Merge #20375: fuzz: Improve coverage for CPartialMerkleTree fuzzing harness

3c77b8009d fuzz: Improve coverage for CPartialMerkleTree fuzzing harness (practicalswift)

Pull request description:

  Improve coverage for `CPartialMerkleTree` fuzzing harness.

  See [`doc/fuzzing.md`](https://github.com/bitcoin/bitcoin/blob/master/doc/fuzzing.md) for information on how to fuzz Bitcoin Core. Don't forget to contribute any coverage increasing inputs you find to the [Bitcoin Core fuzzing corpus repo](https://github.com/bitcoin-core/qa-assets).

  Happy fuzzing :)

ACKs for top commit:
  MarcoFalke:
    ACK 3c77b8009d

Tree-SHA512: a1fa0f7650a5ee5ff83f35e41b9faf6c34671fc304b9af00e5b83073f21d50bcbe91c2428fa64d05dc42a7c521bfd24031e307c7f4abf9ded469d69a55c5d64a

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);
 }