errors in migration process configuration don't give a clear message

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Yes, this would be a nice addition and catch errors early. I have added the code in the IS to Migration.php and added a unit test.

So, there is an existing Kernel test for this, let's just build on that instead of adding a new test.

Looks good to me - and works beautifully.
I've marked as RTBC, however, I think there is a chance, that

        if (!is_array($configurations) && !$this->processPlugins[$index][$property]) {
          throw new MigrateException(sprintf("Process configuration for '$property' must be an array", $property));
        }

from /core/modules/migrate/src/Plugin/Migration.php (line 367 or so) should be removed as unnecessary due to getProcessNormalized() running first. Unless I'm missing some instance that would only run the latter?

thanks, I meant to come back and mention that and clearly I forgot. I see no reason to not remove that code in this patch.

LGTM

Regarding #11 / #13.

+++ b/core/modules/migrate/src/Plugin/Migration.php
@@ -364,9 +364,6 @@ public function getProcessPlugins(array $process = NULL) {
       foreach ($this->getProcessNormalized($process) as $property => $configurations) {
         $this->processPlugins[$index][$property] = [];
-        if (!is_array($configurations) && !$this->processPlugins[$index][$property]) {
-          throw new MigrateException(sprintf("Process configuration for '$property' must be an array", $property));
-        }
         foreach ($configurations as $configuration) {

So, before, we were throwing an exception if $configurations wasn't an array right before foreaching it.
getProcessNormalized() does this:

protected function getProcessNormalized(array $process) {
  $normalized_configurations = [];
  foreach ($process as $destination => $configuration) {
    if (is_string($configuration)) {
      $configuration = [
        'plugin' => 'get',
        'source' => $configuration,
      ];
    }
    if (isset($configuration['plugin'])) {
      $configuration = [
        $configuration,
      ];
    }
    $normalized_configurations[$destination] = $configuration;
  }
  return $normalized_configurations;
}

If any configuration child key of $process is a string, it converts it to an array. But what if it's a different kind of scalar, or an object? Under those circumstances, getProcessNormalized would not be guarantee that the child keys of what it returns are also arrays.

It's slightly confusing because in getProcessNormalized() the return array is called "configurations" (plural) with each child being a configuration (singular). But then in the caller, those individual children are each called "configurations" (plural).

This all might be fine, but I'd suggest leaving the earlier exception in for now and addressing a proposal to remove that hunk in a followup.

RE-uploading the patch from 9 which does not remove the check from getProcessPlugins().

Follow up made, #3133139: Remove is_array check in getProcessPlugins.

re-rtbc-ing.

Add manual testing to the IS

Committed and pushed 93545bd537 to 9.1.x and 145d174fdb to 9.0.x and b1007e34e4 to 8.9.x. Thanks!

This is nice improvement that helps people work out what's going wrong - always a good thing with migrations!

diff --git a/core/modules/migrate/tests/src/Kernel/Plugin/MigrationTest.php b/core/modules/migrate/tests/src/Kernel/Plugin/MigrationTest.php
index 5b730be230..c58df28af5 100644
--- a/core/modules/migrate/tests/src/Kernel/Plugin/MigrationTest.php
+++ b/core/modules/migrate/tests/src/Kernel/Plugin/MigrationTest.php
@@ -42,7 +42,7 @@ public function testGetProcessPluginsException() {
   }
 
   /**
-   * Tests Migration::getDestinationPlugin()
+   * Tests Migration::getProcessPlugins()
    *
    * @param array $process
    *   The migration process pipeline.

Fixed the test description on commit.