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"RuntimeException: Undefined array key" when pasting "a {{{a}}} a" comment (due to regex in PhutilRemarkupInterpreterBlockRule.php)
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Description

Upstreaming from https://phabricator.wikimedia.org/T284397

  1. Phorge at 8d2b481bb5a83d1a4ce82014cabdc83fa3ca432d, PHP 8.2.5 (though irrelevant here)
  2. Go to an existing task.
  3. Write the comment a {{{a}}} a
[2023-05-09 12:12:57] EXCEPTION: (RuntimeException) Undefined array key 1 at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=renderDashboardTabPanelContent, ref.master=d47289622650, ref.renderDashboardTabPanelContent=d47289622650), phorge(head=master, ref.master=8d2b481bb5a8)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/infrastructure/markup/remarkup/PhutilRemarkupEngine.php:243]
  #1 <#2> PhutilRemarkupEngine::splitTextIntoBlocks(array) called at [<phorge>/src/infrastructure/markup/remarkup/PhutilRemarkupEngine.php:136]
  #2 <#2> PhutilRemarkupEngine::preprocessText(string) called at [<phorge>/src/infrastructure/markup/remarkup/PhutilRemarkupEngine.php:106]
  #3 <#2> PhutilRemarkupEngine::markupText(string) called at [<phorge>/src/infrastructure/markup/PhabricatorMarkupEngine.php:614]
  #4 <#2> PhabricatorMarkupEngine::extractPHIDsFromMentions(PhabricatorUser, array) called at [<phorge>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:2059]
  #5 <#2> PhabricatorApplicationTransactionEditor::buildSubscribeTransaction(ManiphestTask, array, array) called at [<phorge>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:2221]
  #6 <#2> PhabricatorApplicationTransactionEditor::expandSupportTransactions(ManiphestTask, array) called at [<phorge>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:1222]
  #7 <#2> PhabricatorApplicationTransactionEditor::applyTransactions(ManiphestTask, array) called at [<phorge>/src/applications/transactions/editengine/PhabricatorEditEngine.php:2031]
  #8 <#2> PhabricatorEditEngine::buildCommentResponse(ManiphestTask) called at [<phorge>/src/applications/transactions/editengine/PhabricatorEditEngine.php:998]
  #9 <#2> PhabricatorEditEngine::buildResponse() called at [<phorge>/src/applications/maniphest/controller/ManiphestTaskEditController.php:12]
  #10 <#2> ManiphestTaskEditController::handleRequest(AphrontRequest) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:284]
  #11 phlog(RuntimeException) called at [<phorge>/src/aphront/handler/PhabricatorAjaxRequestExceptionHandler.php:27]
  #12 PhabricatorAjaxRequestExceptionHandler::handleRequestThrowable(AphrontRequest, RuntimeException) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:751]
  #13 AphrontApplicationConfiguration::handleThrowable(RuntimeException) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:296]
  #14 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:203]
  #15 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phorge>/webroot/index.php:35]

Desired situation (without any crash):

Phorge Cowsay Figlet showcase.png (891×837 px, 66 KB)

Event Timeline

This is not triggered for {{{a}}} a or with a a {{{a}}} a, only for 1 string in front of a string with three squares.

The problem is in getMatchingLineCount() in PhutilRemarkupInterpreterBlockRule.php:
preg_match(self::START_BLOCK_PATTERN) being /^([\w]+)\s*(?:\(([^)]+)\)\s*)?{{{/ is matched ($num_lines++ setting 1), and then while (isset($lines[$cursor])) is true ($num_lines++ setting 2).
Afterwards in PhutilRemarkEngine.php, $block['num_lines'] is one larger than sizeof($blocks), so for ($ii = $min; $ii < $max; $ii++) { $lines .= $text[$ii]; } reaches a non-existing array field.

It returns $num_lines = 0 for the non-testcases above.

The correct fix seems correcting self::START_BLOCK_PATTERN (if anyone is more into regexes than I am) not to trigger for that one-string-prefix situation.

By coincidence, chatting with valerio.bozzolan yesterday made me realize that this rendering rule is merely in place for figlet and cowsay.

Currently, entering a {{{a}}} a markup as a new comment shows as preview the confusing error message "No interpreter found":

phantasstick.png (951×1 px, 59 KB)

Currently, trying to render such markup for existing comments somehow imported from other systems throws a RunTimeException.

Thanks again, I really loved this follow-up on your hack

FYI I proposed a partial patch in D25415 which makes the issue more contained at least.

Cannot reproduce anymore / fixed?