fix: do not proxy traffic to sockets

This commit implements a proxy-exception for traffic targeting local
sockets instead of a "host:port" combination. If such traffic would be
forwarded through the proxy, the socket destination info would be lost,
while a "localhost:80" destination would be added. This means that the
proxy cannot handle such requests correctly either way, which makes
sending traffic to the proxy pointless.

Because of that, if the agent receives traffic destined for a socket, it
will simply use the fallback agent to handle that traffic.

src/classes/Agent.ts

@@ -80,6 +80,19 @@ abstract class Agent {
       requestUrl = this.protocol + '//' + (configuration.hostname ?? configuration.host) + (configuration.port === 80 ?? configuration.port === 443 ? '' : ':' + configuration.port) + request.path;
     }
 
+    // If a request should go to a local socket, proxying it through an HTTP
+    // server does not make sense as the information about the target socket
+    // will be lost and the proxy won't be able to correctly handle the request.
+    if (configuration.socketPath) {
+      log.trace({
+        destination: configuration.socketPath,
+      }, "not proxying request; destination is a socket");
+
+      // @ts-expect-error seems like we are using wrong type for fallbackAgent.
+      this.fallbackAgent.addRequest(request, configuration);
+      return;
+    }
+
     if (!this.isProxyConfigured()) {
       log.trace({
         destination: requestUrl,

test/global-agent/factories/createGlobalProxyAgent.ts

@@ -307,6 +307,33 @@ serial('forwards requests matching NO_PROXY', async (t) => {
   t.is(response.body, 'DIRECT');
 });
 
+serial('forwards requests that go to a socket', async (t) => {
+  const globalProxyAgent = createGlobalProxyAgent();
+
+  // not relevant as traffic shouldn't go through proxy
+  globalProxyAgent.HTTP_PROXY = 'localhost:10324';
+
+  var server = http.createServer(function (req, res) {
+    res.writeHead(200);
+    res.write('OK');
+    res.end();
+  });
+
+  t.teardown(() => {
+    server.close();
+  });
+  server.listen('/tmp/test.sock');
+
+  const response: HttpResponseType = await new Promise((resolve) => {
+    http.get({
+        socketPath: '/tmp/test.sock',
+        path: '/endpoint',
+      }, createHttpResponseResolver(resolve)
+    );
+  });
+  t.is(response.body, 'OK');
+});
+
 serial('proxies HTTP request (using http.get(host))', async (t) => {
   const globalProxyAgent = createGlobalProxyAgent();