Make sure we can't get into a proxy selector loop

I haven't actually seen this happen (real requests seem to be sent
with socket://... URLs for the proxy step) but it seems quite
possible, so it's better to protect against it.

Author: pimterry

src/main/java/tech/httptoolkit/javaagent/advice/OverrideAllProxySelectionAdvice.java

@@ -17,15 +17,21 @@ public static void selectProxy(
     ) {
         String scheme = uri.getScheme();
 
-        // For HTTP URIs only, we override all proxy selection globally to select our proxy instead:
-        if (scheme.equals("http") || scheme.equals("https")) {
+        boolean isHttp = scheme.equals("http") || scheme.equals("https");
+
+        // We read from our custom variables, since we can't access HttpProxyAgent from a bootstrapped
+        // class, and we use namespaced properties to make this extra reliable:
+        String proxyHost = System.getProperty("tech.httptoolkit.proxyHost");
+        int proxyPort = Integer.parseInt(System.getProperty("tech.httptoolkit.proxyPort"));
+
+        boolean isRequestToProxy = uri.getHost().equals(proxyHost) && uri.getPort() == proxyPort;
+
+        // For HTTP URIs going elsewhere, we override all proxy selection globally to go via our proxy:
+        if (isHttp && !isRequestToProxy) {
             returnedProxies = Collections.singletonList(
-                new Proxy(Proxy.Type.HTTP, new InetSocketAddress(
-                    // We read from our custom variables, since we can't access HttpProxyAgent from a bootstrapped
-                    // class, and we use namespaced properties to make this extra reliable:
-                    System.getProperty("tech.httptoolkit.proxyHost"),
-                    Integer.parseInt(System.getProperty("tech.httptoolkit.proxyPort"))
-                ))
+                new Proxy(Proxy.Type.HTTP,
+                    new InetSocketAddress(proxyHost, proxyPort)
+                )
             );
         }
     }