wallet: clearer refusal for an empty or misnamed SQLite backend

Review follow-up on the SQLite activation. Two guards, both fail-closed;
neither is a correctness fix -- FinishLoadedWallet already refuses a
missing default key -- but they make the refusal say what happened:

- an empty or non-wallet wallet.sqlite is refused with "no default key
  record; it may be empty or not a wallet export", instead of the generic
  "no usable default public key" that reads as corruption;
- a misspelled -walletbackend value (e.g. sqlit) is refused instead of
  silently falling through to Berkeley DB; -walletbackend=bdb is accepted
  as the explicit default.

1 files changed, 15 insertions, 0 deletions

diff --git a/src/main_gui.cpp b/src/main_gui.cpp
index 4d77533..4be4995 100644
--- a/src/main_gui.cpp
+++ b/src/main_gui.cpp
@@ -625,6 +625,21 @@ int main(int argc, char* argv[])
         string strWalletSQLite =
             argval2(argc, argv, "/walletsqlite", "-walletsqlite");
 
+        // A misspelled backend must not fall through to Berkeley DB silently --
+        // someone who typed -walletbackend=sqlit meant to run on SQLite, and
+        // opening wallet.dat instead of telling them is the wrong surprise.
+        if (!strWalletBackend.empty() &&
+            strWalletBackend != "sqlite" && strWalletBackend != "bdb")
+        {
+            AttachTerminal();
+            FatalStartupError(fHeadlessStartup,
+                strprintf("Unknown wallet backend '%s'.", strWalletBackend.c_str()),
+                "Use -walletbackend=sqlite for the experimental SQLite backend, "
+                "or omit -walletbackend (or -walletbackend=bdb) to use the "
+                "default Berkeley DB wallet.dat.");
+            return 1;
+        }
+
         if (strWalletBackend == "sqlite")
         {
             // Opt-in, experimental. The node runs on <datadir>/wallet.sqlite and