Fix randomx cache selection for RPCs

Was using the wrong cache slot, and returning invalid PoW hashes to RPC clients

src/crypto/rx-slow-hash.c

@@ -50,7 +50,7 @@
 
 typedef struct rx_state {
   CTHR_MUTEX_TYPE rs_mutex;
-  char rs_hash[32];
+  char rs_hash[HASH_SIZE];
   uint64_t  rs_height;
   randomx_cache *rs_cache;
 } rx_state;
@@ -63,7 +63,8 @@ static rx_state rx_s[2] = {{CTHR_MUTEX_INIT,{0},0,0},{CTHR_MUTEX_INIT,{0},0,0}};
 static randomx_dataset *rx_dataset;
 static uint64_t rx_dataset_height;
 static THREADV randomx_vm *rx_vm = NULL;
-static THREADV int rx_toggle;
+static THREADV int rx_toggle = -1;
+static THREADV char rx_seedhash[HASH_SIZE];
 
 static void local_abort(const char *msg)
 {
@@ -234,21 +235,25 @@ void rx_slow_hash(const uint64_t mainheight, const uint64_t seedheight, const ch
   char *hash, int miners, int is_alt) {
   uint64_t s_height = rx_seedheight(mainheight);
   int changed = 0;
-  int toggle = is_alt ? s_height : seedheight;
+  int toggle = (s_height & SEEDHASH_EPOCH_BLOCKS) != 0;
   randomx_flags flags = RANDOMX_FLAG_DEFAULT;
   rx_state *rx_sp;
   randomx_cache *cache;
 
-  toggle = (toggle & SEEDHASH_EPOCH_BLOCKS) != 0;
   CTHR_MUTEX_LOCK(rx_mutex);
 
   /* if alt block but with same seed as mainchain, no need for alt cache */
-  if (is_alt && s_height == seedheight && !memcmp(rx_s[toggle].rs_hash, seedhash, sizeof(rx_s[toggle].rs_hash)))
-    is_alt = 0;
-
+  if (is_alt) {
+    if (s_height == seedheight && !memcmp(rx_s[toggle].rs_hash, seedhash, HASH_SIZE))
+      is_alt = 0;
+  } else {
   /* RPC could request an earlier block on mainchain */
-  if (!is_alt && s_height > seedheight)
-    is_alt = 1;
+    if (s_height > seedheight)
+      is_alt = 1;
+    /* miner can be ahead of mainchain */
+    else if (s_height < seedheight)
+      toggle ^= 1;
+  }
 
   toggle ^= (is_alt != 0);
   if (toggle != rx_toggle)
@@ -273,13 +278,16 @@ void rx_slow_hash(const uint64_t mainheight, const uint64_t seedheight, const ch
         local_abort("Couldn't allocate RandomX cache");
     }
   }
-  if (rx_sp->rs_height != seedheight || rx_sp->rs_cache == NULL || memcmp(seedhash, rx_sp->rs_hash, sizeof(rx_sp->rs_hash))) {
-    randomx_init_cache(cache, seedhash, 32);
+  if (rx_sp->rs_height != seedheight || rx_sp->rs_cache == NULL || memcmp(seedhash, rx_sp->rs_hash, HASH_SIZE)) {
+    randomx_init_cache(cache, seedhash, HASH_SIZE);
     rx_sp->rs_cache = cache;
     rx_sp->rs_height = seedheight;
-    memcpy(rx_sp->rs_hash, seedhash, sizeof(rx_sp->rs_hash));
+    memcpy(rx_sp->rs_hash, seedhash, HASH_SIZE);
     changed = 1;
   }
+  /* only non-miners use rx_seedhash */
+  if (!miners && !changed && memcmp(seedhash, rx_seedhash, HASH_SIZE))
+    changed = 1;
   if (rx_vm == NULL) {
     randomx_flags flags = RANDOMX_FLAG_DEFAULT;
     if (use_rx_jit()) {
@@ -307,6 +315,9 @@ void rx_slow_hash(const uint64_t mainheight, const uint64_t seedheight, const ch
         mwarning(RX_LOGCAT, "Couldn't allocate RandomX dataset for miner");
       }
       CTHR_MUTEX_UNLOCK(rx_dataset_mutex);
+    } else {
+      /* only non-miners use rx_seedhash */
+      memcpy(rx_seedhash, seedhash, HASH_SIZE);
     }
     rx_vm = randomx_create_vm(flags | RANDOMX_FLAG_LARGE_PAGES, rx_sp->rs_cache, rx_dataset);
     if(rx_vm == NULL) { //large pages failed
@@ -326,6 +337,11 @@ void rx_slow_hash(const uint64_t mainheight, const uint64_t seedheight, const ch
     CTHR_MUTEX_UNLOCK(rx_dataset_mutex);
   } else if (changed) {
     randomx_vm_set_cache(rx_vm, rx_sp->rs_cache);
+    /* remember the seedhash being used by the current VM. Other threads may set
+     * rx_sp->rs_cache to the correct value, but we still have to know if the
+     * current rx_vm has been set to use it or not.
+     */
+    memcpy(rx_seedhash, seedhash, HASH_SIZE);
   }
   /* mainchain users can run in parallel */
   if (!is_alt)