use new function for tracking logs

The new function, called `_send_email_with_mid`, aligns with suggestions
from @polarethene and is heavily simplified compared to its predecessor
`_send_email_and_get_id`. New helpers will be introduced to filter logs
according to the MID constructed in this function.

test/helper/sending.bash

@@ -4,6 +4,8 @@
 
 # ! ATTENTION: This file is loaded by `common.sh` - do not load it yourself!
 # ! ATTENTION: This file requires helper functions from `common.sh`!
+# ! ATTENTION: Functions prefixed with `__` are intended for internal use within
+# !            this file (or other helpers) only, not in tests.
 
 # shellcheck disable=SC2034,SC2155
 
@@ -100,67 +102,22 @@ function _send_email() {
   return "${RETURN_VALUE}"
 }
 
-# Like `_send_email` with two major differences:
+# Contruct the message ID for the 'Message-ID' header.
 #
-# 1. this function waits for the mail to be processed; there is no asynchronicity
+# @param ${1} = message ID part before '@' (later used when filtering logs again)
-#    because filtering the logs in a synchronous way is easier and safer;
+function __construct_mid() {
-# 2. this function takes the name of a variable and inserts IDs one can later
+  echo "<${1:?Message-ID prefix is required}@dms-tests>"
-#    filter by to check logs.
+}
+
+# Like `_send_email` but adds a "Message-Id: ${1}@dms-tests>" header, which
+# allows for filtering logs later.
 #
-# No. 2 is especially useful in case you send more than one email in a single
+# @param ${1} = message ID part before '@' (later used when filtering logs again)
-# test file and need to assert certain log entries for each mail individually.
+function _send_email_with_mid() {
-#
+  local MID=$(__construct_mid "${1:?Left-hand side of MID missing}")
-# The first argument has to be the name of the variable that the e-mail ID is stored in.
-# The second argument **can** be the flag `--expect-rejection`.
-#
-# - If this flag is supplied, the function does not check whether the whole mail delivery
-#   transaction was successful. Additionally the queue ID will be retrieved differently.
-# - CAUTION: It must still be possible to `grep` for the Message-ID that Postfix
-#   generated in the mail log; otherwise this function fails.
-#
-# The rest of the arguments are the same as `_send_email`.
-#
-# ## Attention
-#
-# This function assumes `CONTAINER_NAME` to be properly set (to the container
-# name the command should be executed in)!
-#
-# ## Safety
-#
-# This functions assumes **no concurrent sending of emails to the same container**!
-# If two clients send simultaneously, there is no guarantee the correct ID is
-# chosen. Sending more than one mail at any given point in time with this function
-# is UNDEFINED BEHAVIOR!
-function _send_email_and_get_id() {
-  # Export the variable denoted by ${1} so everyone has access
-  export "${1:?Mail ID must be set for _send_email_and_get_id}"
-  # Get a "reference" to the content of the variable denoted by ${1} so we can manipulate the content
-  local -n ID_ENV_VAR_REF=${1:?}
-  # Prepare the message ID header here because we will shift away ${1} later
-  local MID="<${1}@dms-tests>"
-  # Get rid of ${1} so only the arguments for swaks remain
   shift 1
 
-  # The unique ID Postfix (and other services) use may be different in length
-  # on different systems. Hence, we use a range to safely capture it.
-  local QUEUE_ID_REGEX='[A-Z0-9]{9,12}'
-
-  _wait_for_empty_mail_queue_in_container
   _send_email "${@}" --header "Message-Id: ${MID}"
-  _wait_for_empty_mail_queue_in_container
-
-  # We store Postfix's queue ID first
-  ID_ENV_VAR_REF=$(_exec_in_container tac /var/log/mail.log                    \
-    | grep -E "postfix/cleanup.*: ${QUEUE_ID_REGEX}:.*message-id=${MID}" \
-    | grep -E --only-matching --max-count 1 "${QUEUE_ID_REGEX}" || :)
-  # But we also requre potential Dovecot sieve output, which requires the mesage ID,
-  # so we need to provide the message ID too.
-  ID_ENV_VAR_REF+="|${MID}"
-
-  # Last but not least, we perform plausibility checks on the IDs.
-  assert_not_equal "${ID_ENV_VAR_REF}" ''
-  run echo "${ID_ENV_VAR_REF}"
-  assert_line --regexp "^${QUEUE_ID_REGEX}\|${MID}$"
 }
 
 # Send a spam e-mail by utilizing GTUBE.