Open Radar

 
Summary:
When visiting a website using HTTP/2, certain POST forms cannot submit after the connection has been interrupted and restored (case A).
The same issue also occurs when the initial page is preloaded as Top Hit, and the following action is the form submit (case B).
An example of a specific form that meets the requirements and is served over HTTP/2 is the Wikipedia account signup page (see the URL below).

Steps to Reproduce:
Case A:
1. Open Safari
2. Visit https://en.wikipedia.org/w/index.php?title=Special:UserLogin&type=signup
3. Turn off internet connection
4. Turn internet connection back on
5. Click the "Create your account" button on the Wikipedia website
Case B:
1. Open Safari
2. Make sure "Preload Top Hit in the background" is turned on in Safari Preferences
3. Visit https://en.wikipedia.org/w/index.php?title=Special:UserLogin&type=signup to add it to the browser history
4. Quit Safari
5. Open Safari
6. Paste https://en.wikipedia.org/w/index.php?title=Special:UserLogin&type=signup in the address bar but don't press return; instead, wait for it to show up as Top Hit and get preloaded
7. Click the Top Hit suggestion
8. Click the "Create your account" button on the Wikipedia website

Expected Results:
Returning to the Wikipedia account creation page with a red alert message "Account creation error".

Actual Results:
Safari showing error the following error message:
  Safari Can't Connect to the Server
  Safari can't open the page "..." because Safari can't connect to the server "en.wikipedia.org".

Regression:
The issue occurs on both Safari (Version 9.1.1 (11601.6.17)) and Safari Technology Preview (Version 9.1.1 (11601.6.17, 11602.1.31)).

Notes:
The issue only occurs with HTTP/2; after turning off HTTP/2 support on a test website, the issue went away.
The issue only occurs with certain HTML forms; the form has to have method="POST" and at least 1 named input. Certain forms do not trigger this issue, even if they do POST.
Attached is an AppleScript (http2.applescript) to simulate the issue (case A). Please note that this script temporarily disables your primary WiFi connection.
Also attached is an HTML page (form.html) which has a minimal HTML source that triggers this issue when being served over HTTP/2.
Monitoring the connections to the server with lsof, it can be seen that:
Case A:
- the persistent HTTPS connection (HTTP/2) gets disconnected when the internet connection is interrupted
- when clicking the button a new connection is made
- the first POST request (of the specific form) on this new connection fails; in contrast, a GET request after the interruption succeeds
Case B:
- the persistent HTTPS connection (HTTP/2) used for the Top Hit preload remains open
- for subsequent requests, it does not re-use the connection used by the preload, but instead makes a new connection
- the first POST request (of the specific form) on this new connection fails; in contrast, a GET request after the preload succeeds