better verification on meta tag record upsert

when receiving a new meta tag record upsert request we shouldn't only
validate the given query expressions, but also that the given set of
expressions can be used to instantiate a query. if that's not the case,
then the given meta tag record should be considered invalid and should
get rejected.

Author: replay

expr/tagquery/meta_tag_record.go

@@ -26,6 +26,15 @@ func ParseMetaTagRecord(metaTags []string, expressions []string) (MetaTagRecord,
 		return res, err
 	}
 
+	// we don't actually need to instantiate a query at this point, but we want to verify
+	// that it is possible to instantiate a query from the given meta record expressions.
+	// if we can't instantiate a query from the given expressions, then the meta record
+	// upsert request should be considered invalid and should get rejected.
+	_, err = NewQuery(res.Expressions, 0)
+	if err != nil {
+		return res, fmt.Errorf("Failed to instantiate query from given expressions: %s", err)
+	}
+
 	if len(res.Expressions) == 0 {
 		return res, fmt.Errorf("Meta Tag Record must have at least one query")
 	}

expr/tagquery/meta_tag_record_test.go

@@ -58,3 +58,12 @@ func TestErrorOnParsingMetaTagRecordWithoutQueryy(t *testing.T) {
 		t.Fatalf("Expected an error, but did not get one")
 	}
 }
+
+func TestErrorOnParsingMetaTagRecordWithValidExpressionsButInvalidQuery(t *testing.T) {
+	// the given expression is valid, but as a query this set of expressions is not valid
+	// because every query must have at least one expression requiring a non-empty value
+	_, err := ParseMetaTagRecord([]string{"meta=tag"}, []string{"a!=b"})
+	if err == nil {
+		t.Fatalf("Expected an error, but did not get one")
+	}
+}