Github user ctubbsii commented on a diff in the pull request:
https://github.com/apache/accumulo/pull/68#discussion_r52676712
--- Diff:
server/master/src/main/java/org/apache/accumulo/master/Master.java ---
@@ -1336,7 +1332,11 @@ public void assignedTablet(KeyExtent extent) {
}
@Override
- public Collection<KeyExtent> migrations() {
- return migrations.keySet();
+ public Set<KeyExtent> migrations() {
+ Set<KeyExtent> migrationsCopy = new HashSet<KeyExtent>();
+ synchronized (migrations) {
+ migrationsCopy.addAll(migrations.keySet());
+ }
+ return migrationsCopy;
--- End diff --
I was thinking a bigger risk would be that a user would accidentally
introduce a RuntimeException (UnsupportedOperationException) because they tried
to do something which worked against some implementations of the interface, but
not others, and because wrapped immutability is not something which we can
check at compile-time. It just seems an unnecessary restriction to me.
It's internal-only code, so we can change it any way we like in the
future... and I think the risk you're highlighting is lower than the risk of
somebody accidentally calling an unsupported method... and it's also much
easier to protect against at the time the implementation changes. If it changes
from a protective copy to another implementation, I would think it'd be obvious
for the implementor to check to see if that breaks usage at that time.
For now, I think it makes sense to treat the interface as "snapshot of
migrations", and the protective copy (vs. another implementation) matches up to
that interface semantics. We could change the interface method name to
"migrationsSnapshot()" to be clear, though, and to avoid careless future impl
changes which change that semantics.
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