fix(nfs): add migration 0006 to repair fsid=0 exports

Migration 0005 (MigrateNFSClients) only runs once when first applied,
so any fsid=0 rows present before 0005 was applied never get repaired.

Migration 0006 triggers FixZeroFSIDs() which:
- Queries all nfs_exports rows with fsid=0
- Generates a new random fsid per row
- Updates the row

Version: 0.5.2

internal/db/db.go

@@ -86,6 +86,11 @@ func (d *DB) Migrate() error {
 				return fmt.Errorf("migrate nfs clients: %w", err)
 			}
 		}
+		if name == "0006_nfs_fix_zero_fsid.sql" {
+			if err := d.FixZeroFSIDs(); err != nil {
+				return fmt.Errorf("fix zero fsids: %w", err)
+			}
+		}
 	}
 	return nil
 }
@@ -227,3 +232,38 @@ func (d *DB) MigrateNFSClients() error {
 	}
 	return nil
 }
+
+func (d *DB) FixZeroFSIDs() error {
+	rows, err := d.conn.Query(`SELECT id, fsid FROM nfs_exports WHERE fsid = 0`)
+	if err != nil {
+		return fmt.Errorf("query zero fsid exports: %w", err)
+	}
+	defer rows.Close()
+
+	type fsidRow struct {
+		ID   int64
+		FSID int64
+	}
+	var toFix []fsidRow
+	for rows.Next() {
+		var r fsidRow
+		if err := rows.Scan(&r.ID, &r.FSID); err != nil {
+			return fmt.Errorf("scan fsid row: %w", err)
+		}
+		toFix = append(toFix, r)
+	}
+	if err := rows.Err(); err != nil {
+		return fmt.Errorf("rows iteration: %w", err)
+	}
+
+	for _, r := range toFix {
+		n, err := system.GenerateRandomFSID()
+		if err != nil {
+			return fmt.Errorf("generate fsid for id %d: %w", r.ID, err)
+		}
+		if _, err := d.conn.Exec(`UPDATE nfs_exports SET fsid = ? WHERE id = ?`, int64(n), r.ID); err != nil {
+			return fmt.Errorf("update fsid for id %d: %w", r.ID, err)
+		}
+	}
+	return nil
+}

internal/db/migrations/0006_nfs_fix_zero_fsid.sql

@@ -0,0 +1,3 @@
+-- Repair NFS exports with fsid=0 (exportfs requires a non-zero fsid).
+-- Schema change: none.
+-- Data transformation is done by the Go hook FixZeroFSIDs in db.go.