Create new databases with auto_vacuum incremental (closes #43)

SQLite only accepts auto_vacuum before the database file is first
written. The connection switched to WAL first, which writes the file,
so the PRAGMA auto_vacuum in Initialize came too late and was ignored:
new databases had auto_vacuum off and the periodic incremental vacuum
freed nothing. The setting now goes in the connection string, which the
driver applies on open before the journal mode, and the late PRAGMA is
removed. Tests check that every pooled connection sees auto_vacuum
incremental on a new database and that Vacuum reduces the free page
count after routes are deleted.

Model: opus-5-5

TODO.md

@@ -28,6 +28,11 @@ The other open issue is https://git.eeqj.de/sneak/routewatch/issues/30.
 
 # Completed Steps
 
+- 2026-10-03: a new database is created with `auto_vacuum` set to
+  incremental, through the connection string so it is set before the file
+  is first written, and the periodic incremental vacuum now returns free
+  pages; the late `PRAGMA auto_vacuum` in `Initialize`, which SQLite
+  ignored, is gone (closes #43)
 - 2026-10-02: a plain `docker build .` stamps the commit's tag or short
   commit (`git describe --tags --always`) into the page footer instead of
   `unknown`: `.dockerignore` sends `.git` without `.git/config`, a `VERSION`

internal/database/database.go

@@ -94,8 +94,13 @@ func New(cfg *config.Config, logger *logger.Logger) (*Database, error) {
 	// a transaction that reads before writing starts as a reader and, when it
 	// then writes while another connection holds the write lock, fails at once
 	// with "database is locked" without waiting for _busy_timeout.
+	// _auto_vacuum=incremental lets Vacuum return free pages to the filesystem.
+	// SQLite only accepts it before the database file is first written, and the
+	// switch to WAL writes it, so it must be here: the driver applies it on open,
+	// before _journal_mode. On an existing file it changes nothing.
 	dsn := fmt.Sprintf(
-		"file:%s?_cache_size=%d&_synchronous=OFF&_busy_timeout=%d&_journal_mode=WAL&_txlock=immediate",
+		"file:%s?_cache_size=%d&_synchronous=OFF&_busy_timeout=%d"+
+			"&_auto_vacuum=incremental&_journal_mode=WAL&_txlock=immediate",
 		dbPath,
 		sqliteCacheSizeKiB,
 		sqliteBusyTimeoutMs,
@@ -140,9 +145,8 @@ func (d *Database) Initialize() error {
 	// heap. The heap limits below are process-wide, so setting them once here is
 	// enough for the whole pool.
 	pragmas := []string{
-		"PRAGMA journal_mode=WAL",        // Write-Ahead Logging
-		"PRAGMA analysis_limit=0",        // Disable automatic ANALYZE
-		"PRAGMA auto_vacuum=INCREMENTAL", // Enable incremental vacuum
+		"PRAGMA journal_mode=WAL", // Write-Ahead Logging
+		"PRAGMA analysis_limit=0", // Disable automatic ANALYZE
 		fmt.Sprintf("PRAGMA soft_heap_limit=%d", sqliteSoftHeapLimitBytes),
 		fmt.Sprintf("PRAGMA hard_heap_limit=%d", sqliteHardHeapLimitBytes),
 	}

internal/database/database_test.go

@@ -3,6 +3,7 @@ package database
 import (
 	"context"
 	"database/sql"
+	"fmt"
 	"net"
 	"sync"
 	"testing"
@@ -16,6 +17,13 @@ import (
 // memory"; the DSN change must leave temp_store below this so they spill to disk.
 const tempStoreMemory = 2
 
+// autoVacuumIncremental is the PRAGMA auto_vacuum value meaning "incremental".
+const autoVacuumIncremental = 2
+
+// vacuumTestRoutes is how many routes the vacuum test writes and then deletes,
+// enough to leave many free pages in the file.
+const vacuumTestRoutes = 2000
+
 // heldConnections is how many pooled connections the pragma test holds open at
 // once so each is a distinct SQLite connection that parsed the DSN.
 const heldConnections = 5
@@ -311,7 +319,8 @@ func TestIPv4RangeIntegration(t *testing.T) {
 
