Keep the Request message in memory

We used to free the Request we made after transmitting it.  However, for
GiveBack to work properly, we need to be able to make that request
again.  To do it without having to request the Source_Capabilities
again, it makes sense to keep the Request message in memory.  We have
more messages in the pool than we need anyway.

src/policy_engine.c

 
 /* The received message we're currently working with */
 static union pd_msg *policy_engine_message = NULL;
+/* The most recent Request from the DPM */
+static union pd_msg *last_dpm_request = NULL;
 /* Whether or not the source capabilities match our required power */
 static bool capability_match = false;
 /* Whether or not we have an explicit contract */
 
 static enum policy_engine_state pe_sink_eval_cap(void)
 {
-    /* Get a message object */
-    union pd_msg *request = chPoolAlloc(&pdb_msg_pool);
+    /* Get a message object for the request if we don't have one already */
+    if (last_dpm_request == NULL) {
+        last_dpm_request = chPoolAlloc(&pdb_msg_pool);
+    }
     /* Ask the DPM what to request */
-    capability_match = pdb_dpm_evaluate_capability(policy_engine_message, request);
+    capability_match = pdb_dpm_evaluate_capability(policy_engine_message,
+            last_dpm_request);
     /* Free the Source_Capabilities message */
     chPoolFree(&pdb_msg_pool, policy_engine_message);
-    /* Put the request into policy_engine_message */
-    policy_engine_message = request;
 
     return PESinkSelectCap;
 }
 static enum policy_engine_state pe_sink_select_cap(void)
 {
     /* Transmit the request */
-    chMBPost(&pdb_prltx_mailbox, (msg_t) policy_engine_message, TIME_IMMEDIATE);
+    chMBPost(&pdb_prltx_mailbox, (msg_t) last_dpm_request, TIME_IMMEDIATE);
     chEvtSignal(pdb_prltx_thread, PDB_EVT_PRLTX_MSG_TX);
     eventmask_t evt = chEvtWaitAny(PDB_EVT_PE_TX_DONE | PDB_EVT_PE_TX_ERR
             | PDB_EVT_PE_RESET);
-    /* Free the sent message */
-    chPoolFree(&pdb_msg_pool, policy_engine_message);
-    policy_engine_message = NULL;
+    /* Don't free the request; we might need it again */
     /* If we got reset signaling, transition to default */
     if (evt & PDB_EVT_PE_RESET) {
         return PESinkTransitionDefault;