Don't update CHACallGraph internal target cache from CallGraph getter methods (#1473)

We should only update `targetCache` when computing the call graph, not
when reading its current state. When debugging another issue, I ran into
very confusing behavior where the debugger invoked `toString()` on a
`CHACallGraph` to display its state, which ended up updating
`targetCache` (whose state I was trying to debug) by calling an accessor
method.

Author: msridhar

core/src/main/java/com/ibm/wala/ipa/callgraph/cha/CHACallGraph.java

@@ -148,7 +148,13 @@ public IClassHierarchy getClassHierarchy() {
 
   private final Map<CallSiteReference, Set<IMethod>> targetCache = HashMapFactory.make();
 
-  private Iterator<IMethod> getPossibleTargets(CallSiteReference site) {
+  /**
+   * Gets the possible targets of a call site, caching the result if it has not been computed.
+   *
+   * @param site the call site
+   * @return an iterator of possible targets
+   */
+  private Iterator<IMethod> getOrUpdatePossibleTargets(CallSiteReference site) {
     Set<IMethod> result = targetCache.get(site);
     if (result == null) {
       if (site.isDispatch()) {
@@ -166,11 +172,25 @@ private Iterator<IMethod> getPossibleTargets(CallSiteReference site) {
     return result.iterator();
   }
 
+  /**
+   * Gets the possible targets of a call site from the cache.
+   *
+   * @param site the call site
+   * @return an iterator of possible targets
+   */
+  private Iterator<IMethod> getPossibleTargetsFromCache(CallSiteReference site) {
+    Set<IMethod> result = targetCache.get(site);
+    if (result == null) {
+      return Collections.emptyIterator();
+    }
+    return result.iterator();
+  }
+
   @Override
   public Set<CGNode> getPossibleTargets(CGNode node, CallSiteReference site) {
     return Iterator2Collection.toSet(
         new MapIterator<>(
-            new FilterIterator<>(getPossibleTargets(site), this::isRelevantMethod),
+            new FilterIterator<>(getPossibleTargetsFromCache(site), this::isRelevantMethod),
             object -> {
               try {
                 return findOrCreateNode(object, Everywhere.EVERYWHERE);
@@ -183,7 +203,7 @@ public Set<CGNode> getPossibleTargets(CGNode node, CallSiteReference site) {
 
   @Override
   public int getNumberOfTargets(CGNode node, CallSiteReference site) {
-    return IteratorUtil.count(getPossibleTargets(site));
+    return IteratorUtil.count(getPossibleTargetsFromCache(site));
   }
 
   @Override
@@ -259,7 +279,7 @@ private void closure() throws CancelException {
     while (!newNodes.isEmpty()) {
       CGNode n = newNodes.pop();
       for (CallSiteReference site : Iterator2Iterable.make(n.iterateCallSites())) {
-        Iterator<IMethod> methods = getPossibleTargets(site);
+        Iterator<IMethod> methods = getOrUpdatePossibleTargets(site);
         while (methods.hasNext()) {
           IMethod target = methods.next();
           if (isRelevantMethod(target)) {