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https://issues.apache.org/jira/browse/CASSGO-115?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18069852#comment-18069852
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Bohdan Siryk commented on CASSGO-115:
-------------------------------------
The issue is that when *value* in an interface pointer _*interface{} **_ the
code below initializes a map in the destination variable and never updates it.
What happens:
1. Check if the dst var holds an interface pointer
2. If so, init a map (map A) and set its pointer to the dst var
3. Sets the pointer to the map A to the method variable value
4. Using type-switch check if the value holds *map[string]interface{}, be aware
that switch := v now holds pointer to the map A
5. Inits a new map and sets it to the switch variable v
But, that *switch v* holds a pointer to the *local variable v* from the check
before, so in the switch it overrides the value of the local variable, but not
the destination - destination still holds a pointer to the previously
initialized map
{code:java}
// Unmarshal unmarshals the byte slice into the value.
func (udt UDTTypeInfo) Unmarshal(data []byte, value interface{}) error {
// do this up here so we don't need to duplicate all of the map logic below
if iptr, ok := value.(*interface{}); ok && iptr != nil {
// holds map A
v := map[string]interface{}{}
// set dst pointer to the map A
*iptr = v
// reassign value to the map A
value = &v
}
// v holds a pointer to local var with map A
switch v := value.(type) {
case UDTUnmarshaler:
...
case *map[string]interface{}:
if data == nil {
*v = nil
return nil
}
// create a new map B
m := map[string]interface{}{}
// as the value is points to local var with map A, this actually overrides
the local var with map B
// so map A is never updated, but destination is a map A
*v = m
...
} {code}
> MapScan does not preserve top-level UDT values (returns empty map)
> -------------------------------------------------------------------
>
> Key: CASSGO-115
> URL: https://issues.apache.org/jira/browse/CASSGO-115
> Project: Apache Cassandra Go driver
> Issue Type: Bug
> Components: Core
> Reporter: Stanislav Bychkov
> Priority: Normal
> Fix For: 2.1.0
>
>
> When reading rows that contain a top-level UDT column, MapScan() does not
> preserve the decoded UDT field values.
> For a top-level frozen<udt> column:
> * Scan(...) can decode the UDT correctly
> * MapScan(...) returns the row, but the UDT value is an empty
> map[string]interface{}
> h4. Expected behavior
> MapScan should preserve top-level UDT contents consistently with Scan
> A non-null UDT column should decode into a populated map[string]interface{}
> containing its field values
> h4. This is particularly problematic because:
> The column itself is present
> No error is raised
> The data is silently lost rather than missing or failing loudly
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