cache.php

<?php
// Helper functions for dealing with my amazingly slow homepage.
// Uses a so-called "stale-while-revalidate" caching strategy.

namespace cache;

define('CACHE', match(@$_GET['show']) {
  'all' => "/tmp/cache__all.html",
  'legacy' => "/tmp/cache__legacy.html",
  default => "/tmp/cache__default.html"
});

define('TMP', "/tmp/tmp.html");
define('ENTRYPOINT', __DIR__ . "/index.php");

function capture($closure) {
  ob_start();
  $closure();
  return ob_get_clean();
}

function generate() {
  $cached = capture(fn() => include ENTRYPOINT);

  // We're writing to TMP first, and then renaming
  // atomically to CACHE, to prevent race condition
  // where we'd be serving partials files.
  file_put_contents(TMP, $cached);
  rename(TMP, CACHE);

  return $cached;
}

function serve() {
  if(file_exists(CACHE)) {
    readfile(CACHE);

    // Trigger background process to invalidate cached copy.
    if(!file_exists(TMP)) exec("php cache.php > /dev/null 2>&1 &");
    exit;
  } else {
    echo generate();
    exit;
  }
}

// If this file is called directly (in the case of cache invalidation),
// we generate a new cached copy. Otherwise, we serve a regular page,
// either from cache, or generate it on the fly.

match(php_sapi_name()) {
  'cli' => generate(),
  default => serve(),
};