Removing blocking code async HTTP request

The HTTP Client has a state pool. It blocks when we've exceeded max_concurrency.
This can block processing forever. A simple way to reproduce this is to go into
the demo cdp.js, and execute the XHR request 5 times (loading json/product.json)

To some degree, I think this is a result of weird / non-intuitive execution
flow. If you exec a JS with 100 XHR requests, it'll call our XHR _send function
but none of these will execute until the loop is run (after the script is done
being executed). This can result in poor utilization of our connection and
state pool.

For an async request, getting the *Request object is itself now asynchronous.
If no state is available, we use the Loop's timeout (at 20ms) to keep checking
for an available state.

Author: karlseguin

src/app.zig

@@ -52,7 +52,8 @@ pub const App = struct {
             .telemetry = undefined,
             .app_dir_path = app_dir_path,
             .notification = notification,
-            .http_client = try HttpClient.init(allocator, 5, .{
+            .http_client = try HttpClient.init(allocator, .{
+                .max_concurrent = 3,
                 .http_proxy = config.http_proxy,
                 .tls_verify_host = config.tls_verify_host,
             }),

src/browser/page.zig

@@ -113,7 +113,9 @@ pub const Page = struct {
             .cookie_jar = &session.cookie_jar,
             .microtask_node = .{ .func = microtaskCallback },
             .window_clicked_event_node = .{ .func = windowClicked },
-            .request_factory = browser.http_client.requestFactory(browser.notification),
+            .request_factory = browser.http_client.requestFactory(.{
+                .notification = browser.notification,
+            }),
             .scope = undefined,
             .module_map = .empty,
         };

src/browser/xhr/xhr.zig

@@ -30,6 +30,7 @@ const Mime = @import("../mime.zig").Mime;
 const parser = @import("../netsurf.zig");
 const http = @import("../../http/client.zig");
 const Page = @import("../page.zig").Page;
+const Loop = @import("../../runtime/loop.zig").Loop;
 const CookieJar = @import("../storage/storage.zig").CookieJar;
 
 // XHR interfaces
@@ -78,6 +79,7 @@ const XMLHttpRequestBodyInit = union(enum) {
 
 pub const XMLHttpRequest = struct {
     proto: XMLHttpRequestEventTarget = XMLHttpRequestEventTarget{},
+    loop: *Loop,
     arena: Allocator,
     request: ?*http.Request = null,
 
@@ -91,6 +93,7 @@ pub const XMLHttpRequest = struct {
     sync: bool = true,
     err: ?anyerror = null,
     last_dispatch: i64 = 0,
+    request_body: ?[]const u8 = null,
 
     cookie_jar: *CookieJar,
     // the URI of the page where this request is originating from
@@ -241,12 +244,13 @@ pub const XMLHttpRequest = struct {
     pub fn constructor(page: *Page) !XMLHttpRequest {
         const arena = page.arena;
         return .{
+            .url = null,
             .arena = arena,
+            .loop = page.loop,
             .headers = Headers.init(arena),
             .response_headers = Headers.init(arena),
             .method = undefined,
             .state = .unsent,
-            .url = null,
             .origin_url = &page.url,
             .cookie_jar = page.cookie_jar,
         };
@@ -422,18 +426,24 @@ pub const XMLHttpRequest = struct {
         log.debug(.http, "request", .{ .method = self.method, .url = self.url, .source = "xhr" });
 
         self.send_flag = true;
+        if (body) |b| {
+            self.request_body = try self.arena.dupe(u8, b);
+        }
 
-        self.request = try page.request_factory.create(self.method, &self.url.?.uri);
-        var request = self.request.?;
-        errdefer request.deinit();
+        try page.request_factory.initAsync(page.arena, self.method, &self.url.?.uri, self, onHttpRequestReady, self.loop,);
+    }
+
+    fn onHttpRequestReady(ctx: *anyopaque, request: *http.Request) !void {
+        // on error, our caller will cleanup request
+        const self: *XMLHttpRequest = @alignCast(@ptrCast(ctx));
 
         for (self.headers.list.items) |hdr| {
             try request.addHeader(hdr.name, hdr.value, .{});
         }
 
         {
             var arr: std.ArrayListUnmanaged(u8) = .{};
-            try self.cookie_jar.forRequest(&self.url.?.uri, arr.writer(page.arena), .{
+            try self.cookie_jar.forRequest(&self.url.?.uri, arr.writer(self.arena), .{
                 .navigation = false,
                 .origin_uri = &self.origin_url.uri,
             });
@@ -447,14 +457,15 @@ pub const XMLHttpRequest = struct {
         //  if the request method is GET or HEAD.
         //  https://xhr.spec.whatwg.org/#the-send()-method
         // var used_body: ?XMLHttpRequestBodyInit = null;
-        if (body) |b| {
+        if (self.request_body) |b| {
             if (self.method != .GET and self.method != .HEAD) {
-                request.body = try page.arena.dupe(u8, b);
+                request.body = b;
                 try request.addHeader("Content-Type", "text/plain; charset=UTF-8", .{});
             }
         }
 
-        try request.sendAsync(page.loop, self, .{});
+        try request.sendAsync(self.loop, self, .{});
+        self.request = request;
     }
 
     pub fn onHttpResponse(self: *XMLHttpRequest, progress_: anyerror!http.Progress) !void {
@@ -468,6 +479,7 @@ pub const XMLHttpRequest = struct {
 
         if (progress.first) {
             const header = progress.header;
+
             log.debug(.http, "request header", .{
                 .source = "xhr",
                 .url = self.url,