wallet: coverage for unknown descriptor load

Previously, this was crashing the wallet.

Note: added the test in a separate file intentionally.
Will continue adding further coverage for the wallet load
process in follow-up commits.

src/Makefile.test.include

@@ -174,7 +174,8 @@ BITCOIN_TESTS += \
   wallet/test/availablecoins_tests.cpp \
   wallet/test/init_tests.cpp \
   wallet/test/ismine_tests.cpp \
-  wallet/test/scriptpubkeyman_tests.cpp
+  wallet/test/scriptpubkeyman_tests.cpp \
+  wallet/test/walletload_tests.cpp
 
 FUZZ_SUITE_LD_COMMON +=\
  $(SQLITE_LIBS) \

src/wallet/test/walletload_tests.cpp

@@ -0,0 +1,51 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://www.opensource.org/licenses/mit-license.php.
+
+#include <wallet/wallet.h>
+#include <test/util/setup_common.h>
+
+#include <boost/test/unit_test.hpp>
+
+namespace wallet {
+
+BOOST_AUTO_TEST_SUITE(walletload_tests)
+
+class UnknownDescriptor final : public Descriptor {
+
+public:
+    explicit UnknownDescriptor() {};
+    ~UnknownDescriptor() = default;
+
+    std::string ToString() const override { return "trx(not_a_random_key)"; }
+    std::optional<OutputType> GetOutputType() const override { return OutputType::UNKNOWN; }
+
+    bool IsRange() const override { return false; }
+    bool IsSolvable() const override { return false; }
+    bool IsSingleType() const override { return true; }
+    bool ToPrivateString(const SigningProvider& provider, std::string& out) const override { return false; }
+    bool ToNormalizedString(const SigningProvider& provider, std::string& out, const DescriptorCache* cache = nullptr) const override { return false; }
+    bool Expand(int pos, const SigningProvider& provider, std::vector<CScript>& output_scripts, FlatSigningProvider& out, DescriptorCache* write_cache = nullptr) const override { return false; };
+    bool ExpandFromCache(int pos, const DescriptorCache& read_cache, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const override { return false; }
+    void ExpandPrivate(int pos, const SigningProvider& provider, FlatSigningProvider& out) const override {}
+};
+
+BOOST_FIXTURE_TEST_CASE(wallet_load_unknown_descriptor, TestingSetup)
+{
+    std::unique_ptr<WalletDatabase> database = CreateMockWalletDatabase();
+    {
+        // Write unknown descriptor
+        WalletBatch batch(*database, false);
+        WalletDescriptor wallet_descriptor(std::make_shared<UnknownDescriptor>(), 0, 0, 0, 0);
+        BOOST_CHECK(batch.WriteDescriptor(uint256(), wallet_descriptor));
+    }
+
+    {
+        // Now try to load the wallet and verify the error.
+        const std::shared_ptr<CWallet> wallet(new CWallet(m_node.chain.get(), "", m_args, std::move(database)));
+        BOOST_CHECK_EQUAL(wallet->LoadWallet(), DBErrors::UNKNOWN_DESCRIPTOR);
+    }
+}
+
+BOOST_AUTO_TEST_SUITE_END()
+} // namespace wallet