feat: add ErrNotFound

* Intended to replace github.com/ipfs/go-ipld-format#ErrNotFound
* A new IsNotFound() that uses feature detection rather than type checking so
  it's compatible with old and new forms.

Ref: ipld/go-car#363
Ref: #493

storage/memstore/memstore.go

@@ -3,8 +3,9 @@ package memstore
 import (
 	"bytes"
 	"context"
-	"fmt"
 	"io"
+
+	"github.com/ipld/go-ipld-prime/storage"
 )
 
 // Store is a simple in-memory storage.
@@ -56,7 +57,7 @@ func (store *Store) Get(ctx context.Context, key string) ([]byte, error) {
 	store.beInitialized()
 	content, exists := store.Bag[key]
 	if !exists {
-		return nil, fmt.Errorf("404") // FIXME this needs a standard error type
+		return nil, storage.ErrNotFound{Key: key}
 	}
 	cpy := make([]byte, len(content))
 	copy(cpy, content)
@@ -82,7 +83,7 @@ func (store *Store) Put(ctx context.Context, key string, content []byte) error {
 func (store *Store) GetStream(ctx context.Context, key string) (io.ReadCloser, error) {
 	content, exists := store.Bag[key]
 	if !exists {
-		return nil, fmt.Errorf("404") // FIXME this needs a standard error type
+		return nil, storage.ErrNotFound{Key: key}
 	}
 	return noopCloser{bytes.NewReader(content)}, nil
 }
@@ -91,7 +92,7 @@ func (store *Store) GetStream(ctx context.Context, key string) (io.ReadCloser, e
 func (store *Store) Peek(ctx context.Context, key string) ([]byte, io.Closer, error) {
 	content, exists := store.Bag[key]
 	if !exists {
-		return nil, nil, fmt.Errorf("404") // FIXME this needs a standard error type
+		return nil, nil, storage.ErrNotFound{Key: key}
 	}
 	return content, noopCloser{nil}, nil
 }

storage/notfound.go

@@ -0,0 +1,54 @@
+package storage
+
+import "github.com/ipfs/go-cid"
+
+// compatible with the go-ipld-format ErrNotFound, match against
+// interface{NotFound() bool}
+// this could go into go-ipld-prime, but for now we'll just exercise the
+// feature-test pattern
+
+// ErrNotFound is a 404, but for block storage systems. It is returned when
+// a block is not found. The Key is typically the binary form of a CID
+// (CID#KeyString()).
+//
+// ErrNotFound implements `interface{NotFound() bool}`, which makes it roughly
+// compatible with the legacy github.com/ipfs/go-ipld-format#ErrNotFound.
+// The IsNotFound() function here will test for this and therefore be compatible
+// with this ErrNotFound, and the legacy ErrNotFound. The same is not true for
+// the legacy github.com/ipfs/go-ipld-format#IsNotFound.
+type ErrNotFound struct {
+	Key string
+}
+
+func (e ErrNotFound) Error() string {
+	if c, err := cid.Cast([]byte(e.Key)); err == nil && c != cid.Undef {
+		return "ipld: could not find " + c.String()
+	}
+	return "ipld: could not find " + e.Key
+}
+
+// NotFound always returns true, and is used to feature-test for ErrNotFound
+// errors.
+func (e ErrNotFound) NotFound() bool {
+	return true
+}
+
+// Is allows errors.Is to work with this error type.
+func (e ErrNotFound) Is(err error) bool {
+	switch err.(type) {
+	case ErrNotFound:
+		return true
+	default:
+		return false
+	}
+}
+
+// IsNotFound returns true if the error is a ErrNotFound. As it uses a
+// feature-test, it is also compatible with the legacy
+// github.com/ipfs/go-ipld-format#ErrNotFound.
+func IsNotFound(err error) bool {
+	if nf, ok := err.(interface{ NotFound() bool }); ok {
+		return nf.NotFound()
+	}
+	return false
+}

storage/notfound_test.go

@@ -0,0 +1,58 @@
+package storage_test
+
+import (
+	"testing"
+
+	"github.com/ipfs/go-cid"
+	"github.com/ipld/go-ipld-prime/storage"
+)
+
+func TestNotFound(t *testing.T) {
+	nf := storage.ErrNotFound{Key: "foo"}
+	if !storage.IsNotFound(nf) {
+		t.Fatal("expected ErrNotFound to be a NotFound error")
+	}
+	if nf.Error() != "ipld: could not find foo" {
+		t.Fatal("unexpected error message")
+	}
+
+	nf = storage.ErrNotFound{Key: cid.MustParse("bafybeigdyrzt5sfp7udm7hu76uh7y26nf3efuylqabf3oclgtqy55fbzdi").KeyString()}
+	if !storage.IsNotFound(nf) {
+		t.Fatal("expected ErrNotFound to be a NotFound error")
+	}
+	if nf.Error() != "ipld: could not find bafybeigdyrzt5sfp7udm7hu76uh7y26nf3efuylqabf3oclgtqy55fbzdi" {
+		t.Fatal("unexpected error message")
+	}
+
+	wnf := &weirdNotFoundError{}
+	if !storage.IsNotFound(wnf) {
+		t.Fatal("expected weirdNotFoundError to be a NotFound error")
+	}
+
+	// a weirder case, this one implements `NotFound()` but it returns false, so
+	// this shouldn't be a NotFound error
+	wnnf := &weirdNotNotFoundError{}
+	if storage.IsNotFound(wnnf) {
+		t.Fatal("expected weirdNotNotFoundError to NOT be a NotFound error")
+	}
+}
+
+type weirdNotFoundError struct{}
+
+func (weirdNotFoundError) NotFound() bool {
+	return true
+}
+
+func (weirdNotFoundError) Error() string {
+	return "weird not found error"
+}
+
+type weirdNotNotFoundError struct{}
+
+func (weirdNotNotFoundError) NotFound() bool {
+	return false
+}
+
+func (weirdNotNotFoundError) Error() string {
+	return "weird not NOT found error"
+}