Tramp using su fails with powerline prompt

Question

I'm having a weird problem opening files as root. From emacs -Q, I issue (require 'tramp), customize tramp-verbose to 10, and try to open a file as root: C-x C-f /su::/etc/fstab. I'm then prompted for my root password. Entering the password, Emacs pauses for ca. 10 seconds, then appears to open a new file called fstab, but it's not the fstab that already exists in /etc/fstab. The message about calling make-directory to create the parent directories shows up, so I've apparently opened a new file somewhere. I can't see where, exactly, as C-x C-f, or even M-x shell-commmand pwd both prompt me for the root password, and then fail.

Despite having turned tramp-verbose to the maximum value, there's not much in my *Messages* buffer after all this:

Creating customization setup...done
Tramp: Opening connection for root@onottra672626p using su...
Tramp: Sending command ‘exec su - root’
Tramp: Waiting for prompts from remote shell...
Tramp: Sending Password
Tramp: Waiting for prompts from remote shell...failed
Tramp: Opening connection for root@onottra672626p using su...failed
Use M-x make-directory RET RET to create the directory and its parents

Tramp: Checking ‘vc-registered’ for /su:root@onottra672626p: /etc/fstab...failed
Use M-x make-directory RET RET to create the directory and its parents
Tramp: Opening connection for root@ using su...
Tramp: Sending command ‘exec su - root’
Tramp: Waiting for prompts from remote shell...
Tramp: Sending Password
Tramp: Waiting for prompts from remote shell...failed
Tramp: Opening connection for root@onottra672626p using su...failed
Quit

This is all on a laptop, I'm not trying to connect to another machine.

Emacs 25.1.91.1, and confirmed again with Emacs 25.2.1.

Any suggestions for further troubleshooting, or is this just a regular bug?

Update

The problem was due to my use of the powerline prompt. The debug-log contained the lines:

08:34:24.189824 tramp-accept-process-output (10) # *tramp/su root@....* run *tramp/su root@....*

[30;41m~ [0;31m [m

The  symbol is provided by powerline. Tramp needs to detect shell prompts, and this unexpected character was confusing it. The fix required modifying the variable tramp-shell-prompt-pattern to include the new symbol:

Original value was: "\(?:^\| \)[^]#$%>\n]*#?[]#$%>] \(\[[0-9;][a-zA-Z] \)"

New new, corrected value is "\(?:^\| \)[^]#$%>\n]*#?[]#$%>] \(\[[0-9;][a-zA-Z] \)"

Answer 1

For the records, this has been discussed on the Emacs bugtracker. It's not a Tramp error, but an unusual prompt from the remote machine.