fix: populate fsid for legacy NFS exports on migration

Migration 0002 added fsid column with DEFAULT 0 but never
populated existing rows. Now PopulateLegacyFSIDs() runs after 0002
to extract fsid from legacy options string or generate random.

Also: move generateRandomFSID to internal/system to avoid import
cycles. Migration 0004 drops the legacy options column.

Version: 0.3.1 -> 0.3.2

internal/db/queries_nfs.go

@@ -3,6 +3,10 @@ package db
 import (
 	"database/sql"
 	"fmt"
+	"regexp"
+	"strconv"
+
+	"github.com/darroyo/nasctl/internal/system"
 )
 
 func scanNFSExport(row interface {
@@ -199,3 +203,55 @@ func (d *DB) FSIDExists(fsid int64) (bool, error) {
 	}
 	return count > 0, nil
 }
+
+var fsidLegacyPattern = regexp.MustCompile(`fsid=(\d+)`)
+
+func (d *DB) PopulateLegacyFSIDs() error {
+	rows, err := d.conn.Query(`SELECT id, fsid, options FROM nfs_exports WHERE fsid = 0`)
+	if err != nil {
+		return fmt.Errorf("query legacy fsids: %w", err)
+	}
+	defer rows.Close()
+
+	type pending struct {
+		id   int64
+		fsid int64
+	}
+	var updates []pending
+
+	for rows.Next() {
+		var id, fsid int64
+		var options string
+		if err := rows.Scan(&id, &fsid, &options); err != nil {
+			return fmt.Errorf("scan row: %w", err)
+		}
+		newFSID := int64(0)
+
+		if m := fsidLegacyPattern.FindStringSubmatch(options); m != nil {
+			if n, err := strconv.ParseInt(m[1], 10, 64); err == nil && n > 0 {
+				newFSID = n
+			}
+		}
+
+		if newFSID == 0 {
+			n, err := system.GenerateRandomFSID()
+			if err != nil {
+				return fmt.Errorf("generate fsid: %w", err)
+			}
+			newFSID = int64(n)
+		}
+
+		updates = append(updates, pending{id, newFSID})
+	}
+	if err := rows.Err(); err != nil {
+		return fmt.Errorf("rows iteration: %w", err)
+	}
+
+	for _, u := range updates {
+		if _, err := d.conn.Exec(`UPDATE nfs_exports SET fsid = ? WHERE id = ?`, u.fsid, u.id); err != nil {
+			return fmt.Errorf("update fsid for id %d: %w", u.id, err)
+		}
+	}
+	return nil
+}
+