chaintopology: Simplify rescan offset computation

Simplification of the offset calculation to use the rescan parameter, and rename
of `wallet_first_blocknum`. We now use either relative rescan from our last
known location, or absolute if a negative rescan was given. It's all handled in
a single location (except the case in which the blockcount is below our
precomputed offset), so this should reduce surprises.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

lightningd/chaintopology.c

@@ -519,17 +519,18 @@ static void get_init_block(struct bitcoind *bitcoind,
 static void get_init_blockhash(struct bitcoind *bitcoind, u32 blockcount,
 			       struct chain_topology *topo)
 {
-	/* This can happen if bitcoind still syncing, or first_blocknum is MAX */
-	if (blockcount < topo->first_blocknum)
-		topo->first_blocknum = blockcount;
-
-	/* For fork protection (esp. because we don't handle our own first
-	 * block getting reorged out), we always go 100 blocks further back
-	 * than we need. */
-	if (topo->first_blocknum < 100)
-		topo->first_blocknum = 0;
-	else
-		topo->first_blocknum -= 100;
+	/* If bitcoind's current blockheight is below the requested height, just
+	 * go back to that height. This might be a new node catching up, or
+	 * bitcoind is processing a reorg. */
+	if (blockcount < topo->first_blocknum) {
+		if (bitcoind->ld->config.rescan < 0) {
+			/* Absolute blockheight, but bitcoind's blockheight isn't there yet */
+			topo->first_blocknum = blockcount - 1;
+		} else if (topo->first_blocknum == UINT32_MAX) {
+			/* Relative rescan, but we didn't know the blockheight */
+			topo->first_blocknum = blockcount - bitcoind->ld->config.rescan;
+		}
+	}
 
 	/* Rollback to the given blockheight, so we start track
 	 * correctly again */

lightningd/lightningd.c

@@ -282,7 +282,7 @@ int main(int argc, char *argv[])
 {
 	struct lightningd *ld;
 	bool newdir;
-	u32 first_blocknum;
+	u32 blockheight;
 
 	daemon_setup(argv[0], log_backtrace_print, log_backtrace_exit);
 	ld = new_lightningd(NULL);
@@ -357,10 +357,20 @@ int main(int argc, char *argv[])
 	if (!wallet_htlcs_reconnect(ld->wallet, &ld->htlcs_in, &ld->htlcs_out))
 		fatal("could not reconnect htlcs loaded from wallet, wallet may be inconsistent.");
 
-	/* Worst case, scan back to the first lightning deployment */
-	first_blocknum = wallet_first_blocknum(ld->wallet,
-					       get_chainparams(ld)
-					       ->when_lightning_became_cool);
+	/* Get the blockheight we are currently at, UINT32_MAX is used to signal
+	 * an unitialized wallet and that we should start off of bitcoind's
+	 * current height */
+	blockheight = wallet_blocks_height(ld->wallet, UINT32_MAX);
+
+	/* If we were asked to rescan from an absolute height (--rescan < 0)
+	 * then just go there. Otherwise take compute the diff to our current
+	 * height, lowerbounded by 0. */
+	if (ld->config.rescan < 0)
+		blockheight = -ld->config.rescan;
+	else if (blockheight < (u32)ld->config.rescan)
+		blockheight = 0;
+	else if (blockheight != UINT32_MAX)
+		blockheight -= ld->config.rescan;
 
 	db_commit_transaction(ld->wallet->db);
 
@@ -368,7 +378,7 @@ int main(int argc, char *argv[])
 	setup_topology(ld->topology,
 		       &ld->timers,
 		       ld->config.poll_time,
-		       first_blocknum);
+		       blockheight);
 
 	/* Replay transactions for all running onchainds */
 	onchaind_replay_channels(ld);

lightningd/test/run-find_my_path.c

@@ -107,12 +107,12 @@ struct txfilter *txfilter_new(const tal_t *ctx UNNEEDED)
 /* Generated stub for version */
 const char *version(void)
 { fprintf(stderr, "version called!\n"); abort(); }
+/* Generated stub for wallet_blocks_height */
+u32 wallet_blocks_height(struct wallet *w UNNEEDED, u32 def UNNEEDED)
+{ fprintf(stderr, "wallet_blocks_height called!\n"); abort(); }
 /* Generated stub for wallet_channels_load_active */
 bool wallet_channels_load_active(const tal_t *ctx UNNEEDED, struct wallet *w UNNEEDED)
 { fprintf(stderr, "wallet_channels_load_active called!\n"); abort(); }
-/* Generated stub for wallet_first_blocknum */
-u32 wallet_first_blocknum(struct wallet *w UNNEEDED, u32 first_possible UNNEEDED)
-{ fprintf(stderr, "wallet_first_blocknum called!\n"); abort(); }
 /* Generated stub for wallet_htlcs_load_for_channel */
 bool wallet_htlcs_load_for_channel(struct wallet *wallet UNNEEDED,
 				   struct channel *chan UNNEEDED,

tests/test_lightningd.py

@@ -3607,6 +3607,7 @@ class LightningDTests(BaseLightningDTests):
     def test_funding_while_offline(self):
         l1 = self.node_factory.get_node()
         addr = l1.rpc.newaddr()['address']
+        sync_blockheight([l1])
 
         # l1 goes down.
         l1.stop()

wallet/wallet.c

@@ -767,14 +767,7 @@ void wallet_channel_stats_load(struct wallet *w,
 	stats->out_msatoshi_fulfilled = sqlite3_column_int64(stmt, 7);
 }
 
-/* We want the earlier of either:
- * 1. The first channel we're still watching (it might have closed),
- * 2. The last block we scanned for UTXO (might have new incoming payments)
- *
- * chaintopology actually subtracts another 100 blocks to make sure we
- * catch chain forks.
- */
-u32 wallet_first_blocknum(struct wallet *w, u32 first_possible)
+u32 wallet_blocks_height(struct wallet *w, u32 def)
 {
 	u32 blockheight;
 	sqlite3_stmt *stmt = db_prepare(w->db, "SELECT MAX(height) FROM blocks;");
@@ -785,7 +778,7 @@ u32 wallet_first_blocknum(struct wallet *w, u32 first_possible)
 		sqlite3_finalize(stmt);
 		return blockheight;
 	} else
-		return first_possible;
+		return def;
 }
 
 static void wallet_channel_config_insert(struct wallet *w,

wallet/wallet.h

@@ -314,12 +314,15 @@ void wallet_channel_stats_incr_out_fulfilled(struct wallet *w, u64 cdbid, u64 ms
 void wallet_channel_stats_load(struct wallet *w, u64 cdbid, struct channel_stats *stats);
 
 /**
- * wallet_first_blocknum - get first block we're interested in.
+ * Retrieve the blockheight of the last block processed by lightningd.
+ *
+ * Will return either the maximal blockheight or the default value if the wallet
+ * was never used before.
  *
  * @w: wallet to load from.
- * @first_possible: when c-lightning may have been active from
+ * @def: the default value to return if we've never used the wallet before
  */
-u32 wallet_first_blocknum(struct wallet *w, u32 first_possible);
+u32 wallet_blocks_height(struct wallet *w, u32 def);
 
 /**
  * wallet_extract_owned_outputs - given a tx, extract all of our outputs