storage_proxy: send hints to pending replicas

Consider the following scenario:
- Current replica set is [A, B, C]
- write succeeds on [A, B], and a hint is logged for node C
- before the hint is replayed, D bootstraps and the token migrates from C to D
- hint is replayed to node C while D is pending, but it's too late, since streaming for that token is already done
- C is cleaned up, replayed data is lost, and D has a stale copy until next repair.
In the scenario we effectively fail to send the hint. This scenario is also more likely to happen with tablets,
as it can happen for every tablet migration.

This issue is particularly detrimental to materialized views. View updates use hints by default and a specific
view update may be sent to just one view replica (when a single base replica has a different row state due to
reordering or missed writes). When we lose a hint for such a view update, we can generate a persistent inconsistency
between the base and view - ghost rows can appear due to a lost tombstone and rows may be missing in the view due
to a lost row update. Such inconsistencies can't be fixed neither by repairing the view or the base table.

To handle this, in this patch we add the pending replicas to the list of targets of each hint, even if the original
target is still alive.

This will cause some updates to be redundant. These updates are probably unavoidable for now, but they shouldn't
be too common either. The scenarios for them are:
1. managing to send the hint to the source of a migrating replica before streaming that its token - the write will
arrive on the pending replica anyway in streaming
2. the hint target not being the source of the migration - if we managed to apply the original write of the hint to
the actual source of the migration, the pending replica will get it during streaming
3. sending the same hint to many targets at a similar time - while sending to each target, we'll see the same pending
replica for the hint so we'll send it multiple times
4. possible retries where even though the hint was successfully sent to the main target, we failed to send it to the
pending replica, so we need to retry the entire write

This patch handles both tablet migrations and tablet rebuilds. In the future, for tablet migrations, we can avoid
sending the hint to pending replias if the hint target is not the source fo the migration, which would allow us to
avoid the redundant writes 2 and 3. For rack-aware RF, this will be as simple as checking whether the replicas are
in the same rack.

We also add a test case reproducing the issue.

Co-Authored-By: Raphael S. Carvalho <raphaelsc@scylladb.com>

Fixes https://github.com/scylladb/scylladb/issues/19835

Closes scylladb/scylladb#25590

(cherry picked from commit 10b8e1c51c)

Closes scylladb/scylladb#25882

test/cluster/test_hints.py

@@ -16,6 +16,7 @@ from cassandra.query import SimpleStatement, ConsistencyLevel
 from test.pylib.internal_types import ServerInfo
 from test.pylib.manager_client import ManagerClient
 from test.pylib.rest_client import inject_error
+from test.pylib.tablets import get_tablet_replicas
 from test.pylib.util import wait_for
 
 from test.cluster.conftest import skip_mode
@@ -321,3 +322,59 @@ async def test_canceling_hint_draining(manager: ManagerClient):
     # Make sure draining finishes successfully.
     assert await_sync_point(s1, sync_point, 60)
     await s1_log.wait_for(f"Removed hint directory for {host_id2}")
+
+@pytest.mark.asyncio
+@skip_mode("release", "error injections are not supported in release mode")
+async def test_hint_to_pending(manager: ManagerClient):
+    """
+    This test reproduces the scenario where sending a hint to a pending replica is needed
+    for consistency as in https://github.com/scylladb/scylladb/issues/19835.
+    In the test, we have 2 servers and a table with RF=1. One server is stopped, and we
+    perform a write generating a hint to it. Then, we start the stopped server again and
+    immediately request a tablet migration from that server. The hint is sent after the
+    tablet migration performs streaming but before it completes. The order of operations
+    is induced using error injections.
+    At the end, we verify that the hint was successfully applied.
+    """
+    servers = await manager.servers_add(2, property_file=[
+        {"dc": "dc1", "rack": "r1"},
+        {"dc": "dc1", "rack": "r1"},
+    ])
+    cql = await manager.get_cql_exclusive(servers[0])
+    await manager.api.disable_tablet_balancing(servers[0].ip_addr)
+    await manager.api.disable_tablet_balancing(servers[1].ip_addr)
+
+    async with new_test_keyspace(manager, "WITH replication = {'class': 'NetworkTopologyStrategy', 'replication_factor': 1} AND tablets = {'initial': 1}") as ks:
+        table = f"{ks}.t"
+        await cql.run_async(f"CREATE TABLE {table} (pk int primary key, v int)")
+        replica = (await get_tablet_replicas(manager, servers[0], ks, "t", 0))[0]
+        host_ids = [await manager.get_host_id(server.server_id) for server in servers]
+        if replica[0] != host_ids[1]:
+            # We'll use server 0 as the source of the hint, so the tablet replica needs to be on server 1
+            await manager.api.move_tablet(servers[0].ip_addr, ks, "t", replica[0], replica[1], host_ids[1], 0, 0)
+
+        await manager.server_stop_gracefully(servers[1].server_id)
+        await manager.others_not_see_server(servers[1].ip_addr)
+
+        await cql.run_async(SimpleStatement(f"INSERT INTO {table} (pk, v) VALUES (0, 0)", consistency_level=ConsistencyLevel.ANY))
+
+        await manager.api.enable_injection(servers[0].ip_addr, "hinted_handoff_pause_hint_replay", False)
+        await manager.server_start(servers[1].server_id)
+        sync_point = create_sync_point(servers[0])
+
+        await manager.api.enable_injection(servers[0].ip_addr, "pause_after_streaming_tablet", False)
+        tablet_migration = asyncio.create_task(manager.api.move_tablet(servers[0].ip_addr, ks, "t", host_ids[1], 0, host_ids[0], 0, 0))
+
+        async def migration_reached_streaming():
+            stages = await cql.run_async(f"SELECT stage FROM system.tablets WHERE keyspace_name='{ks}' ALLOW FILTERING")
+            logger.info(f"Current stages: {[row.stage for row in stages]}")
+            return set(["streaming"]) == set([row.stage for row in stages]) or None
+        await wait_for(migration_reached_streaming, time.time() + 60)
+
+        await manager.api.disable_injection(servers[0].ip_addr, "hinted_handoff_pause_hint_replay")
+        assert await_sync_point(servers[0], sync_point, 30)
+
+        await manager.api.message_injection(servers[0].ip_addr, "pause_after_streaming_tablet")
+        await asyncio.wait([tablet_migration])
+
+        assert list(await cql.run_async(f"SELECT v FROM {table} WHERE pk = 0")) == [(0,)]