Refactor utility io conversions into one internal package

Improve reader type conversion by checking if type satisfies
ReaderAt to avoid unnecessary wrapping.

Move io converters into one place.

Fixes:
- ipld/go-car#145


This commit was moved from ipld/go-car@f3fc595

ipld/car/v2/index_gen.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"sync"
 
 	internalio "github.com/ipld/go-car/v2/internal/io"
 
@@ -97,22 +96,6 @@ func GenerateIndexFromFile(path string) (index.Index, error) {
 	return GenerateIndex(f)
 }
 
-var _ io.ReaderAt = (*readSeekerAt)(nil)
-
-type readSeekerAt struct {
-	rs io.ReadSeeker
-	mu sync.Mutex
-}
-
-func (rsa *readSeekerAt) ReadAt(p []byte, off int64) (n int, err error) {
-	rsa.mu.Lock()
-	defer rsa.mu.Unlock()
-	if _, err := rsa.rs.Seek(off, io.SeekStart); err != nil {
-		return 0, err
-	}
-	return rsa.rs.Read(p)
-}
-
 // ReadOrGenerateIndex accepts both CAR v1 and v2 format, and reads or generates an index for it.
 // When the given reader is in CAR v1 format an index is always generated.
 // For a payload in CAR v2 format, an index is only generated if Header.HasIndex returns false.
@@ -137,7 +120,7 @@ func ReadOrGenerateIndex(rs io.ReadSeeker) (index.Index, error) {
 		return GenerateIndex(rs)
 	case 2:
 		// Read CAR v2 format
-		v2r, err := NewReader(&readSeekerAt{rs: rs})
+		v2r, err := NewReader(internalio.ToReaderAt(rs))
 		if err != nil {
 			return nil, err
 		}

ipld/car/v2/internal/io/converter.go

@@ -3,13 +3,15 @@ package io
 import (
 	"io"
 	"io/ioutil"
+	"sync"
 )
 
 var (
 	_ io.ByteReader = (*readerPlusByte)(nil)
 	_ io.ByteReader = (*readSeekerPlusByte)(nil)
 	_ io.ByteReader = (*discardingReadSeekerPlusByte)(nil)
 	_ io.ReadSeeker = (*discardingReadSeekerPlusByte)(nil)
+	_ io.ReaderAt   = (*readSeekerAt)(nil)
 )
 
 type (
@@ -30,6 +32,11 @@ type (
 		io.ReadSeeker
 		io.ByteReader
 	}
+
+	readSeekerAt struct {
+		rs io.ReadSeeker
+		mu sync.Mutex
+	}
 )
 
 func ToByteReader(r io.Reader) io.ByteReader {
@@ -49,6 +56,13 @@ func ToByteReadSeeker(r io.Reader) ByteReadSeeker {
 	return &discardingReadSeekerPlusByte{Reader: r}
 }
 
+func ToReaderAt(rs io.ReadSeeker) io.ReaderAt {
+	if ra, ok := rs.(io.ReaderAt); ok {
+		return ra
+	}
+	return &readSeekerAt{rs: rs}
+}
+
 func (rb readerPlusByte) ReadByte() (byte, error) {
 	return readByte(rb)
 }
@@ -89,3 +103,12 @@ func readByte(r io.Reader) (byte, error) {
 	_, err := io.ReadFull(r, p[:])
 	return p[0], err
 }
+
+func (rsa *readSeekerAt) ReadAt(p []byte, off int64) (n int, err error) {
+	rsa.mu.Lock()
+	defer rsa.mu.Unlock()
+	if _, err := rsa.rs.Seek(off, io.SeekStart); err != nil {
+		return 0, err
+	}
+	return rsa.rs.Read(p)
+}