fuzz: add a new, more efficient, descriptor parsing target

This new target focuses on fuzzing the actual descriptor parsing logic
by not requiring the fuzzer to produce valid keys (nor a valid checksum
for that matter).
This should make it much more efficient to find bugs we could introduce
moving forward.

Using a character as a marker (here '%') to be able to search and
replace in the string without having to mock the actual descriptor
parsing logic was an insight from Pieter Wuille.

src/test/fuzz/descriptor_parse.cpp

@@ -3,11 +3,109 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <chainparams.h>
+#include <key_io.h>
 #include <pubkey.h>
 #include <script/descriptor.h>
 #include <test/fuzz/fuzz.h>
 #include <util/chaintype.h>
 
+//! Types are raw compressed pubkeys, raw xonly pubkeys, raw privkeys (WIF), xpubs, xprvs.
+static constexpr uint8_t KEY_TYPES_COUNT{5};
+//! How many keys we'll generate in total.
+static constexpr uint8_t TOTAL_KEYS_GENERATED{std::numeric_limits<uint8_t>::max()};
+
+/**
+ * Converts a mocked descriptor string to a valid one. Every key in a mocked descriptor key is
+ * represented by 2 hex characters preceded by the '%' character. We parse the two hex characters
+ * as an index in a list of pre-generated keys. This list contains keys of the various types
+ * accepted in descriptor keys expressions.
+ */
+class MockedDescriptorConverter {
+    //! 256 keys of various types.
+    std::array<std::string, TOTAL_KEYS_GENERATED> keys_str;
+
+public:
+    // We derive the type of key to generate from the 1-byte id parsed from hex.
+    bool IdIsCPubKey(uint8_t idx) const { return idx % KEY_TYPES_COUNT == 0; }
+    bool IdIsXOnlyPubKey(uint8_t idx) const { return idx % KEY_TYPES_COUNT == 1; }
+    bool IdIsConstPrivKey(uint8_t idx) const { return idx % KEY_TYPES_COUNT == 2; }
+    bool IdIsXpub(uint8_t idx) const { return idx % KEY_TYPES_COUNT == 3; }
+    bool IdIsXprv(uint8_t idx) const { return idx % KEY_TYPES_COUNT == 4; }
+
+    //! When initializing the target, populate the list of keys.
+    void Init() {
+        // The data to use as a private key or a seed for an xprv.
+        uint8_t key_data[32] = {1};
+        // Generate keys of all kinds and store them in the keys array.
+        for (uint8_t i{0}; i < TOTAL_KEYS_GENERATED; i++) {
+            key_data[31] = i;
+
+            // If this is a "raw" key, generate a normal privkey. Otherwise generate
+            // an extended one.
+            if (IdIsCPubKey(i) || IdIsXOnlyPubKey(i) || IdIsConstPrivKey(i)) {
+                CKey privkey;
+                privkey.Set(key_data, key_data + 32, true);
+                if (IdIsCPubKey(i)) {
+                    const CPubKey pubkey{privkey.GetPubKey()};
+                    keys_str[i] = HexStr(pubkey);
+                } else if (IdIsXOnlyPubKey(i)) {
+                    const XOnlyPubKey pubkey{privkey.GetPubKey()};
+                    keys_str[i] = HexStr(pubkey);
+                } else {
+                    keys_str[i] = EncodeSecret(privkey);
+                }
+            } else {
+                CExtKey ext_privkey;
+                ext_privkey.SetSeed({(std::byte *)key_data, 32});
+                if (IdIsXprv(i)) {
+                    keys_str[i] = EncodeExtKey(ext_privkey);
+                } else {
+                    const CExtPubKey ext_pubkey{ext_privkey.Neuter()};
+                    keys_str[i] = EncodeExtPubKey(ext_pubkey);
+                }
+            }
+        }
+    }
+
+    //! Parse an id in the keys vectors from a 2-characters hex string.
+    std::optional<uint8_t> IdxFromHex(std::string_view hex_characters) const {
+        if (hex_characters.size() != 2) return {};
+        auto idx = ParseHex(hex_characters);
+        if (idx.size() != 1) return {};
+        return idx[0];
+    }
+
+    //! Get a valid descriptor string from a descriptor string whose keys were mocked.
+    std::optional<std::string> GetDescriptor(std::string_view mocked_desc) const {
+        // The smallest fragment would be "pk(%00)"
+        if (mocked_desc.size() < 7) return {};
+
+        // The actual, valid, descriptor string to be returned.
+        std::string desc;
+        desc.reserve(mocked_desc.size());
+
+        // Replace all occurences of '%' followed by two hex characters with the corresponding key.
+        for (size_t i = 0; i < mocked_desc.size();) {
+            if (mocked_desc[i] == '%') {
+                if (i + 1 >= mocked_desc.size()) return {};
+                if (const auto idx = IdxFromHex(mocked_desc.substr(i + 1, 2))) {
+                    desc += keys_str[*idx];
+                    i += 3;
+                } else {
+                    return {};
+                }
+            } else {
+                desc += mocked_desc[i++];
+            }
+        }
+
+        return desc;
+    }
+};
+
+//! The converter of mocked descriptors, needs to be initialized when the target is.
+MockedDescriptorConverter MOCKED_DESC_CONVERTER;
+
 /** Test a successfully parsed descriptor. */
 static void TestDescriptor(const Descriptor& desc)
 {
@@ -22,6 +120,23 @@ void initialize_descriptor_parse()
     SelectParams(ChainType::MAIN);
 }
 
+void initialize_mocked_descriptor_parse()
+{
+    initialize_descriptor_parse();
+    MOCKED_DESC_CONVERTER.Init();
+}
+
+FUZZ_TARGET_INIT(mocked_descriptor_parse, initialize_mocked_descriptor_parse)
+{
+    const std::string mocked_descriptor{buffer.begin(), buffer.end()};
+    if (const auto descriptor = MOCKED_DESC_CONVERTER.GetDescriptor(mocked_descriptor)) {
+        FlatSigningProvider signing_provider;
+        std::string error;
+        const auto desc = Parse(*descriptor, signing_provider, error);
+        if (desc) TestDescriptor(*desc);
+    }
+}
+
 FUZZ_TARGET_INIT(descriptor_parse, initialize_descriptor_parse)
 {
     const std::string descriptor(buffer.begin(), buffer.end());