refactor: tighten up cloudflare detection

The previous approach to detecting whether to use Cloudflare's sockets was to check for missing polyfills.
But as we improve the polyfills that Wrangler can provide these checks are no longer valid.

Now we just try to use the Cloudflare API first and fallback to Node.js if those are not available.

packages/pg/lib/stream.js

@@ -1,14 +1,19 @@
+let isCloudflareRuntime
+
 /**
  * Get a socket stream compatible with the current runtime environment.
  * @returns {Duplex}
  */
 module.exports.getStream = function getStream(ssl) {
-  const net = require('net')
-  if (typeof net.Socket === 'function') {
-    return new net.Socket()
-  } else {
+  if (isCloudflareRuntime === undefined) {
+    isCloudflareRuntime = computeIsCloudflareRuntime()
+  }
+  if (isCloudflareRuntime) {
     const { CloudflareSocket } = require('pg-cloudflare')
     return new CloudflareSocket(ssl)
+  } else {
+    const net = require('net')
+    return new net.Socket()
   }
 }
 
@@ -18,11 +23,35 @@ module.exports.getStream = function getStream(ssl) {
  * @returns {Duplex}
  */
 module.exports.getSecureStream = function getSecureStream(options) {
-  var tls = require('tls')
-  if (tls.connect) {
-    return tls.connect(options)
-  } else {
+  if (isCloudflareRuntime === undefined) {
+    isCloudflareRuntime = computeIsCloudflareRuntime()
+  }
+  if (isCloudflareRuntime) {
     options.socket.startTls(options)
     return options.socket
+  } else {
+    var tls = require('tls')
+    return tls.connect(options)
+  }
+}
+
+/**
+ * Are we running in a Cloudflare Worker?
+ *
+ * @returns true if the code is currently running inside a Cloudflare Worker.
+ */
+function computeIsCloudflareRuntime() {
+  // Since 2022-03-21 the `global_navigator` compatibility flag is on for Cloudflare Workers
+  // which means that `navigator.userAgent` will be defined.
+  if (typeof navigator === 'object' && navigator !== null && typeof navigator.userAgent === 'string') {
+    return navigator.userAgent === 'Cloudflare-Workers'
+  }
+  // In case `navigator` or `navigator.userAgent` is not defined then try a more sneaky approach
+  if (typeof Response === 'function') {
+    const resp = new Response(null, { cf: { thing: true } })
+    if (typeof resp.cf === 'object' && resp.cf !== null && resp.cf.thing) {
+      return true
+    }
   }
+  return false
 }