Raise S20 for every generation refusal, ask for a push when the counter was behind, and write a send's generation with its digest (hq issue 234 review)

A refusal of a send the mesh never recorded went only to stderr; it is now kept
with its sender said to be unknown, and raised. A counter behind the machine (a
store put back) is raised and the condition names push, since the receive loop
must not push. The node's digest and generation are one transaction, and a send
record that fails is said loudly without failing the send. The trigger ignores
an update that changes nothing, and the put-back mark is dropped: a put-back is
composed afresh with the current generation.

internal/inventory/migrations/0094-a-declaration-says-the-generation-it-came-from.sql

@@ -24,26 +24,33 @@ begin
 end
 $$;
 
--- Per row, so a statement that changes nothing (an assignment repeated, `on conflict do nothing`) raises
--- nothing; a statement that changes several raises once per row, which only ever moves it forward.
+-- Per row, so a statement that changes nothing raises nothing: an assignment repeated (`on conflict do
+-- nothing` inserts no row) and an update that leaves a row as it was (the WHEN below; a plain UPDATE fires a
+-- row trigger whether or not it changed anything). A statement that changes several rows raises once per
+-- row, which only ever moves it forward. Two triggers, because only an update has both OLD and NEW.
 create trigger assignment_generation_raised
-    after insert or update or delete on assignment
+    after insert or delete on assignment
     for each row execute function raise_assignment_generation();
+create trigger assignment_generation_raised_by_update
+    after update on assignment
+    for each row when (old is distinct from new) execute function raise_assignment_generation();
 
--- What a machine was last sent of it: the generation, and whether that send was a gate's put-back, so
+-- The generation a machine was last sent, written in the same transaction as the digest of that send, so
 -- what the mesh WOULD send is stamped the same way and reads as byte for byte what it DID send when
--- nothing else changed (as sent_epoch, migration 0068). Null and false for a send without.
+-- nothing else changed (as sent_epoch, migration 0068). Null for a send without one.
 alter table node add column sent_generation bigint;
-alter table node add column sent_put_back boolean not null default false;
 -- Whether the machine's node-engine said it reads a generation (its reports' `reads_generation`): until
 -- it has, it is sent none, because an older node-engine refuses a key it does not know, whole.
 alter table node add column reads_generation boolean not null default false;
 
 -- Every send: the machine, its sequence, who sent it — the lease epoch the sender acted under and the
--- caller, process and build — the generation it was composed from, whether it was a put-back, its
--- digest and the modules it named. Written after the declaration is away, as the node row's record is.
--- A refusal by the machine of a send for its generation is kept on the send it refused, so the
--- condition it raises names the sender from here. The newest 200 per machine are kept.
+-- caller, process and build — the generation it was composed from, its digest and the modules it named.
+-- Written after the declaration is away, with the node row's record of it. A refusal by the machine of a
+-- send for its generation is kept on the send it refused, so the condition it raises names the sender
+-- from here; a refusal of a sequence the mesh has no send for is kept as a row of its own, `recorded`
+-- false, whose sender is said to be unknown. `counter_raised_to` is the generation the mesh's counter was
+-- raised to when the refusal showed the counter behind the machine (a store put back from a backup). The
+-- newest 200 per machine are kept.
 create table declaration_send (
     id           bigserial   primary key,
     node         uuid        not null references node (id) on delete cascade,
@@ -51,13 +58,14 @@ create table declaration_send (
     epoch        bigint,
     sender       text        not null,
     generation   bigint,
-    put_back     boolean     not null default false,
     digest       text        not null,
     modules      text[]      not null default '{}',
     sent_at      timestamptz not null default now(),
     refused_at   timestamptz,
     -- refused_applied is the generation the machine said it had applied when it refused this send.
-    refused_applied bigint
+    refused_applied bigint,
+    counter_raised_to bigint,
+    recorded     boolean     not null default true
 );
 create index declaration_send_node on declaration_send (node, id desc);
 create index declaration_send_sequence on declaration_send (node, sequence);