mempool: notifyTxsAvailable if there're txs left, but recheck=false (#2991)

(left after committing a block)

Fixes #2961

--------------
ORIGINAL ISSUE

Tendermint version : 0.26.4-b771798d

ABCI app : kv-store

Environment:

OS (e.g. from /etc/os-release): macOS 10.14.1 What happened: Set
mempool.recheck = false and create empty block = false in config.toml.
When transactions get added right between a new empty block is being
proposed and committed, the proposer won't propose new block for that
transactions immediately after. That transactions are stuck in the
mempool until a new transaction is added and trigger the proposer.

What you expected to happen: If there is a transaction left in the
mempool, new block should be proposed immediately.

Have you tried the latest version: yes

How to reproduce it (as minimally and precisely as possible): Fire two
transaction using broadcast_tx_sync with specific delay between them.
(You may need to do it multiple time before the right delay is found, on
my machine the delay is 0.98s)

Logs (paste a small part showing an error (< 10 lines) or link a
pastebin, gist, etc. containing more of the log file):
https://pastebin.com/0Wt6uhPF

Config (you can paste only the changes you've made): [mempool] recheck =
false create_empty_block = false

Anything else we need to know: In mempool.go, we found that proposer
will immediately propose new block if

Last committed block has some transaction (causing AppHash to changed)
or mem.notifyTxsAvailable() is called. Our scenario is as followed.

A transaction is fired, it will create 1 block with 1 tx (line 1-4 in
the log) and 1 empty block. After the empty block is proposed but before
it is committed, second transaction is fired and added to mempool. (line
8-16) Now, since the last committed block is empty and
mem.notifyTxsAvailable() will be called only if mempool.recheck = true.
The proposer won't immediately propose new block, causing the second
transaction to stuck in mempool until another transaction is added to
mempool and trigger mem.notifyTxsAvailable().

CHANGELOG_PENDING.md

@@ -24,5 +24,6 @@ Special thanks to external contributors on this release:
 
 ### BUG FIXES:
 - [kv indexer] \#2912 don't ignore key when executing CONTAINS
+- [mempool] \#2961 notifyTxsAvailable if there're txs left after committing a block, but recheck=false
 - [mempool] \#2994 Don't allow txs with negative gas wanted
 - [p2p] \#2715 fix a bug where seeds don't disconnect from a peer after 3h

mempool/mempool.go

@@ -556,13 +556,18 @@ func (mem *Mempool) Update(
 	// Remove committed transactions.
 	txsLeft := mem.removeTxs(txs)
 
-	// Recheck mempool txs if any txs were committed in the block
-	if mem.config.Recheck && len(txsLeft) > 0 {
-		mem.logger.Info("Recheck txs", "numtxs", len(txsLeft), "height", height)
-		mem.recheckTxs(txsLeft)
-		// At this point, mem.txs are being rechecked.
-		// mem.recheckCursor re-scans mem.txs and possibly removes some txs.
-		// Before mem.Reap(), we should wait for mem.recheckCursor to be nil.
+	// Either recheck non-committed txs to see if they became invalid
+	// or just notify there're some txs left.
+	if len(txsLeft) > 0 {
+		if mem.config.Recheck {
+			mem.logger.Info("Recheck txs", "numtxs", len(txsLeft), "height", height)
+			mem.recheckTxs(txsLeft)
+			// At this point, mem.txs are being rechecked.
+			// mem.recheckCursor re-scans mem.txs and possibly removes some txs.
+			// Before mem.Reap(), we should wait for mem.recheckCursor to be nil.
+		} else {
+			mem.notifyTxsAvailable()
+		}
 	}
 
 	// Update metrics