tx: pre-allocate the full number of witnesses when deserializing

Prevents quadratic resizing for deserializing non-standard txs with
many witnesses. Note in the transaction.c case, analyze_tx has already
run and validated that the number of witnesses is sane.

Author: jgriffiths

src/internal.h

@@ -132,6 +132,12 @@ const struct wally_map_item *map_find_equal_integer(const struct wally_map *lhs,
 /* Clamp initial witness stack allocation sizing */
 #define MAX_WITNESS_ITEMS_ALLOC 100u /* Non-Taproot standardness limit */
 
+/* Allows allocating a larger witness for e.g deserializing */
+struct wally_tx_witness_stack;
+int tx_witness_stack_init_alloc(size_t allocation_len,
+                                size_t max_allocation_len,
+                                struct wally_tx_witness_stack **output);
+
 /* Absolute maximum number of inputs and outputs for BTC.
  * Liquid numbers are smaller; we use the upper limit */
 #define TX_MAX_INPUTS (TX_MAX_INPUTS_ALLOC * 10)

src/pullpush.c

@@ -260,7 +260,8 @@ int pull_witness(const unsigned char **cursor, size_t *max,
         /* Not enough bytes remaining for num_witnesses empty witnesses */
         return WALLY_EINVAL;
     }
-    ret = wally_tx_witness_stack_init_alloc(num_witnesses, witness_out);
+    ret = tx_witness_stack_init_alloc(num_witnesses, num_witnesses,
+                                      witness_out);
 
     for (i = 0; ret == WALLY_OK && i < num_witnesses; ++i) {
         const unsigned char *wit;

src/transaction.c

@@ -138,7 +138,8 @@ int wally_tx_witness_stack_clone_alloc(const struct wally_tx_witness_stack *stac
     if (!stack)
         return WALLY_EINVAL;
 
-    ret = wally_tx_witness_stack_init_alloc(stack->items_allocation_len, output);
+    ret = tx_witness_stack_init_alloc(stack->items_allocation_len,
+                                      stack->items_allocation_len, output);
     for (i = 0; ret == WALLY_OK && i < stack->num_items; ++i) {
         ret = wally_tx_witness_stack_set(*output, i,
                                          stack->items[i].witness,
@@ -151,15 +152,16 @@ int wally_tx_witness_stack_clone_alloc(const struct wally_tx_witness_stack *stac
     return ret;
 }
 
-int wally_tx_witness_stack_init_alloc(size_t allocation_len,
-                                      struct wally_tx_witness_stack **output)
+int tx_witness_stack_init_alloc(size_t allocation_len,
+                                size_t max_allocation_len,
+                                struct wally_tx_witness_stack **output)
 {
     OUTPUT_CHECK;
     OUTPUT_ALLOC(struct wally_tx_witness_stack);
 
     if (allocation_len) {
-        if (allocation_len > MAX_WITNESS_ITEMS_ALLOC)
-            allocation_len = MAX_WITNESS_ITEMS_ALLOC;
+        if (allocation_len > max_allocation_len)
+            allocation_len = max_allocation_len;
         (*output)->items = wally_calloc(allocation_len * sizeof(struct wally_tx_witness_item));
         if (!(*output)->items) {
             wally_free(*output);
@@ -172,6 +174,16 @@ int wally_tx_witness_stack_init_alloc(size_t allocation_len,
     return WALLY_OK;
 }
 
+int wally_tx_witness_stack_init_alloc(size_t allocation_len,
+                                      struct wally_tx_witness_stack **output)
+{
+    /* The public interface is limited to pre-allocating enough
+     * witness items for a standard tx, and will be slow if adding more
+     */
+    return tx_witness_stack_init_alloc(allocation_len,
+                                       MAX_WITNESS_ITEMS_ALLOC, output);
+}
+
 static int tx_witness_stack_free(struct wally_tx_witness_stack *stack,
                                  bool free_parent)
 {
@@ -2371,7 +2383,7 @@ static int witness_stack_from_bytes(const unsigned char *bytes, struct wally_tx_
     const unsigned char *p = bytes;
     p += varint_from_bytes(p, &num_witnesses);
     if (num_witnesses) {
-        ret = wally_tx_witness_stack_init_alloc(num_witnesses, witness);
+        ret = tx_witness_stack_init_alloc(num_witnesses, num_witnesses, witness);
         if (ret != WALLY_OK)
             goto cleanup;
 
@@ -2485,8 +2497,8 @@ static int tx_from_bytes(const unsigned char *bytes, size_t bytes_len,
             p += varint_from_bytes(p, &num_witnesses);
             if (!num_witnesses)
                 continue;
-            ret = wally_tx_witness_stack_init_alloc(num_witnesses,
-                                                    &(*output)->inputs[i].witness);
+            ret = tx_witness_stack_init_alloc(num_witnesses, num_witnesses,
+                                              &(*output)->inputs[i].witness);
             if (ret != WALLY_OK)
                 goto fail;