PR(IDENTITY-FIX): Fix Panic On Private Doc Update

This was  bug that caused a panic when a request without an identity tried
to update a document that was private (registered with an identity).

The test that asserts the fix is in the next few commits. The test name
is called: `TestACP_CreateWithIdentityAndUpdateWithoutIdentity_CanNotUpdate`

client/collection.go

@@ -185,6 +185,8 @@ type Collection interface {
 	// Get returns the document with the given DocID.
 	//
 	// Returns an ErrDocumentNotFound if a document matching the given DocID is not found.
+	//
+	// Returns an ErrDocumentNotAccessible if DocID is inaccessible.
 	Get(
 		ctx context.Context,
 		identity immutable.Option[string],

client/errors.go

@@ -45,6 +45,7 @@ var (
 	ErrFieldNotObject                      = errors.New("trying to access field on a non object type")
 	ErrValueTypeMismatch                   = errors.New("value does not match indicated type")
 	ErrDocumentNotFound                    = errors.New("no document for the given ID exists")
+	ErrDocumentNotAccessible               = errors.New("document is not accessible")
 	ErrPolicyAddFailureACPModuleNotFound   = errors.New("failure adding policy because ACP module was not found")
 	ErrInvalidACPPermToDeleteDocument      = errors.New("invalid acp permission to delete the document")
 	ErrInvalidACPPermToUpdateDocument      = errors.New("invalid acp permission to update the document")

db/collection_get.go

@@ -48,6 +48,11 @@ func (c *collection) Get(
 	if err != nil {
 		return nil, err
 	}
+
+	if doc == nil {
+		return nil, client.ErrDocumentNotAccessible
+	}
+
 	return doc, c.commitImplicitTxn(ctx, txn)
 }
 

db/collection_index.go

@@ -134,9 +134,11 @@ func (c *collection) updateIndexedDoc(
 	if err != nil {
 		return err
 	}
+	// TODO-ACP: https://github.com/sourcenetwork/defradb/issues/2365 - ACP <> Indexing, possibly also check
+	// and handle the case of when oldDoc == nil (will be nil if inaccessible document).
 	oldDoc, err := c.get(
 		ctx,
-		acpIdentity.NoIdentity, // TODO-ACP: https://github.com/sourcenetwork/defradb/issues/2365 - ACP <> Indexing
+		acpIdentity.NoIdentity,
 		txn,
 		c.getPrimaryKeyFromDocID(doc.ID()), c.Definition().CollectIndexedFields(),
 		false,