Add the chain package and its buckets: builds, rollouts and assignments with history 64 (issue 1089)

internal/chain holds typed Build, Rollout and Assignment and a Backend with no delete or purge, in memory and on the bus. The builds and rollouts buckets are asserted with history 64, no age and values up to just under a mebibyte; the assignments bucket gets the same, keeping its marker age because a stream cannot be made to disallow message ages. Migration 0104 makes the one-row chain_moved switch. The controller keeps no consolidated schema, so 0104 is the whole schema change. Nothing reads the buckets yet.

internal/inventory/chain_moved_test.go

@@ -0,0 +1,40 @@
+package inventory
+
+import (
+	"strings"
+	"testing"
+)
+
+// **The move of the chain has a switch, made by the last migration and written by no migration** (ADR 0408 §8, issue
+// 1089): migration 0104 makes the one-row table chain_moved, and does not touch a table of the chain, which a later
+// delivery drops once the move is read back.
+func TestMigration0104MakesTheSwitchOfTheChainsMoveAndNothingElse(t *testing.T) {
+	migrations, err := Migrations()
+	if err != nil {
+		t.Fatal(err)
+	}
+	var found bool
+	for _, m := range migrations {
+		if m.Number != 104 {
+			continue
+		}
+		found = true
+		sql := strings.ToLower(m.SQL)
+		if !strings.Contains(sql, "create table chain_moved") || !strings.Contains(sql, "check (only_one)") {
+			t.Errorf("migration 0104 does not make the one-row chain_moved:\n%s", m.SQL)
+		}
+		for _, line := range strings.Split(sql, "\n") {
+			if strings.HasPrefix(strings.TrimSpace(line), "--") {
+				continue
+			}
+			for _, verb := range []string{"drop ", "insert ", "delete ", "update ", "alter "} {
+				if strings.Contains(line, verb) {
+					t.Errorf("migration 0104 does more than make the switch: %q", line)
+				}
+			}
+		}
+	}
+	if !found {
+		t.Fatal("no migration 0104")
+	}
+}

internal/inventory/migrations/0104-the-chain-moves-to-the-bus.sql

@@ -0,0 +1,16 @@
+-- Builds, rollouts and assignments move to the bus (novox/hq ADR 0408 §8, design 47 milestone 5, issue 1089).
+--
+-- This migration makes the switch alone. The rows of build, build_request, release_plan, batched_merge, assignment and
+-- pending_assignment are written into the chain's buckets by the serving controller at its start, holding the lease,
+-- because a migration runs when the store is opened, before any connection to the bus. This row is the switch: while it
+-- is absent nothing reads the buckets; once the move has counted every row against the keys it wrote, it is written, and
+-- nothing reads the tables. One row at most; never written by a migration, so a delivery alone changes nothing. The
+-- controller keeps no consolidated schema, so this migration is the whole of the change; the tables go by a later one.
+create table chain_moved (
+    only_one boolean     primary key default true check (only_one),
+    moved_at timestamptz not null default now(),
+    -- The counts the move read from the tables and wrote as keys, per kind, and the largest value it wrote: what a
+    -- reader of the move reads back.
+    counts   jsonb       not null,
+    largest  jsonb       not null
+);