fix: error handling for when array_combine would fail

Add message to make the error more actionable and include context in question

Resolves #842

classes/local/execution/engine.php

@@ -535,7 +535,14 @@ public function abort(?\Throwable $reason = null) {
         }
 
         $this->set_current_step(null);
-        $this->log('Engine: aborting steps', $message ? ['reason' => $message] : [], Logger::NOTICE);
+        $context = [];
+        if ($reason->debuginfo) {
+            $context = (array) json_decode($reason->debuginfo);
+            if (json_last_error() !== JSON_ERROR_NONE) {
+                $context = [];
+            }
+        }
+        $this->log('Engine: aborting steps', $message ? array_merge(['reason' => $message], $context) : [], Logger::NOTICE);
         $this->exception = $reason;
         foreach ($this->enginesteps as $enginestep) {
             $status = $enginestep->status;

classes/local/step/reader_csv.php

@@ -91,7 +91,18 @@ public function csv_contents_generator() {
 
             // Convert header string to an actual headers array.
             $headers = str_getcsv($strheaders, $delimiter);
+            $numheaders = count($headers);
             while (($data = fgetcsv($handle, $maxlinelength, $delimiter)) !== false) {
+                $numfields = count($data);
+                if ($numfields !== $numheaders) {
+                    throw new \moodle_exception('reader_csv:header_field_count_mismatch', 'tool_dataflows', '', (object) [
+                        'numfields' => $numfields,
+                        'numheaders' => $numheaders,
+                    ], json_encode([
+                        'fields' => $data,
+                        'headers' => $headers,
+                    ]));
+                }
                 $record = array_combine($headers, $data);
                 yield (object) $record;
             }

lang/en/tool_dataflows.php

@@ -370,6 +370,7 @@
 $string['reader_csv:headers_help'] = 'If populated, then this will act as the header to map field to keys. If left blank, it will be populated automatically using the first read row.';
 $string['reader_csv:overwriteheaders'] = 'Overwrite existing headers';
 $string['reader_csv:overwriteheaders_help'] = 'If checked, the headers supplied above will be used instead of the ones in the file, effectively ignoring the first row.';
+$string['reader_csv:header_field_count_mismatch'] = 'Number of headers ({$a->numheaders}) do not match number of fields ({$a->numfields})';
 
 // Reader JSON.
 $string['reader_json:arrayexpression_help'] = 'Nested array to extract from JSON. For example, {$a->expression} will return the users array from the following JSON (If empty it is assumed the starting point of the JSON file is an array):{$a->jsonexample}';