Fix packet counter accuracy after TRUNCATE operations

The packet counter uses a PostgreSQL sequence that doesn't get reset
when the packets table is truncated, causing inaccurate counts.

This migration:
- Creates an event trigger that automatically resets the packet counter
  sequence when the packets table is truncated
- Adds a manual reset function reset_packet_counter() that can be called
  if needed
- Improves the get_packet_count() function to properly handle when the
  sequence is at 0

After applying this migration, the counter will automatically reset to 0
whenever TRUNCATE is used on the packets table.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

priv/repo/migrations/20250719202358_add_truncate_reset_for_packet_counter.exs

@@ -0,0 +1,113 @@
+defmodule Aprsme.Repo.Migrations.AddTruncateResetForPacketCounter do
+  use Ecto.Migration
+
+  def up do
+    # Create a function to reset the packet counter sequence
+    execute """
+    CREATE OR REPLACE FUNCTION reset_packet_counter_on_truncate()
+    RETURNS event_trigger AS $$
+    DECLARE
+      cmd RECORD;
+    BEGIN
+      FOR cmd IN SELECT * FROM pg_event_trigger_ddl_commands()
+      LOOP
+        IF cmd.command_tag = 'TRUNCATE TABLE' AND cmd.object_identity = 'public.packets' THEN
+          -- Reset the sequence to 0
+          PERFORM setval('packet_count_seq', 0, false);
+          
+          -- Also update the deprecated packet_counters table for backward compatibility
+          UPDATE packet_counters 
+          SET count = 0, updated_at = NOW()
+          WHERE counter_type = 'total_packets';
+        END IF;
+      END LOOP;
+    END;
+    $$ LANGUAGE plpgsql;
+    """
+
+    # Create an event trigger for TRUNCATE operations
+    execute """
+    CREATE EVENT TRIGGER reset_packet_counter_trigger
+    ON ddl_command_end
+    WHEN TAG IN ('TRUNCATE TABLE')
+    EXECUTE FUNCTION reset_packet_counter_on_truncate();
+    """
+
+    # Also create a manual reset function that can be called if needed
+    execute """
+    CREATE OR REPLACE FUNCTION reset_packet_counter()
+    RETURNS void AS $$
+    BEGIN
+      -- Reset the sequence
+      PERFORM setval('packet_count_seq', 0, false);
+      
+      -- Update the deprecated table
+      UPDATE packet_counters 
+      SET count = 0, updated_at = NOW()
+      WHERE counter_type = 'total_packets';
+    END;
+    $$ LANGUAGE plpgsql;
+    """
+
+    # Fix the get_packet_count function to handle the case when sequence is at 0
+    execute """
+    CREATE OR REPLACE FUNCTION get_packet_count()
+    RETURNS BIGINT AS $$
+    DECLARE
+      seq_exists BOOLEAN;
+      current_val BIGINT;
+    BEGIN
+      -- Check if sequence exists
+      SELECT EXISTS (
+        SELECT 1 FROM pg_sequences 
+        WHERE schemaname = 'public' AND sequencename = 'packet_count_seq'
+      ) INTO seq_exists;
+      
+      IF NOT seq_exists THEN
+        -- Fallback to counting if sequence doesn't exist
+        RETURN (SELECT COUNT(*) FROM packets);
+      END IF;
+      
+      -- Try to get current value
+      BEGIN
+        current_val := currval('packet_count_seq');
+      EXCEPTION
+        WHEN object_not_in_prerequisite_state THEN
+          -- If currval hasn't been called yet, get last_value
+          SELECT last_value INTO current_val FROM packet_count_seq;
+      END;
+      
+      -- If sequence is at 0 and hasn't been used (is_called = false), return 0
+      IF current_val = 0 THEN
+        RETURN 0;
+      ELSE
+        RETURN current_val;
+      END IF;
+    END;
+    $$ LANGUAGE plpgsql STABLE;
+    """
+  end
+
+  def down do
+    # Drop the event trigger
+    execute "DROP EVENT TRIGGER IF EXISTS reset_packet_counter_trigger;"
+
+    # Drop the reset functions
+    execute "DROP FUNCTION IF EXISTS reset_packet_counter_on_truncate();"
+    execute "DROP FUNCTION IF EXISTS reset_packet_counter();"
+
+    # Restore the original get_packet_count function
+    execute """
+    CREATE OR REPLACE FUNCTION get_packet_count()
+    RETURNS BIGINT AS $$
+    BEGIN
+      RETURN currval('packet_count_seq');
+    EXCEPTION
+      WHEN object_not_in_prerequisite_state THEN
+        -- If currval hasn't been called yet in this session, get the last value
+        RETURN last_value FROM packet_count_seq;
+    END;
+    $$ LANGUAGE plpgsql STABLE;
+    """
+  end
+end