Ensure all displayed addresses are imported

Fixes #469. Prior to this commit, using the now default
version of sync (earlier was called "fast sync"), imports
of addresses beyond those already used in the application
was not occurring, resulting in addresses displayed for
deposit that had not been imported as watch-only in Bitcoin
Core. This meant that a user may deposit but not see the
balance in Joinmarket.
This fix ensures that every address displayed (via any
interface) will always have been pre-imported).

jmclient/jmclient/wallet_service.py

@@ -401,9 +401,12 @@ class WalletService(Service):
         #    and then caching allows a small sync to complete *reasonably*
         #    quickly while a larger one is not really negatively affected.
         #    The downside is another free variable, batch size, but this need
-        #    not be exposed to the user; it is not the same as gap limit, in fact,
-        #    the concept of gap limit does not apply to this kind of sync, which
-        #    *assumes* that the most recent usage of addresses is indeed recorded.
+        #    not be exposed to the user; it is not the same as gap limit.
+        #    The assumption is that usage of addresses occurs only if already
+        #    imported, either through in-app usage during coinjoins, or because
+        #    deposit by user will be based on wallet_display() which is only
+        #    showing imported addresses. Hence the gap-limit import at the end
+        #    to ensure this is always true.
         remaining_used_addresses = used_addresses.copy()
         addresses, saved_indices = self.collect_addresses_init()
         for addr in addresses:
@@ -415,8 +418,11 @@ class WalletService(Service):
         for j in range(MAX_ITERATIONS):
             if not remaining_used_addresses:
                 break
-            for addr in \
-                self.collect_addresses_gap(gap_limit=BATCH_SIZE):
+            gap_addrs = self.collect_addresses_gap(gap_limit=BATCH_SIZE)
+            # note: gap addresses *not* imported here; we are still trying
+            # to find the highest-index used address, and assume that imports
+            # are up to that index (at least) - see above main rationale.
+            for addr in gap_addrs:
                 remaining_used_addresses.discard(addr)
 
             # increase wallet indices for next iteration
@@ -433,6 +439,12 @@ class WalletService(Service):
         # overall performance gain is probably negligible
         used_indices = self.get_used_indices(used_addresses)
         self.rewind_wallet_indices(used_indices, saved_indices)
+        # at this point we have the correct up to date index at each branch;
+        # we ensure that all addresses that will be displayed (see wallet_utils.py,
+        # function wallet_display()) are imported by importing gap limit beyond current
+        # index:
+        self.bci.import_addresses(self.collect_addresses_gap(), self.get_wallet_name(),
+                                  self.restart_callback)
         self.synced = True
 
     def sync_addresses(self):