HashMap is is not thread safe. Field fileDetails is synchronized in 20 locations, but not in 1 (the 21st) location

[paulward24]

I submitted a CR for this issue:

#43839

The field fileDetails (a HashMap, i.e., not thread safe)

https://github.com/elastic/elasticsearch/blob/master/server/src/main/java/org/elasticsearch/indices/recovery/RecoveryState.java#L679

is used only in synchronzied methods (in about 20 locations), e.g.,:

https://github.com/elastic/elasticsearch/blob/master/server/src/main/java/org/elasticsearch/indices/recovery/RecoveryState.java#L767-L768

i.e., including .size().

This is correct, because according to JDK:

If multiple threads access a hash map concurrently, and at least
one of the threads modifies the map structurally, it must be
synchronized externally.

https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html

However, in the 21st location, here:

https://github.com/elastic/elasticsearch/blob/master/server/src/main/java/org/elasticsearch/indices/recovery/RecoveryState.java#L958

the method is not synchronized.

This CR simply adds the keyword synchronized to the method, just like for all the other places.

[elasticmachine]

Pinging @elastic/es-distributed