fuzz: Improve coverage for CPartialMerkleTree fuzzing harness

src/test/fuzz/merkleblock.cpp

@@ -16,12 +16,36 @@
 void test_one_input(const std::vector<uint8_t>& buffer)
 {
     FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
-    std::optional<CPartialMerkleTree> partial_merkle_tree = ConsumeDeserializable<CPartialMerkleTree>(fuzzed_data_provider);
-    if (!partial_merkle_tree) {
-        return;
+    CPartialMerkleTree partial_merkle_tree;
+    switch (fuzzed_data_provider.ConsumeIntegralInRange<int>(0, 1)) {
+    case 0: {
+        const std::optional<CPartialMerkleTree> opt_partial_merkle_tree = ConsumeDeserializable<CPartialMerkleTree>(fuzzed_data_provider);
+        if (opt_partial_merkle_tree) {
+            partial_merkle_tree = *opt_partial_merkle_tree;
+        }
+        break;
     }
-    (void)partial_merkle_tree->GetNumTransactions();
+    case 1: {
+        CMerkleBlock merkle_block;
+        const std::optional<CBlock> opt_block = ConsumeDeserializable<CBlock>(fuzzed_data_provider);
+        CBloomFilter bloom_filter;
+        std::set<uint256> txids;
+        if (opt_block && !opt_block->vtx.empty()) {
+            if (fuzzed_data_provider.ConsumeBool()) {
+                merkle_block = CMerkleBlock{*opt_block, bloom_filter};
+            } else if (fuzzed_data_provider.ConsumeBool()) {
+                while (fuzzed_data_provider.ConsumeBool()) {
+                    txids.insert(ConsumeUInt256(fuzzed_data_provider));
+                }
+                merkle_block = CMerkleBlock{*opt_block, txids};
+            }
+        }
+        partial_merkle_tree = merkle_block.txn;
+        break;
+    }
+    }
+    (void)partial_merkle_tree.GetNumTransactions();
     std::vector<uint256> matches;
     std::vector<unsigned int> indices;
-    (void)partial_merkle_tree->ExtractMatches(matches, indices);
+    (void)partial_merkle_tree.ExtractMatches(matches, indices);
 }