fix(schema): log error instead of panic if schema not found for predi…

…cate (#7502) (#7509)

This PR fixes alpha panic when drop operation and the query is run concurrently.
This may happen when some query that needs indexing over this predicate is executing while the predicate is dropped from the state (using drop operation).

(cherry picked from commit 96073fc)

schema/schema.go

@@ -313,7 +313,12 @@ func (s *state) Tokenizer(ctx context.Context, pred string) []tok.Tokenizer {
 			su = schema
 		}
 	}
-	x.AssertTruef(su != nil, "schema state not found for %s", pred)
+	if su == nil {
+		// This may happen when some query that needs indexing over this predicate is executing
+		// while the predicate is dropped from the state (using drop operation).
+		glog.Errorf("Schema state not found for %s.", pred)
+		return nil
+	}
 	tokenizers := make([]tok.Tokenizer, 0, len(su.Tokenizer))
 	for _, it := range su.Tokenizer {
 		t, found := tok.GetTokenizer(it)

worker/tokens.go

@@ -84,6 +84,9 @@ func pickTokenizer(ctx context.Context, attr string, f string) (tok.Tokenizer, e
 	}
 
 	tokenizers := schema.State().Tokenizer(ctx, attr)
+	if tokenizers == nil {
+		return nil, errors.Errorf("Schema state not found for %s.", attr)
+	}
 	for _, t := range tokenizers {
 		// If function is eq and we found a tokenizer that's !Lossy(), lets return it
 		switch f {