Block TLS handshake until TCP connection established

If a TLS handshake is attempted before the completion of an
asynchronous TCP connection has been ascertained, our local state will
not be set up correctly for further progress and the sentinel "open"
event will never be sent.  This can occur if sufficient time passes
after the initiation of an async TCP connection so that by the time
`wait_reading_process_output` is called, the connection has already
been established on the TCP level.

This somewhat timing-sensitive bug has plagued HTTPS connections on
some platforms, notably macOS, for a long time (bug#49449).

* src/process.c (wait_reading_process_output): Gate the TLS handshake
by the NON_BLOCKING_CONNECT_FD flag.  The flag will be cleared as soon
as the TCP socket is found to be writable.
* test/src/process-tests.el (process-async-https-with-delay):
New test.

1 files changed, 30 insertions, 0 deletions

diff --git a/test/src/process-tests.el b/test/src/process-tests.el
index 1774f2fc74e..9bab523708e 100644
--- a/test/src/process-tests.el
+++ b/test/src/process-tests.el
@@ -28,6 +28,7 @@
 (require 'puny)
 (require 'subr-x)
 (require 'dns)
+(require 'url-http)
 
 ;; Timeout in seconds; the test fails if the timeout is reached.
 (defvar process-test-sentinel-wait-timeout 2.0)
@@ -916,5 +917,34 @@ Return nil if FILENAME doesn't exist."
       ;; ...and the change description should be "interrupt".
       (should (equal '("interrupt\n") events)))))
 
+(ert-deftest process-async-https-with-delay ()
+  "Bug#49449: asynchronous TLS connection with delayed completion."
+  (skip-unless (and internet-is-working (gnutls-available-p)))
+  (let* ((status nil)
+         (buf (url-http
+                 #s(url "https" nil nil "elpa.gnu.org" nil
+                        "/packages/archive-contents" nil nil t silent t t)
+                 (lambda (s) (setq status s))
+                 '(nil) nil 'tls)))
+    (unwind-protect
+        (progn
+          ;; Busy-wait for 1 s to allow for the TCP connection to complete.
+          (let ((delay 1.0)
+                (t0 (float-time)))
+            (while (< (float-time) (+ t0 delay))))
+          ;; Wait for the entire operation to finish.
+          (let ((limit 4.0)
+                (t0 (float-time)))
+            (while (and (null status)
+                        (< (float-time) (+ t0 limit)))
+              (sit-for 0.1)))
+          (should status)
+          (should-not (assq :error status))
+          (should buf)
+          (should (> (buffer-size buf) 0))
+          )
+      (when buf
+        (kill-buffer buf)))))
+
 (provide 'process-tests)
 ;;; process-tests.el ends here