Checkable: Don't skip redundancy group checks for parent dependencies

Previously, all parent dependencies were recursively checked right at
the beginning of `Checkable::IsReachable()` for reachability and
immediately returned `false` if one of them fails. However, since the
introduction of `redundancy_group`, that failed parent might be within a
redundancy group, which shouldn't necessarily cause the dependency to
fail completely. This PR changes this insofar as not all parents are
checked at the beginning of the method, but only a single parent per
a dependency object is evaluated.

lib/icinga/checkable-dependency.cpp

@@ -55,11 +55,6 @@ bool Checkable::IsReachable(DependencyType dt, Dependency::Ptr *failedDependency
 		return false;
 	}
 
-	for (const Checkable::Ptr& checkable : GetParents()) {
-		if (!checkable->IsReachable(dt, failedDependency, rstack + 1))
-			return false;
-	}
-
 	/* implicit dependency on host if this is a service */
 	const auto *service = dynamic_cast<const Service *>(this);
 	if (service && (dt == DependencyState || dt == DependencyNotification)) {
@@ -78,10 +73,14 @@ bool Checkable::IsReachable(DependencyType dt, Dependency::Ptr *failedDependency
 	std::unordered_map<std::string, Dependency::Ptr> violated; // key: redundancy group, value: nullptr if satisfied, violating dependency otherwise
 
 	for (const Dependency::Ptr& dep : deps) {
-		std::string redundancy_group = dep->GetRedundancyGroup();
+		bool parentReachable{true};
+		if (Checkable::Ptr parent = dep->GetParent(); parent.get() != this) {
+			parentReachable = parent->IsReachable(dt, failedDependency, rstack + 1);
+		}
 
-		if (!dep->IsAvailable(dt)) {
-			if (redundancy_group.empty()) {
+		std::string redundancyGroup = dep->GetRedundancyGroup();
+		if (!parentReachable || !dep->IsAvailable(dt)) {
+			if (redundancyGroup.empty()) {
 				Log(LogDebug, "Checkable")
 					<< "Non-redundant dependency '" << dep->GetName() << "' failed for checkable '" << GetName() << "': Marking as unreachable.";
 
@@ -92,11 +91,11 @@ bool Checkable::IsReachable(DependencyType dt, Dependency::Ptr *failedDependency
 			}
 
 			// tentatively mark this dependency group as failed unless it is already marked;
-			//  so it either passed before (don't overwrite) or already failed (so don't care)
+			// so it either passed before (don't overwrite) or already failed (so don't care)
 			// note that std::unordered_map::insert() will not overwrite an existing entry
-			violated.insert(std::make_pair(redundancy_group, dep));
-		} else if (!redundancy_group.empty()) {
-			violated[redundancy_group] = nullptr;
+			violated.insert(std::make_pair(redundancyGroup, dep));
+		} else if (!redundancyGroup.empty()) {
+			violated[redundancyGroup] = nullptr;
 		}
 	}