JITServer: handle non-null compilation

When a JITServer stream failure is thrown, sometimes setting
the compilation object to NULL can be skiped to head straight
into exception handling. This causes an assert to be tripped.

In this particular case, there is a front-end call in
outOfProcessCompilationEnd (within computeDataForCHTableCommit)
which can cause a stream error. Since it isn't guarded
by a global try-catch, it results in the given
assert being tripped.

Addresses this issue:
#23845

Author: KavinSatheeskumar

runtime/compiler/control/CompilationThread.cpp

@@ -8747,6 +8747,13 @@ TR_MethodMetaData *TR::CompilationInfoPerThreadBase::wrappedCompile(J9PortLibrar
                 diagnostic("Forced Option Set %d\n", optionSetIndex);
             }
         }
+        if (that->_compiler) {
+            // If JITServer is enabled, this will send messages
+            // to the client to get the retained method set and
+            // cache it locally. This is needed for
+            // postCompilationTasks, see issue #23845
+            that->_compiler->retainedMethods();
+        }
     } catch (const std::exception &e) {
         // TODO: we must handle OOM cases when we abort the compilation right from the start.
         // Or eliminate the code that throws (or the code could also look at how the expensive the compilation is)

runtime/compiler/env/J9RetainedMethodSet.cpp

@@ -583,6 +583,8 @@ void J9::RepeatRetainedMethodsAnalysis::getDataForClient(TR::Compilation *comp,
     }
 
     TR_ResolvedMethod *keepaliveMethod = NULL;
+    // retainedMethods is guaranteed to exist, so no messages
+    // will be sent to the server. See issue #23845
     auto keepaliveMethodsIter = comp->retainedMethods()->keepaliveMethods();
     while (keepaliveMethodsIter.next(&keepaliveMethod)) {
         keepaliveMethods.push_back(static_cast<TR_ResolvedJ9JITServerMethod *>(keepaliveMethod)->getRemoteMirror());