Fix regression by returning error on unknown alloc

client/client.go

@@ -487,9 +487,10 @@ func (c *Client) Stats() map[string]map[string]string {
 	return stats
 }
 
-// CollectAllocation garbage collects a single allocation
-func (c *Client) CollectAllocation(allocID string) {
-	c.garbageCollector.Collect(allocID)
+// CollectAllocation garbage collects a single allocation on a node. Returns
+// true if alloc was found and garbage collected; otherwise false.
+func (c *Client) CollectAllocation(allocID string) bool {
+	return c.garbageCollector.Collect(allocID)
 }
 
 // CollectAllAllocs garbage collects all allocations on a node in the terminal

client/gc.go

@@ -196,14 +196,16 @@ func (a *AllocGarbageCollector) Stop() {
 	close(a.shutdownCh)
 }
 
-// Collect garbage collects a single allocation on a node
-func (a *AllocGarbageCollector) Collect(allocID string) {
+// Collect garbage collects a single allocation on a node. Returns true if
+// alloc was found and garbage collected; otherwise false.
+func (a *AllocGarbageCollector) Collect(allocID string) bool {
 	if gcAlloc := a.allocRunners.Remove(allocID); gcAlloc != nil {
 		a.destroyAllocRunner(gcAlloc.allocRunner, "forced collection")
-		return
+		return true
 	}
 
 	a.logger.Printf("[DEBUG] client.gc: alloc %s is invalid or was already garbage collected", allocID)
+	return false
 }
 
 // CollectAll garbage collects all termianated allocations on a node

command/agent/alloc_endpoint.go

@@ -133,7 +133,10 @@ func (s *HTTPServer) allocGC(allocID string, resp http.ResponseWriter, req *http
 		return nil, structs.ErrPermissionDenied
 	}
 
-	s.agent.Client().CollectAllocation(allocID)
+	if !s.agent.Client().CollectAllocation(allocID) {
+		// Could not find alloc
+		return nil, fmt.Errorf("unable to collect allocation: not present")
+	}
 	return nil, nil
 }