implement CidFromReader

And reuse two CidFromBytes tests for it, which includes both CIDv0 and
CIDv1 cases as inputs, as well as some inputs that should error.

Fixes #126.

cid.go

@@ -680,3 +680,139 @@ func CidFromBytes(data []byte) (int, Cid, error) {
 
 	return l, Cid{string(data[0:l])}, nil
 }
+
+func toBufByteReader(r io.Reader, dst []byte) *bufByteReader {
+	// If the reader already implements ByteReader, use it directly.
+	// Otherwise, use a fallback that does 1-byte Reads.
+	if br, ok := r.(io.ByteReader); ok {
+		return &bufByteReader{direct: br, dst: dst}
+	}
+	return &bufByteReader{fallback: r, dst: dst}
+}
+
+type bufByteReader struct {
+	direct   io.ByteReader
+	fallback io.Reader
+
+	dst []byte
+}
+
+func (r *bufByteReader) ReadByte() (byte, error) {
+	// The underlying reader has ReadByte; use it.
+	if br := r.direct; br != nil {
+		b, err := br.ReadByte()
+		if err != nil {
+			return 0, err
+		}
+		r.dst = append(r.dst, b)
+		return b, nil
+	}
+
+	// Fall back to a one-byte Read.
+	// TODO: consider reading straight into dst,
+	// once we have benchmarks and if they prove that to be faster.
+	var p [1]byte
+	if _, err := io.ReadFull(r.fallback, p[:]); err != nil {
+		return 0, err
+	}
+	r.dst = append(r.dst, p[0])
+	return p[0], nil
+}
+
+// CidFromReader reads a precise number of bytes for a CID from a given reader.
+// It returns the number of bytes read, the CID, and any error encountered.
+// The number of bytes read is accurate even if a non-nil error is returned.
+//
+// It's recommended to supply a reader that buffers and implements io.ByteReader,
+// as CidFromReader has to do many single-byte reads to decode varints.
+// If the argument only implements io.Reader, single-byte Read calls are used instead.
+func CidFromReader(r io.Reader) (int, Cid, error) {
+	// 64 bytes is enough for any CIDv0,
+	// and it's enough for most CIDv1s in practice.
+	// If the digest is too long, we'll allocate more.
+	br := toBufByteReader(r, make([]byte, 0, 64))
+
+	// We read the first varint, to tell if this is a CIDv0 or a CIDv1.
+	// The varint package wants a io.ByteReader, so we must wrap our io.Reader.
+	vers, err := varint.ReadUvarint(br)
+	if err != nil {
+		return len(br.dst), Undef, err
+	}
+
+	// If we have a CIDv0, read the rest of the bytes and cast the buffer.
+	if vers == mh.SHA2_256 {
+		if n, err := io.ReadFull(r, br.dst[1:34]); err != nil {
+			return len(br.dst) + n, Undef, err
+		}
+
+		br.dst = br.dst[:34]
+		h, err := mh.Cast(br.dst)
+		if err != nil {
+			return len(br.dst), Undef, err
+		}
+
+		return len(br.dst), Cid{string(h)}, nil
+	}
+
+	if vers != 1 {
+		return len(br.dst), Undef, fmt.Errorf("expected 1 as the cid version number, got: %d", vers)
+	}
+
+	// CID block encoding multicodec.
+	_, err = varint.ReadUvarint(br)
+	if err != nil {
+		return len(br.dst), Undef, err
+	}
+
+	// We could replace most of the code below with go-multihash's ReadMultihash.
+	// Note that it would save code, but prevent reusing buffers.
+	// Plus, we already have a ByteReader now.
+	mhStart := len(br.dst)
+
+	// Multihash hash function code.
+	_, err = varint.ReadUvarint(br)
+	if err != nil {
+		return len(br.dst), Undef, err
+	}
+
+	// Multihash digest length.
+	mhl, err := varint.ReadUvarint(br)
+	if err != nil {
+		return len(br.dst), Undef, err
+	}
+
+	// Refuse to make large allocations to prevent OOMs due to bugs.
+	const maxDigestAlloc = 32 << 20 // 32MiB
+	if mhl > maxDigestAlloc {
+		return len(br.dst), Undef, fmt.Errorf("refusing to allocate %d bytes for a digest", mhl)
+	}
+
+	// Fine to convert mhl to int, given maxDigestAlloc.
+	prefixLength := len(br.dst)
+	cidLength := prefixLength + int(mhl)
+	if cidLength > cap(br.dst) {
+		// If the multihash digest doesn't fit in our initial 64 bytes,
+		// efficiently extend the slice via append+make.
+		br.dst = append(br.dst, make([]byte, cidLength-cap(br.dst))...)
+	} else {
+		// The multihash digest fits inside our buffer,
+		// so just extend its capacity.
+		br.dst = br.dst[:cidLength]
+	}
+
+	if n, err := io.ReadFull(r, br.dst[prefixLength:cidLength]); err != nil {
+		// We can't use len(br.dst) here,
+		// as we've only read n bytes past prefixLength.
+		return prefixLength + n, Undef, err
+	}
+
+	// This simply ensures the multihash is valid.
+	// TODO: consider removing this bit, as it's probably redundant;
+	// for now, it helps ensure consistency with CidFromBytes.
+	_, _, err = mh.MHFromBytes(br.dst[mhStart:])
+	if err != nil {
+		return len(br.dst), Undef, err
+	}
+
+	return len(br.dst), Cid{string(br.dst)}, nil
+}

cid_test.go

@@ -4,10 +4,12 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"io"
 	"math/rand"
 	"reflect"
 	"strings"
 	"testing"
+	"testing/iotest"
 
 	mbase "github.com/multiformats/go-multibase"
 	mh "github.com/multiformats/go-multihash"
@@ -692,51 +694,98 @@ func TestReadCidsFromBuffer(t *testing.T) {
 	if cur != len(buf) {
 		t.Fatal("had trailing bytes")
 	}
-}
 
