wallet: make SQLite the default backend for new wallets

A fresh install with no wallet.dat now starts on the SQLite backend and
creates wallet.sqlite, instead of a Berkeley DB wallet.dat. The choice is
recorded in a plain-text <datadir>/wallet-backend marker so it survives a
restart without a command-line flag.

Backend resolution when no -walletbackend is given: an existing
wallet.sqlite (or a "sqlite" marker) stays on SQLite; an existing
wallet.dat stays on Berkeley DB -- nothing is converted behind the user;
a fresh datadir starts on SQLite. An explicit -walletbackend=bdb or a
"bdb" marker forces Berkeley DB and overrides the default.

Adds CreateNewSQLiteWallet() (WalletSQLiteRuntimeOpen gains fMustExist so
the create path can make a new file) which mints the first default key
straight into the SQLite store via the same FinishLoadedWallet() call the
Berkeley DB create path uses, then cleans up on failure. Marker read/write
helpers are plain text, hand-editable.

New wallet-backend-default self-test proves the resolution in child
processes (fresh->SQLite+marker, existing wallet.dat->BDB, "bdb"
marker->BDB, explicit flag->wallet.dat). The three wallet.dat-specific
suites now ask for -walletbackend=bdb explicitly, since a flagless fresh
datadir is SQLite now. All wallet suites pass.

1 files changed, 73 insertions, 7 deletions

diff --git a/src/main_gui.cpp b/src/main_gui.cpp
index af6053f..6cd8b75 100644
--- a/src/main_gui.cpp
+++ b/src/main_gui.cpp
@@ -743,14 +743,80 @@ int main(int argc, char* argv[])
             DBFlush(true);
             return 0;
         }
-        else if (!LoadWallet())
+        else if (strWalletBackend == "bdb")
         {
-            // LoadWallet() explains itself into strWalletLoadError when it
-            // knows why -- an unsupported wallet format, for one, which is the
-            // case this whole path exists to make visible.
-            FatalStartupError(fHeadlessStartup, "Cannot open wallet.dat.",
-                              strWalletLoadError);
-            return 1;
+            // Explicit -walletbackend=bdb overrides the marker and the default:
+            // force Berkeley DB (LoadWallet creates wallet.dat if none exists).
+            if (!LoadWallet())
+            {
+                FatalStartupError(fHeadlessStartup, "Cannot open wallet.dat.",
+                                  strWalletLoadError);
+                return 1;
+            }
+        }
+        else
+        {
+            // No explicit -walletbackend on the command line. Resolve it from
+            // the persistent marker, then defaults: an existing wallet.sqlite
+            // stays on SQLite, an existing wallet.dat stays on Berkeley DB (the
+            // GUI offers to convert), and a fresh install starts on SQLite --
+            // the new default. wallet.dat is never touched by any of this.
+            string strSQLitePath = GetAppDir() + "/wallet.sqlite";
+            bool fSQLiteExists = FileExists(strSQLitePath.c_str());
+            bool fWalletDatExists = FileExists((GetAppDir() + "/wallet.dat").c_str());
+            string strMarker = ReadWalletBackendMarker();
+
+            bool fUseSQLite = false;
+            bool fCreateSQLite = false;
+            if (strMarker == "sqlite")
+            {
+                if (fSQLiteExists)          fUseSQLite = true;
+                else if (fWalletDatExists)  fUseSQLite = false;  // marker stale; real wallet is BDB
+                else { fUseSQLite = true; fCreateSQLite = true; }
+            }
+            else if (strMarker == "bdb")
+            {
+                fUseSQLite = false;
+            }
+            else  // no marker recorded yet
+            {
+                if (fSQLiteExists && !fWalletDatExists) fUseSQLite = true;
+                else if (fWalletDatExists)              fUseSQLite = false;
+                else { fUseSQLite = true; fCreateSQLite = true; }
+            }
+
+            if (fUseSQLite)
+            {
+                AttachTerminal();
+                bool fOk = fCreateSQLite
+                    ? CreateNewSQLiteWallet(strSQLitePath)
+                    : LoadWalletFromSQLiteRuntime(strSQLitePath);
+                if (!fOk)
+                {
+                    FatalStartupError(fHeadlessStartup,
+                        fCreateSQLite ? "Cannot create the SQLite wallet."
+                                      : "Cannot open the SQLite wallet.",
+                        strWalletLoadError);
+                    return 1;
+                }
+                // Record the choice so it is stable from here on.
+                if (ReadWalletBackendMarker() != "sqlite")
+                    WriteWalletBackendMarker("sqlite");
+                fprintf(stderr, "Wallet backend: SQLite\n  %s\n%s",
+                        strSQLitePath.c_str(),
+                        fCreateSQLite ? "A new wallet was created here.\n" : "");
+                fflush(stderr);
+                // Fall through and run the node; CWalletDB routes to this file.
+            }
+            else if (!LoadWallet())
+            {
+                // LoadWallet() explains itself into strWalletLoadError when it
+                // knows why -- an unsupported wallet format, for one, which is
+                // the case this whole path exists to make visible.
+                FatalStartupError(fHeadlessStartup, "Cannot open wallet.dat.",
+                                  strWalletLoadError);
+                return 1;
+            }
         }
 
         // After the block index, so there is a chain to compare the wallet