Allow source counts to be cached: implement ::doCount() instead of ::count()

I cannot reproduce those failures. Re-testing against 9.1 in case that makes a difference.

Ah, right, this requires #3190815: Source count caching broken: impossible to enable source count caching for SqlBase-based source plugins (plus, unneeded cache I/O) 😬

@DamienMcKenna: wow that was fast! 😄

+++ b/src/Plugin/migrate/source/d7/MetatagFieldInstance.php
@@ -137,7 +137,7 @@ class MetatagFieldInstance extends DrupalSqlBase {
-  public function count($refresh = FALSE) {
+  public function doCount() {

Nit: should be protected.

#3190815: Source count caching broken: impossible to enable source count caching for SqlBase-based source plugins (plus, unneeded cache I/O) landed! 🥳

+++ b/src/Plugin/migrate/source/d7/MetatagFieldInstance.php
@@ -141,4 +141,11 @@ public function count($refresh = FALSE) {
+  public function doCount() {

Should still be changed to protected 😅

FYI: the two failures in #12 on PHP 8 + Drupal 9.2 are the same two that exist in HEAD: https://www.drupal.org/pift-ci-job/2057145.

Also, the change in #10 + #12 compared to #12 breaks the performance improvement that #3190815: Source count caching broken: impossible to enable source count caching for SqlBase-based source plugins (plus, unneeded cache I/O) made possible.

If you want to support both simultaneously, you'll need to make count() smarter: you'll need to update its logic to be conditional depending on the Drupal core version…

With #3190818: Allow source counts to be cached: implement ::doCount() instead of ::count() also committed … and with time having passed … I'm wondering if you'd be willing to commit this without BC layer for Drupal 8.9, @DamienMcKenna?