-func TestBadCidFromBytes(t *testing.T) {
-	l, c, err := CidFromBytes([]byte{mh.SHA2_256, 32, 0x00})
-	if err == nil {
-		t.Fatal("expected not-enough-bytes for V0 CidFromBytes")
-	}
-	if l != 0 {
-		t.Fatal("expected length=0 from bad CidFromBytes")
-	}
-	if c != Undef {
-		t.Fatal("expected Undef CID from bad CidFromBytes")
-	}
+	// The same, but now with CidFromReader.
+	// In multiple forms, to catch more io interface bugs.
+	for _, r := range []io.Reader{
+		// implements io.ByteReader
+		bytes.NewReader(buf),
 
-	c, err = Decode("bafkreie5qrjvaw64n4tjm6hbnm7fnqvcssfed4whsjqxzslbd3jwhsk3mm")
-	if err != nil {
-		t.Fatal(err)
-	}
-	byts := make([]byte, c.ByteLen())
-	copy(byts, c.Bytes())
-	byts[1] = 0x80 // bad codec varint
-	byts[2] = 0x00
-	l, c, err = CidFromBytes(byts)
-	if err == nil {
-		t.Fatal("expected not-enough-bytes for V1 CidFromBytes")
-	}
-	if l != 0 {
-		t.Fatal("expected length=0 from bad CidFromBytes")
-	}
-	if c != Undef {
-		t.Fatal("expected Undef CID from bad CidFromBytes")
+		// tiny reads, no io.ByteReader
+		iotest.OneByteReader(bytes.NewReader(buf)),
+	} {
+		cur = 0
+		for _, expc := range cids {
+			n, c, err := CidFromReader(r)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if c != expc {
+				t.Fatal("cids mismatched")
+			}
+			cur += n
+		}
+		if cur != len(buf) {
+			t.Fatal("had trailing bytes")
+		}
 	}
+}
 
-	copy(byts, c.Bytes())
-	byts[2] = 0x80 // bad multihash varint
-	byts[3] = 0x00
-	l, c, err = CidFromBytes(byts)
-	if err == nil {
-		t.Fatal("expected not-enough-bytes for V1 CidFromBytes")
-	}
-	if l != 0 {
-		t.Fatal("expected length=0 from bad CidFromBytes")
-	}
-	if c != Undef {
-		t.Fatal("expected Undef CID from bad CidFromBytes")
+func TestBadCidInput(t *testing.T) {
+	for _, name := range []string{
+		"FromBytes",
+		"FromReader",
+	} {
+		t.Run(name, func(t *testing.T) {
+			usingReader := name == "FromReader"
+
+			fromBytes := CidFromBytes
+			if usingReader {
+				fromBytes = func(data []byte) (int, Cid, error) {
+					return CidFromReader(bytes.NewReader(data))
+				}
+			}
+
+			l, c, err := fromBytes([]byte{mh.SHA2_256, 32, 0x00})
+			if err == nil {
+				t.Fatal("expected not-enough-bytes for V0 CID")
+			}
+			if !usingReader && l != 0 {
+				t.Fatal("expected length==0 from bad CID")
+			} else if usingReader && l == 0 {
+				t.Fatal("expected length!=0 from bad CID")
+			}
+			if c != Undef {
+				t.Fatal("expected Undef CID from bad CID")
+			}
+
+			c, err = Decode("bafkreie5qrjvaw64n4tjm6hbnm7fnqvcssfed4whsjqxzslbd3jwhsk3mm")
+			if err != nil {
+				t.Fatal(err)
+			}
+			byts := make([]byte, c.ByteLen())
+			copy(byts, c.Bytes())
+			byts[1] = 0x80 // bad codec varint
+			byts[2] = 0x00
+			l, c, err = fromBytes(byts)
+			if err == nil {
+				t.Fatal("expected not-enough-bytes for V1 CID")
+			}
+			if !usingReader && l != 0 {
+				t.Fatal("expected length==0 from bad CID")
+			} else if usingReader && l == 0 {
+				t.Fatal("expected length!=0 from bad CID")
+			}
+			if c != Undef {
+				t.Fatal("expected Undef CID from bad CID")
+			}
+
+			copy(byts, c.Bytes())
+			byts[2] = 0x80 // bad multihash varint
+			byts[3] = 0x00
+			l, c, err = fromBytes(byts)
+			if err == nil {
+				t.Fatal("expected not-enough-bytes for V1 CID")
+			}
+			if !usingReader && l != 0 {
+				t.Fatal("expected length==0 from bad CID")
+			} else if usingReader && l == 0 {
+				t.Fatal("expected length!=0 from bad CID")
+			}
+			if c != Undef {
+				t.Fatal("expected Undef CID from bad CidFromBytes")
+			}
+		})
 	}
 }