rpc: getwalletinfo, return wallet 'birthtime'

And add coverage for it

Github-Pull: bitcoin#28920
Rebased-From: 1ce45ba

src/wallet/rpc/wallet.cpp

@@ -69,6 +69,7 @@ static RPCHelpMan getwalletinfo()
                         {RPCResult::Type::BOOL, "descriptors", "whether this wallet uses descriptors for scriptPubKey management"},
                         {RPCResult::Type::BOOL, "external_signer", "whether this wallet is configured to use an external signer such as a hardware wallet"},
                         {RPCResult::Type::BOOL, "blank", "Whether this wallet intentionally does not contain any keys, scripts, or descriptors"},
+                        {RPCResult::Type::NUM_TIME, "birthtime", /*optional=*/true, "The start time for blocks scanning. It could be modified by (re)importing any descriptor with an earlier timestamp."},
                         RESULT_LAST_PROCESSED_BLOCK,
                     }},
                 },
@@ -132,6 +133,9 @@ static RPCHelpMan getwalletinfo()
     obj.pushKV("descriptors", pwallet->IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS));
     obj.pushKV("external_signer", pwallet->IsWalletFlagSet(WALLET_FLAG_EXTERNAL_SIGNER));
     obj.pushKV("blank", pwallet->IsWalletFlagSet(WALLET_FLAG_BLANK_WALLET));
+    if (int64_t birthtime = pwallet->GetBirthTime(); birthtime != UNKNOWN_TIME) {
+        obj.pushKV("birthtime", birthtime);
+    }
 
     AppendLastProcessedBlock(obj, *pwallet);
     return obj;

src/wallet/wallet.h

@@ -877,6 +877,9 @@ class CWallet final : public WalletStorage, public interfaces::Chain::Notificati
     /* Returns true if the wallet can give out new addresses. This means it has keys in the keypool or can generate new keys */
     bool CanGetAddresses(bool internal = false) const;
 
+    /* Returns the time of the first created key or, in case of an import, it could be the time of the first received transaction */
+    int64_t GetBirthTime() const { return m_birth_time; }
+
     /**
      * Blocks until the wallet state is up-to-date to /at least/ the current
      * chain at the time this function is entered

test/functional/wallet_reindex.py

@@ -44,15 +44,27 @@ def birthtime_test(self, node, miner_wallet):
             self.advance_time(node, BLOCK_TIME)
 
         # Now create a new wallet, and import the descriptor
-        node.createwallet(wallet_name='watch_only', disable_private_keys=True, blank=True, load_on_startup=True)
+        node.createwallet(wallet_name='watch_only', disable_private_keys=True, load_on_startup=True)
         wallet_watch_only = node.get_wallet_rpc('watch_only')
+        # Blank wallets don't have a birth time
+        assert 'birthtime' not in wallet_watch_only.getwalletinfo()
 
         # For a descriptors wallet: Import address with timestamp=now.
         # For legacy wallet: There is no way of importing a script/address with a custom time. The wallet always imports it with birthtime=1.
         # In both cases, disable rescan to not detect the transaction.
         wallet_watch_only.importaddress(wallet_addr, rescan=False)
         assert_equal(len(wallet_watch_only.listtransactions()), 0)
 
+        # Depending on the wallet type, the birth time changes.
+        wallet_birthtime = wallet_watch_only.getwalletinfo()['birthtime']
+        if self.options.descriptors:
+            # As blocks were generated every 10 min, the chain MTP timestamp is node_time - 60 min.
+            assert_equal(self.node_time - BLOCK_TIME * 6, wallet_birthtime)
+        else:
+            # No way of importing scripts/addresses with a custom time on a legacy wallet.
+            # It's always set to the beginning of time.
+            assert_equal(wallet_birthtime, 1)
+
         # Rescan the wallet to detect the missing transaction
         wallet_watch_only.rescanblockchain()
         assert_equal(wallet_watch_only.gettransaction(tx_id)['confirmations'], 50)
@@ -65,7 +77,16 @@ def birthtime_test(self, node, miner_wallet):
 
         # Verify the transaction is still 'confirmed' after reindex
         wallet_watch_only = node.get_wallet_rpc('watch_only')
-        assert_equal(wallet_watch_only.gettransaction(tx_id)['confirmations'], 50)
+        tx_info = wallet_watch_only.gettransaction(tx_id)
+        assert_equal(tx_info['confirmations'], 50)
+
+        # Depending on the wallet type, the birth time changes.
+        if self.options.descriptors:
+            # For descriptors, verify the wallet updated the birth time to the transaction time
+            assert_equal(tx_info['time'], wallet_watch_only.getwalletinfo()['birthtime'])
+        else:
+            # For legacy, as the birth time was set to the beginning of time, verify it did not change
+            assert_equal(wallet_birthtime, 1)
 
         wallet_watch_only.unloadwallet()