drupal_mail() should support using different MailSystemInterface classes for format() and mail()

+++ b/includes/mail.inc
@@ -220,10 +234,13 @@ function drupal_mail($module, $key, $to, $language, $params = array(), $from = N
+function drupal_mail_system($module, $key, $method='format') {

@@ -232,15 +249,33 @@ function drupal_mail_system($module, $key) {
-  if (isset($configuration[$id])) {
+  if ( isset($configuration[$id])
+    && ( is_string($configuration[$id])
+      || ( is_array($configuration[$id])
+        && isset($configuration[$id][$method])
+      )
+    )
+  ) {
     $class = $configuration[$id];
   }
-  elseif (isset($configuration[$module])) {
+  elseif (isset($configuration[$module])
+    && ( is_string($configuration[$module])
+      || ( is_array($configuration[$module])
+        && isset($configuration[$module][$method])
+      )
+    )
+  ) {
     $class = $configuration[$module];

Various coding style issues. See http://drupal.org/coding-standards

+++ b/includes/mail.inc
@@ -232,15 +249,33 @@ function drupal_mail_system($module, $key) {
+  if (is_array($class)) {
+    if (empty($class[$method])) {
+      throw new Exception(t('The %mail_system variable does not specify a %interface class to use for the %method method.', array('%mail_system' => 'mail_system', '%interface' => 'MailSystemInterface', '%method' => $method)));
+    }
+    $class = $class[$method];
+  }

It looks like this patch tries to retain support for strings to some extent, but then again not.

For D8, if this patch is going to be considered, a decision needs to be made as to whether both data types are allowed, or whether all variables should be arrays for consistency.

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1) a) Missing spaces in function argument default value. b) Superfluous white-space in control structure conditions. c) The conditions should not be wrapped.

2) Whether this API change can be backported to D7 is a question that has to be deferred. For now, only a proper implementation for D8 matters. I'd like to hear feedback from others, but it likely means that we want to have consistent system variable values; i.e., all being arrays, no support for string values. Otherwise, modules that are trying to adjust these variables (and potentially variable values of other modules) would have to care for the different data types on their own all over the place. Ironically, this very Mail API had a very similar weirdness that allowed modules to set the mail body to a string or to an array, which was a major pain.

#7: drupal_mail_system-1135262-7.patch queued for re-testing.

Is this issue still relevant? If so, I'd be happy to re-roll the patch in #7.

next step, verify the issue still exists, write/update steps to reproduce

beginning to manually test

it appears that the patch in #7 refers to the mail.inc file in the wrong location
the patch points to includes/mail.inc
but the current d8 has the file in core/includes/mail.inc

the coding standards also may be debated and we should review to make sure they are up to standard.

I took the patch provided by @pillarsdotnet and applied it to the current mail.inc.
I copied and pasted the code exactly without any changes of my own.

The only unclear change is with the reference to 'MailSystemInterface' vs 'Drupal\Core\Mail\MailInterface'.
It is referenced in the examples and the comments.
I chose to use the MailSystemInterface as posted by @pillarsdotnet.

I think I should have changed the status.
changing status to needs review.

Removing 'needs reroll' tag.

+++ b/core/includes/mail.inc
@@ -272,15 +289,44 @@ function drupal_mail_system($module, $key) {
+  do {
...
+++ b/core/includes/mail.inc
@@ -272,15 +289,44 @@ function drupal_mail_system($module, $key) {
+  } while (FALSE);

I'm betting this entire control structure will need to be redone.

I also don't think, at this late stage, it will make it into 8.x.

See also #1889644: Convert drupal_mail_system() to a MailFactory service to allow more flexible replacements, which does not directly resolve this but makes it much easier and less ugly to support this in mailsystem.module.

Ought to be possible with a bc layer.

Thank you for sharing your idea for improving Drupal.

We are working to decide if this proposal meets the Criteria for evaluating proposed changes. There hasn't been any discussion here for over 8 years which suggests that this has either been implemented or there is no community support. Your thoughts on this will allow a decision to be made.

Since we need more information to move forward with this issue, the status is now Postponed (maintainer needs more info). If we don't receive additional information to help with the issue, it may be closed after three months.

Thanks!

Lets close this, mailsystem supports this and the core system will be replaced with something that looks very different eventually.