ESI blocks don't appear when they are added to regions using context.

subscribing

A better implementation would be to use something like the following:

/**
 * Implements hook_context_plugins to define our custom block context plugin
 * 
 * @return array
 */
function esi_context_plugins() {
  $plugins = array();
  $plugins['context_reaction_esi_block'] = array(
    'handler' => array(
      'path' => drupal_get_path('module', 'esi') .'/plugins',
      'file' => 'context_reaction_esi_block.inc',
      'class' => 'context_reaction_esi_block',
      'parent' => 'context_reaction_block',
  ),
  );

  return $plugins;
}

/**
 * Implements hook_context_registry_alter to use our custom block context plugin
 * to output ESI tags instead of the block
 * 
 * @param $registry array
 */
function esi_context_registry_alter(&$registry) {
  if (isset($registry['reactions']['block'])) {
    $registry['reactions']['block']['plugin'] = 'context_reaction_esi_block';
  }
}

And then have

<?php
class context_reaction_esi_block extends context_reaction_block {
  protected function build_block($block, $reset = FALSE) {
    $esi_config = _esi__block_settings($block->module, $block->delta);

    if ($esi_config->enabled) {
      $block->content = theme('esi_tag', $block);
    }
    else {
      $array = module_invoke($block->module, 'block', 'view', $block->delta);
      if (isset($array) && is_array($array)) {
        foreach ($array as $k => $v) {
          $block->$k = $v;
        }
      }
    }

    if (!empty($block->content)) {
      // Only query for custom block title if block core compatibility is enabled.
      if (!variable_get('context_reaction_block_disable_core', FALSE)) {
        global $user, $theme_key;
        $block->title = db_result(db_query("SELECT title FROM {blocks} WHERE module = '%s' AND delta = '%s' AND theme = '%s'", $block->module, $block->delta, $theme_key));
      }
      // Override default block title if a custom display title is present.
      if (!empty($block->title)) {
        // Check plain here to allow module generated titles to keep any markup.
        $block->subject = $block->title == '<none>' ? '' : check_plain($block->title);
      }
      if (!isset($block->subject)) {
        $block->subject = '';
      }
    }
    return $block;
  }
}

However to make that work the callback for the block has to change to:

/**
 * Get the themed HTML for a particular block
 *
 * @param String $theme
 *   The name of the theme: e.g. "garland"
 * @param String $region
 *   The name of the region the block is in: e.g. "left"
 * @param String $module
 * @param String $delta
 */
function _esi__get_block($theme, $region, $module, $delta) {
  global $theme_key;
  $theme_key = $theme;
  init_theme();

  $block = db_fetch_object(db_query("SELECT * FROM {blocks} WHERE module = '%s' AND delta = '%s' AND theme = '%s'", $module, $delta, $theme));
  $block->context = $region;
  $block->status = 1;
  $array = module_invoke($block->module, 'block', 'view', $block->delta);
  if (isset($array) && is_array($array)) {
    foreach ($array as $k => $v) {
      $block->$k = $v;
    }
  }

  if (!empty($block->title)) {
    // Check plain here to allow module generated titles to keep any markup.
    $block->subject = $block->title == '<none>' ? '' : check_plain($block->title);
  }
  if (!isset($block->subject)) {
    $block->subject = '';
  }

  return $block;
}

That all works, though you have to be careful about blocks that claim to be BLOCK_CACHE_NO_CACHE when they really don't need to be ... I'm looking at you, Boxes. Which brings me to an interesting question about why BLOCK_CACHE_PER_PAGE is implemented as a hash on the callback but BLOCK_CACHE_PER_USER and BLOCK_CACHE_PER_ROLE are handled as cookies.

We have a working version of this in production on a site. If there is any interest I could probably roll it as a patch to the existing release and/or fork the repo and do it there.

Hmm, I couldn't get #3 to work, but I'd love to see a proper patch so I know I'm not missing something here.

Removing the function esi_theme_registry_alter(&$theme_registry) got #3 working for me.

Thanks for keeping me honest, PeterC. I did completely miss that step. Anyone know anything about the hash for BLOCK_CACHE_PER_PAGE and cookies for BLOCK_CACHE_PER_USER and BLOCK_CACHE_PER_ROLE?

The BLOCK_CACHE_PER_PAGE setting is used to provide a different ESI URL for each page where the block is displayed. This ensures Varnish will cache a different version of the block for each page it's displayed on.

The BLOCK_CACHE_PER_USER and BLOCK_CACHE_PER_ROLE cookies work with a custom vcl_hash routine which splits each block into a separate per-user/per-role cache-bin within Varnish. This means that a per-user block (such as a welcome message saying "Hello Bob") is only displayed to the user, and similarly for role-based block control.

BLOCK_CACHE_PER_PAGE can be implemented as a static hash because the block's contents only changes on a page-URL basis. Blocks which use BLOCK_CACHE_PER_ROLE and BLOCK_CACHE_PER_USER can vary their contents according to the user-session, so Varnish must be capable of varying them by cookie data.

Here's a patch. Seems to be working for me.

Tried a patch from #9. I have layouts assigned to pages using context_layouts module, a submodule of context, and using the patch does include an esi tag for the blocks but does not use the proper template chosen using context_lauouts.

#9 is in the 2.x branch. This should be fixed. If I am mistaken please re-open.

last -dev results in a fatal error, because context_reaction_esi_block.inc is in the module root folder and should be in /plugins.

And

      $block->content = theme('esi_tag', $block);

should be:

      $block->content = theme('esi_tag', 'block', $block);

or else $type is empty and src won't get build

This patch has been committed.

This one as well