDONTNEED opens file with FADV_DONTNEED
+func OpenFileFADVDONTNEED(path string) (*os.File, error) {
+	// Open the file
+	file, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("error opening file: %w", err)
+	}
+
+	// File descriptor
+	fd := int(file.Fd())
+
+	// Use FADV_DONTNEED to suggest not caching pages
+	err = unix.Fadvise(fd, 0, 0, unix.FADV_DONTNEED)
+	if err != nil {
+		return nil, fmt.Errorf("error setting FADV_DONTNEED: %w", err)
+	}
+
+	return file, nil
+}

pkg/file/file_notlinux.go

@@ -0,0 +1,12 @@
+//go:build !linux
+
+package file
+
+import (
+	"os"
+)
+
+// OpenFileFADVDONTNEED opens file with FADV_DONTNEED
+func OpenFileFADVDONTNEED(path string) (*os.File, error) {
+	return os.Open(path)
+}