How can I retrieve all context variables of a Firebird database connection

Firebird has a number of context variables which are accessible with the RDB$GET_CONTEXT function, and - for user contexts - can be set with RDB$SET_CONTEXT.

The user context variables can be found in the monitoring table MON$CONTEXT_VARIABLES (administrators can see them for all connections), but variables of the SYSTEM context are absent in this table. Is there a way to list all context variables, including the variables from the SYSTEM context?

Right now there is no way to query all SYSTEM context variables, nor a way to discover which SYSTEM context variable names are valid for a Firebird version. I have filed an improvement request to add something for this, https://github.com/FirebirdSQL/firebird/issues/7623.

As a workaround, you can use a list of known SYSTEM context variable names and check all of them (skipping the ones which don't exist for a version). The code below is an EXECUTE BLOCK, but this can easily be modified to a selectable stored procedure. The code should work on Firebird 2.5 and higher.

execute block
  returns (
    CONTEXT_NAME varchar(20) character set ASCII, 
    CONTEXT_VAR_NAME varchar(80) character set NONE, 
    CONTEXT_VAR_VALUE varchar(32765) character set NONE)
as
  declare CONTEXT_VAR_NAMES varchar(500) character set ASCII;
  declare CONTEXT_VAR_NAMES_LENGTH integer;
  declare SEPARATOR_POS integer = 0;
  declare NAME_START integer = 1;
begin
  CONTEXT_NAME = 'SYSTEM';
  -- SYSTEM context names as of Firebird 5.0.0 beta-2 snapshot
  CONTEXT_VAR_NAMES = 'CLIENT_ADDRESS,CLIENT_HOST,CLIENT_PID,CLIENT_PROCESS,'
    || 'CURRENT_ROLE,CURRENT_USER,DB_FILE_ID,DB_GUID,DB_NAME,EFFECTIVE_USER,' 
    || 'ENGINE_VERSION,EXT_CONN_POOL_ACTIVE_COUNT,EXT_CONN_POOL_IDLE_COUNT,' 
    || 'EXT_CONN_POOL_LIFETIME,EXT_CONN_POOL_SIZE,GLOBAL_CN,ISOLATION_LEVEL,' 
    || 'LOCK_TIMEOUT,NETWORK_PROTOCOL,READ_ONLY,REPLICA_MODE,' 
    || 'REPLICATION_SEQUENCE,PARALLEL_WORKERS,SESSION_ID,SESSION_IDLE_TIMEOUT,' 
    || 'SESSION_TIMEZONE,SNAPSHOT_NUMBER,STATEMENT_TIMEOUT,TRANSACTION_ID,' 
    || 'WIRE_COMPRESSED,WIRE_CRYPT_PLUGIN,WIRE_ENCRYPTED';
  CONTEXT_VAR_NAMES_LENGTH = char_length(CONTEXT_VAR_NAMES);
  -- Extract individual context var names
  while (SEPARATOR_POS < CONTEXT_VAR_NAMES_LENGTH) do
  begin
	-- find next comma in list
  	SEPARATOR_POS = position(',', CONTEXT_VAR_NAMES, NAME_START);
    if (SEPARATOR_POS = 0) then
    begin
	  -- comma not found, position after end of string
      SEPARATOR_POS = CONTEXT_VAR_NAMES_LENGTH + 1;
    end
    CONTEXT_VAR_NAME = substring(CONTEXT_VAR_NAMES from NAME_START for SEPARATOR_POS - NAME_START);
    -- Position at first character of next context var name (or beyond end of string)
    NAME_START = SEPARATOR_POS + 1;
    CONTEXT_VAR_VALUE = rdb$get_context('SYSTEM', CONTEXT_VAR_NAME);
    suspend;
   
    -- when gdscode ctx_var_not_found do continue; /* continue doesn't work on 2.5 */
    when gdscode ctx_var_not_found do
    begin
    end
  end
  -- Retrieve user context variables for current connection and transaction
  for select 
        case when MON$TRANSACTION_ID is null then 'USER_SESSION' else 'USER_TRANSACTION' end, 
        MON$VARIABLE_NAME, 
        MON$VARIABLE_VALUE 
      from MON$CONTEXT_VARIABLES 
      where MON$ATTACHMENT_ID = current_connection 
      and (MON$TRANSACTION_ID is null or MON$TRANSACTION_ID = current_transaction)
      order by MON$TRANSACTION_ID nulls first, MON$VARIABLE_NAME
      into CONTEXT_NAME, CONTEXT_VAR_NAME, CONTEXT_VAR_VALUE do
   begin
     suspend;
   end
end