Add unit tests for WriteWitnessCache

Requires moving implementation into header

src/wallet/wallet.h

@@ -15,6 +15,7 @@
 #include "primitives/transaction.h"
 #include "tinyformat.h"
 #include "ui_interface.h"
+#include "util.h"
 #include "utilstrencodings.h"
 #include "validationinterface.h"
 #include "wallet/crypter.h"
@@ -605,7 +606,40 @@ protected:
                                 const CBlock* pblock,
                                 ZCIncrementalMerkleTree tree);
     void DecrementNoteWitnesses();
-    void WriteWitnessCache();
+
+    template <typename WalletDB>
+    void WriteWitnessCache(WalletDB& walletdb) {
+        if (!walletdb.TxnBegin()) {
+            // This needs to be done atomically, so don't do it at all
+            LogPrintf("WriteWitnessCache(): Couldn't start atomic write\n");
+            return;
+        }
+        try {
+            for (std::pair<const uint256, CWalletTx>& wtxItem : mapWallet) {
+                if (!walletdb.WriteTx(wtxItem.first, wtxItem.second)) {
+                    LogPrintf("WriteWitnessCache(): Failed to write CWalletTx, aborting atomic write\n");
+                    walletdb.TxnAbort();
+                    return;
+                }
+            }
+            if (!walletdb.WriteWitnessCacheSize(nWitnessCacheSize)) {
+                LogPrintf("WriteWitnessCache(): Failed to write nWitnessCacheSize, aborting atomic write\n");
+                walletdb.TxnAbort();
+                return;
+            }
+        } catch (const std::exception &exc) {
+            // Unexpected failure
+            LogPrintf("WriteWitnessCache(): Unexpected error during atomic write:\n");
+            LogPrintf("%s\n", exc.what());
+            walletdb.TxnAbort();
+            return;
+        }
+        if (!walletdb.TxnCommit()) {
+            // Couldn't commit all to db, but in-memory state is fine
+            LogPrintf("WriteWitnessCache(): Couldn't commit atomic write\n");
+            return;
+        }
+    }
 
 private:
     template <class T>