Mailfromd
	General-Purpose Mail Filter	Sergey Poznyakoff

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4.18 Exceptional Conditions

When the running program encounters a condition it is not able to handle, it signals an exception. To illustrate the concept, let's consider the execution of the following code fragment:

  if primitive_hasmx(domainpart($f))
    accept
  fi

The function primitive_hasmx (see primitive_hasmx) tests whether the domain name given as its argument has any ‘MX’ records. It should return a boolean value. However, when querying the Domain Name System, it may fail to get a definite result. For example, the DNS server can be down or temporary unavailable. In other words, primitive_hasmx can be in a situation when, instead of returning ‘yes’ or ‘no’, it has to return ‘don't know’. That is exactly then when it signals an exception.

Exceptions are identified by exception types, the decimal numbers associated with them. The header file ‘status.mfh’ defines symbolic exception names for each of these. The following table summarizes all the exception types implemented by mailfromd version 6.0:

e_dbfailure

General database failure. For example, the database cannot be opened. This exception can be signaled by any function that queries any DBM database.

e_divzero

Division by zero.

e_eof

Function reached end of file while reading. See section I/O functions, for a description of functions that can signal this exception.

e_failure

failure

e_failure

A general failure has occurred. In particular, this exception is signaled by DNS lookup functions when any permanent failure occurs. This exception can be signaled by any DNS-related function (hasmx, poll, etc.) or operation (mx matches).

e_invcidr

Invalid CIDR notation. This is signaled by match_cidr function when its second argument is not a valid CIDR.

e_invip

Invalid IP address. This is signaled by match_cidr function when its first argument is not a valid IP address.

e_invtime

Invalid time interval specification. It is signaled by interval function if its argument is not a valid time interval (see time interval specification).

e_io

An error occurred during the input-output operation. See section I/O functions, for a description of functions that can signal this exception.

e_macroundef

A Sendmail macro is undefined.

e_noresolve

The argument of a DNS-related function cannot be resolved to host name or IP address. Currently only ismx (see ismx) raises this exception.

e_range

The supplied argument is outside the allowed range. This is signalled, for example, by substring function (see substring).

e_regcomp

Regular expression cannot be compiled. This can happen when a regular expression (a right-hand argument of a matches operator) is built at the runtime and the produced string is an invalid regex.

e_ston_conv

String-to-number conversion failed. This can be signaled when a string is used in numeric context which cannot be converted to the numeric data type. For example:

set x "10a" if %x / 2 …

The if condition will signal ston_conv, since ‘10a’ cannot be converted to a number.

e_temp_failure

temp_failure

e_temp_failure

A temporary failure has occurred. This can be signaled by DNS-related functions or operations.

e_url

The supplied URL is invalid. See section Interfaces to Third-Party Programs.

In addition to these, two symbols are defined that are not exception types in the strict sense of the world, but are provided to make writing filter scripts more convenient. These are success, meaning successful return from a function, and not_found, meaning that the required entity (e.g. domain name or email address) was not found. See figure-poll-wrapper, for an illustration on how these can be used. For consistency with other exception codes, these can be spelled as e_success and e_not_found.

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 section Logging and Debugging). However, the user can intercept any exception by installing his own exception-handling routines.

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

catch exception-list
do
  handler-body
done

where exception-list is the list of exception types, separated by the word or. A special form ‘*’, which stands for all exceptions, is allowed as well. The handler-body is the list of statements comprising the handler body. For example, the code below installs a handler for exceptions 2 (‘e_failure’) and 3 (‘e_temp_failure’):

catch 2 or 3
do
  …
done

However, using numeric exception codes is not convenient and is not in fact recommended. We recommend to use exception symbolic names instead. These are declared in the header file ‘status.mfh’, which should be included in your startup script. Using this approach, the above code snippet will look like:

#include <status.mfh>

catch e_failure or e_temp_failure
do
  …
done

The 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 catch. Each exception redefined by a catch remains in force until either another catch appears further in the same function or handler that redefines it or an end of the containing block is encountered, whichever occurs first. Upon exit from a function or milter handler, all exceptions are restored to the state they had when it has been entered.

Upon entry to a catch code, it receives two positional arguments, which can be referenced in handler-body as $1 and $2. 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. A 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 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.

The following example shows an envfrom handler that catches eventual exceptions, signaled by primitive_hasmx. The handler accepts the message if the sender domain has any ‘MX’ records, rejects them if it does not, and continues execution if an exception occurs. In the latter case, a log message is also issued:

#include <status.mfh>

prog envfrom
do
  catch e_failure or e_temp_failure
  do
    echo "Caught exception $1: $2"
    continue
  done
  
  if primitive_hasmx(domainpart($f))
    accept
  else
    reject
  fi
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 (21).

func hasmx (s) returns n
do
  catch *
  do
    return 0
  done
  return primitive_hasmx(domainpart($1))
done

All variables remain visible within catch body, with the exception of positional arguments of the enclosing function or handler. So, if you wish to access function arguments from a catch, define your function using new style (see section User-Defined Functions) declaration. To access positional arguments of a handler, you will have to assign them to local variables prior to installing a catch, e.g.:

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

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

throw excode descr

The arguments correspond exactly to the positional arguments 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 several limitations of throw: first, the excode argument must be an immediate value, you cannot use an expression in its place. Secondly, its range is limited to exception codes, described in status.mfh. In other words, you cannot (yet) invent your own exception types.

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