Lib-storage Error Handling
==========================

'src/lib-storage/mail-error.h' describes different types of errors and has some
other error-related functions and macros.

Only errors returning "int" can actually return a failure.

 * Functions that return a pointer will never return failure with NULL. Only
   "find" type of functions can return NULL, which means "not found".
 * Iterators usually work by the init() returning iterator pointer and next()
   returning a boolean. If there were any errors in either init() or next(),
   deinit() finally returns a failure.

Getting lib-storage errors
--------------------------

 * 'mailbox_list_*()' functions set their errors to the given mailbox_list
   structure. You can get these errors with 'mailbox_list_get_last_error()'.
 * All other functions that have some way of accessing mail_storage (mailbox,
   mail, transactions, etc.) set their errors to the storage. You can get these
   errors with 'mail_storage_get_last_error()'.
    * Mail user and namespace functions have their own error handling,
      typically by returning error strings as parameters.
 * Both '*_get_last_error()' functions should be called soon after the error is
   noticed, before other failing lib-storage calls overwrite the error.
    * In deinit failures it usually doesn't matter if you get the first or the
      last error, so it's easier to just call all the different deinit
      functions and finally look up what the last failure was.

Setting lib-storage errors
--------------------------

Errors can be set with two calls:

 * 'mail_storage_set_error()' and 'mailbox_list_set_error()' should be used
   when the error is user's fault in some way. For example invalid mailbox
   name, out of quota, etc. The error string will be shown to user. It won't be
   written to a log file.
 * 'mail_storage_set_critical()' and 'mailbox_list_set_critical()' should be
   used when the error is a problem in the system and sysadmin should be
   notified. For example out of disk space or just in general an unexpected
   syscall failure. The error string that will be shown to user is the
   "Internal error occurred", but it will be logged as an error.
 * The reason for the separation of these two is:
    1. Only log errors that sysadmin can do something about.
    2. Never show user anything even potentially sensitive about the system,
       such as path names.

There are also a few other calls that aren't used as often, but can be helpful:

 * 'mail_storage_set_internal_error()' and 'mailbox_list_set_internal_error()'
   simply set the user-visible error message to "Internal error occurred".
   These can be used if the actual error was already logged.
 * 'mail_storage_set_error_from_errno()' and
   'mailbox_list_set_error_from_errno()' set the user-visible error message
   based on some common 'errno' values. Currently:
    * EACCESS, EPERM, EROFS: Permission denied
    * ENOSPC, EDQUOT: Not enough disk space
    * ENOENT, ENOTDIR: Not found
    * ELOOP: Directory structure is broken

(This file was created from the wiki on 2016-03-16 04:44)
