4.22.3 Exception Handling

Normally when an exception is signalled, the program execution is terminated and the MTA is returned a tempfail status. Additional information regarding the exception is then output to the logging channel (see Logging and Debugging). However, the user can intercept any exception by installing his own exception-handling routines.

An exception-handling routine is introduced by a try–catch statement, which has the following syntax:

try
do
  stmtlist
done
catch exception-list
do
  handler-body
done

where stmtlist and handler-body are sequences of MFL statements and exception-list is the list of exception types, separated by the word or. A special exception-list ‘*’ is allowed and means all exceptions.

This construct works as follows. First, the statements from stmtlist are executed. If the execution finishes successfully, control is passed to the first statement after the ‘catch’ block. Otherwise, if an exception is signalled and this exception is listed in exception-list, the execution is passed to the handler-body. If the exception is not listed in exception-list, it is handled as usual.

The following example shows a ‘try--catch’ construct used for handling eventual exceptions, signalled by primitive_hasmx.

try
do
  if primitive_hasmx(domainpart($f))
    accept
  else
    reject
  fi
done
catch e_failure or e_temp_failure
do
  echo "primitive_hasmx failed"
  continue
done

The ‘try--catch’ statement can appear anywhere inside a function or a handler, but it cannot appear outside of them. It can also be nested within another ‘try--catch’, in either of its parts. Upon exit from a function or milter handler, all exceptions are restored to the state they had when it has been entered.

A catch block can also be used alone, without preceding try part. Such a construct is called a standalone catch. It is mostly useful for setting global exception handlers in a begin statement (see begin/end). When used within a usual function or handler, the exception handlers set by a standalone catch remain in force until either another standalone catch appears further in the same function or handler, or an end of the function is encountered, whichever occurs first.

A standalone catch defined within a function must return from it by executing return statement. If it does not do that explicitly, the default value of 1 is returned. A standalone catch defined within a milter handler must end execution with any of the following actions: accept, continue, discard, reject, tempfail. By default, continue is used.

It is not recommended to mix ‘try--catch’ constructs and standalone catches. If a standalone catch appears within a ‘try--catch’ statement, its scope of visibility is undefined.

Upon entry to a handler-body, two implicit positional arguments are defined, which can be referenced in handler-body as $1 and $2¹⁷. The first argument gives the numeric code of the exception that has occurred. The second argument is a textual string containing a human-readable description of the exception.

The following is an improved version of the previous example, which uses these parameters to supply more information about the failure:

try
do
  if primitive_hasmx(domainpart($f))
    accept
  else
    reject
  fi
done
catch e_failure or e_temp_failure
do
  echo "Caught exception $1: $2"
  continue
done

The following example defines the function hasmx that returns true if the domain part of its argument has any ‘MX’ records, and false if it does not or if an exception occurs ¹⁸.

func hasmx (string s)
  returns number
do
  try
  do
    return primitive_hasmx(domainpart(s))
  done
  catch *
  do
    return 0
  done
done

The same function can written using standalone catch:

func hasmx (string s)
  returns number
do
  catch *
  do
    return 0
  done
  return primitive_hasmx(domainpart(s))
done

All variables remain visible within catch body, with the exception of positional arguments of the enclosing handler. To access positional arguments of a handler from the catch body, assign them to local variables prior to the ‘try--catch’ construct, e.g.:

prog header
do
  string hname $1
  string hvalue $2
  try
  do
    …
  done
  catch *
  do
    echo "Exception $1 while processing header %hname: %hvalue"
    echo $2
    tempfail
  done

You can also generate (or raise) exceptions explicitly in the code, using throw statement:

throw excode descr

The arguments correspond exactly to the positional parameters of the catch statement: excode gives the numeric code of the exception, descr gives its textual description. This statement can be used in complex scripts to create non-local exits from deeply nested statements.

Notice, that the the excode argument must be an immediate value: an exception identifier (either a built-in one or one declared previously using a dclex statement).