Protect against transfer.abort() being called during callback

This was already handled in most cases, but not for a body-less response. It's
safe to call transfer.abort() during a callback, so long as the performing flag
is set to true. This was set during the normal libcurl callbacks, but for a
body-less response, we manually invoke the header_done_callback and were not
setting the performing flag.

src/browser/tests/net/xhr.html

@@ -252,3 +252,34 @@
     testing.expectEqual(XMLHttpRequest.UNSENT, req.readyState);
   });
 </script>
+
+
+<script id=xhr_abort_callback_nobody>
+  testing.async(async (restore) => {
+    const req = new XMLHttpRequest();
+    let abortFired = false;
+    let errorFired = false;
+    let loadEndFired = false;
+
+    await new Promise((resolve) => {
+      req.onabort = () => { abortFired = true; };
+      req.onerror = () => { errorFired = true; };
+      req.onloadend = () => {
+        loadEndFired = true;
+        resolve();
+      };
+
+      req.open('GET', 'http://127.0.0.1:9582/xhr_empty');
+      req.onreadystatechange = (e) => {
+        req.abort();
+      }
+      req.send();
+    });
+
+    restore();
+    testing.expectEqual(true, abortFired);
+    testing.expectEqual(true, errorFired);
+    testing.expectEqual(true, loadEndFired);
+    testing.expectEqual(XMLHttpRequest.UNSENT, req.readyState);
+  });
+</script>

src/http/Client.zig

@@ -789,25 +789,30 @@ fn processMessages(self: *Client) !bool {
         if (msg.err) |err| {
             requestFailed(transfer, err, true);
         } else blk: {
-            // In case of request w/o data, we need to call the header done
-            // callback now.
-            if (!transfer._header_done_called) {
-                const proceed = transfer.headerDoneCallback(&msg.conn) catch |err| {
-                    log.err(.http, "header_done_callback", .{ .err = err });
+            {
+                self.handles.performing = true;
+                defer self.handles.performing = false;
+
+                // In case of request w/o data, we need to call the header done
+                // callback now.
+                if (!transfer._header_done_called) {
+                    const proceed = transfer.headerDoneCallback(&msg.conn) catch |err| {
+                        log.err(.http, "header_done_callback", .{ .err = err });
+                        requestFailed(transfer, err, true);
+                        continue;
+                    };
+                    if (!proceed) {
+                        requestFailed(transfer, error.Abort, true);
+                        break :blk;
+                    }
+                }
+                transfer.req.done_callback(transfer.ctx) catch |err| {
+                    // transfer isn't valid at this point, don't use it.
+                    log.err(.http, "done_callback", .{ .err = err });
                     requestFailed(transfer, err, true);
                     continue;
                 };
-                if (!proceed) {
-                    requestFailed(transfer, error.Abort, true);
-                    break :blk;
-                }
             }
-            transfer.req.done_callback(transfer.ctx) catch |err| {
-                // transfer isn't valid at this point, don't use it.
-                log.err(.http, "done_callback", .{ .err = err });
-                requestFailed(transfer, err, true);
-                continue;
-            };
 
             transfer.req.notification.dispatch(.http_request_done, &.{
                 .transfer = transfer,
@@ -1041,10 +1046,6 @@ pub const Transfer = struct {
 
     pub fn abort(self: *Transfer, err: anyerror) void {
         requestFailed(self, err, true);
-        if (self._conn == null) {
-            self.deinit();
-            return;
-        }
 
         const client = self.client;
         if (client.handles.performing) {

src/testing.zig

@@ -561,6 +561,14 @@ fn testHTTPHandler(req: *std.http.Server.Request) !void {
         });
     }
 
+    if (std.mem.eql(u8, path, "/xhr_empty")) {
+        return req.respond("", .{
+            .extra_headers = &.{
+                .{ .name = "Content-Type", .value = "text/html; charset=utf-8" },
+            },
+        });
+    }
+
     if (std.mem.eql(u8, path, "/xhr/json")) {
         return req.respond("{\"over\":\"9000!!!\",\"updated_at\":1765867200000}", .{
             .extra_headers = &.{