| ## gpssh developer notes |
| |
| This directory contains code that overrides our vendored pexpect module. |
| |
| In particular, we override the way the ssh command prompt is validated |
| on a remote machine. The original, vendored pexpect tries to match 2 successive prompts |
| from an interactive bash shell. However, if the target host is slow, because of |
| CPU loading or network loading, those prompts may returned to the ssh client with some delay. |
| |
| In the case of an SSH session with delay, the overridden method in this module retries several times, |
| extending the timeout from the default 0.1 second expectation between characters to up to 16x that. |
| |
| Experimentally, the retries and extended timeout are enough to tolerate a 1 second delay between characters, at least on a localhost connection using the "tc" tool to delay network on that loopback. |
| |
| The number of retries can be configured via a configuration setting. |
| |
| ### Testing the retry logic |
| |
| `tc` itself needs permissions beyond sudo, so it does not seem to work in an |
| unprivileged container (i.e., via Concourse). |
| |
| |
| On a Linux VM with root, use the `tc` library to delay network traffic as follows. |
| |
| First, a delay of 500ms should exercise the retry logic, but still succeed: |
| |
| ``` |
| sudo tc qdisc add dev lo root netem delay 500ms |
| ``` |
| |
| To reconfigure the delay, first cancel the existing delay and then set a new one: |
| |
| ``` |
| sudo tc qdisc del dev lo root |
| sudo tc qdisc add dev lo root netem delay 1200ms |
| ``` |
| |
| |
| |
