wallet: fix legacy spkm default birth time

To avoid scanning blocks, as assumed by a wallet with no
generated keys or imported scripts, the default value for
the birth time needs to be set to the maximum int64_t value.

Once the first key is generated or the first script is imported,
the legacy SPKM will update the birth time automatically.

Author: furszy

src/wallet/scriptpubkeyman.cpp

@@ -709,7 +709,7 @@ void LegacyScriptPubKeyMan::UpdateTimeFirstKey(int64_t nCreateTime)
         // Cannot determine birthday information, so set the wallet birthday to
         // the beginning of time.
         nTimeFirstKey = 1;
-    } else if (!nTimeFirstKey || nCreateTime < nTimeFirstKey) {
+    } else if (nTimeFirstKey == UNKNOWN_TIME || nCreateTime < nTimeFirstKey) {
         nTimeFirstKey = nCreateTime;
     }
 

src/wallet/scriptpubkeyman.h

@@ -51,6 +51,9 @@ class WalletStorage
     virtual bool IsLocked() const = 0;
 };
 
+//! Constant representing an unknown spkm creation time
+static constexpr int64_t UNKNOWN_TIME = std::numeric_limits<int64_t>::max();
+
 //! Default for -keypool
 static const unsigned int DEFAULT_KEYPOOL_SIZE = 1000;
 
@@ -286,7 +289,8 @@ class LegacyScriptPubKeyMan : public ScriptPubKeyMan, public FillableSigningProv
     WatchOnlySet setWatchOnly GUARDED_BY(cs_KeyStore);
     WatchKeyMap mapWatchKeys GUARDED_BY(cs_KeyStore);
 
-    int64_t nTimeFirstKey GUARDED_BY(cs_KeyStore) = 0;
+    // By default, do not scan any block until keys/scripts are generated/imported
+    int64_t nTimeFirstKey GUARDED_BY(cs_KeyStore) = UNKNOWN_TIME;
 
     //! Number of pre-generated keys/scripts (part of the look-ahead process, used to detect payments)
     int64_t m_keypool_size GUARDED_BY(cs_KeyStore){DEFAULT_KEYPOOL_SIZE};