 // TestConnectionPoolPragmas holds several pooled connections open at once and
 // checks each one carries the per-connection settings from the DSN, plus the
-// process-wide hard heap limit.
+// process-wide hard heap limit, and sees the new file with auto_vacuum
+// incremental.
 func TestConnectionPoolPragmas(t *testing.T) {
 	cfg := &config.Config{StateDir: t.TempDir()}
 
@@ -372,6 +381,72 @@ func TestConnectionPoolPragmas(t *testing.T) {
 		if hardHeapLimit != sqliteHardHeapLimitBytes {
 			t.Errorf("conn %d: hard_heap_limit = %d, want %d", i, hardHeapLimit, sqliteHardHeapLimitBytes)
 		}
+
+		var autoVacuum int
+		if err := c.QueryRowContext(ctx, "PRAGMA auto_vacuum").Scan(&autoVacuum); err != nil {
+			t.Fatalf("conn %d: failed to read auto_vacuum: %v", i, err)
+		}
+		if autoVacuum != autoVacuumIncremental {
+			t.Errorf("conn %d: auto_vacuum = %d, want %d (incremental)", i, autoVacuum, autoVacuumIncremental)
+		}
+	}
+}
+
+// TestVacuumReturnsFreePages checks that after routes are deleted from a new
+// database, Vacuum returns the pages they used. With auto_vacuum off (issue
+// https://git.eeqj.de/sneak/routewatch/issues/43) the free pages stayed in the
+// file and Vacuum left the count unchanged.
+func TestVacuumReturnsFreePages(t *testing.T) {
+	cfg := &config.Config{StateDir: t.TempDir()}
+
+	db, err := New(cfg, logger.New())
+	if err != nil {
+		t.Fatalf("failed to create database: %v", err)
+	}
+	defer func() { _ = db.Close() }()
+
+	ctx := context.Background()
+	ts := time.Now().UTC()
+	const asn = 64500
+
+	routes := make([]*LiveRoute, 0, vacuumTestRoutes)
+	deletions := make([]LiveRouteDeletion, 0, vacuumTestRoutes)
+	for i := range vacuumTestRoutes {
+		route := mkV4Route(t, fmt.Sprintf("10.%d.%d.0/24", i/256, i%256), asn, ts)
+		routes = append(routes, route)
+		deletions = append(deletions, LiveRouteDeletion{
+			Prefix:    route.Prefix,
+			OriginASN: asn,
+			PeerIP:    route.PeerIP,
+			IPVersion: ipVersionV4,
+		})
+	}
+
+	if err := db.UpsertLiveRouteBatch(routes); err != nil {
+		t.Fatalf("failed to write routes: %v", err)
+	}
+	if err := db.DeleteLiveRouteBatch(deletions); err != nil {
+		t.Fatalf("failed to delete routes: %v", err)
+	}
+
+	var before int
+	if err := db.db.QueryRowContext(ctx, "PRAGMA freelist_count").Scan(&before); err != nil {
+		t.Fatalf("failed to read freelist_count: %v", err)
+	}
+	if before == 0 {
+		t.Fatalf("no free pages after deleting %d routes", vacuumTestRoutes)
+	}
+
+	if err := db.Vacuum(ctx); err != nil {
+		t.Fatalf("Vacuum failed: %v", err)
+	}
+
+	var after int
+	if err := db.db.QueryRowContext(ctx, "PRAGMA freelist_count").Scan(&after); err != nil {
+		t.Fatalf("failed to read freelist_count: %v", err)
+	}
+	if after >= before {
+		t.Errorf("free pages after Vacuum = %d, want fewer than %d", after, before)
 	}
 }