bug#42706: 27.0.50; commands breaking on unreachable remote directory

[Ken Raeburn]

A couple commands I've tried have failed if the default directory for a
buffer is remote and the server is unreachable. One was list-buffers
when one of my buffers (not the current one) was visiting a remote file
on a server that was down.  The other was cd, when the current default
directory was remote. In both cases, the file-error error ("Timeout
reached") was raised and not trapped. The cd command did not get to the
point of prompting me for a new directory. Stack traces are below. Looks
like a common failure point is calling abbreviate-file-name, which calls
file-name-case-insensitive-p, which under tramp/scp runs remote
commands, or tries to.

While file-name-case-insensitive-p can't guarantee a reasonable answer
in the case of a failed connection, it looks to me like
abbreviate-file-name could probably fail safe by assuming case
sensitivity and continuing on.

I haven't noticed these problems before trying Emacs 27 pretests, but I
may have just gotten lucky.

Debugger entered--Lisp error: (file-error "Timeout reached, see buffer 
‘*tramp/scp root@thomp...")
  signal(file-error ("Timeout reached, see buffer ‘*tramp/scp root@thomp..."))
  tramp-signal-hook-function(file-error ("Timeout reached, see buffer 
‘*tramp/scp root@thomp..."))
  signal(file-error ("Timeout reached, see buffer ‘*tramp/scp root@thomp..."))
  tramp-maybe-open-connection((tramp-file-name "scp" "root" nil "[host]" nil 
"[path]..." nil))
  tramp-send-command((tramp-file-name "scp" "root" nil "[host]" nil "[path]..." 
nil) "test 0 2>/dev/null; echo tramp_exit_status $?")
  tramp-send-command-and-check((tramp-file-name "scp" "root" nil "[host]" nil 
"[path]..." nil) "test 0")
  tramp-get-test-command((tramp-file-name "scp" "root" nil "[host]" nil 
"[path]..." nil))
  tramp-run-test("-d" "[scp-path]")
  tramp-sh-handle-file-directory-p("[scp-path]")
  apply(tramp-sh-handle-file-directory-p "[scp-path]")
  tramp-sh-file-name-handler(file-directory-p "[scp-path]")
  apply(tramp-sh-file-name-handler file-directory-p "[scp-path]")
  tramp-file-name-handler(file-directory-p "[scp-path]")
  file-directory-p("[scp-path]")
  apply(file-directory-p "[scp-path]")
  tramp-archive-run-real-handler(file-directory-p ("[scp-path]"))
  tramp-archive-file-name-handler(file-name-case-insensitive-p "[scp-path]")
  file-name-case-insensitive-p("[scp-path]")
  abbreviate-file-name("[scp-path]")
  Buffer-menu--pretty-file-name("[scp-path]")
  list-buffers--refresh(nil #<buffer *scratch*>)
  list-buffers-noselect(nil)
  list-buffers(nil)
  funcall-interactively(list-buffers nil)
  call-interactively(list-buffers nil nil)
  command-execute(list-buffers)

Debugger entered--Lisp error: (file-error "Timeout reached, see buffer 
‘*tramp/scp root@thomp...")
  signal(file-error ("Timeout reached, see buffer ‘*tramp/scp root@thomp..."))
  tramp-signal-hook-function(file-error ("Timeout reached, see buffer 
‘*tramp/scp root@thomp..."))
  signal(file-error ("Timeout reached, see buffer ‘*tramp/scp root@thomp..."))
  tramp-maybe-open-connection((tramp-file-name "scp" "root" nil "[host]" nil 
"[path]..." nil))
  tramp-send-command((tramp-file-name "scp" "root" nil "[host]" nil "[path]..." 
nil) "test 0 2>/dev/null; echo tramp_exit_status $?")
  tramp-send-command-and-check((tramp-file-name "scp" "root" nil "[host]" nil 
"[path]..." nil) "test 0")
  tramp-get-test-command((tramp-file-name "scp" "root" nil "[host]" nil 
"[path]..." nil))
  tramp-run-test("-d" "[scp-path]")
  tramp-sh-handle-file-directory-p("[scp-path]")
  apply(tramp-sh-handle-file-directory-p "[scp-path]")
  tramp-sh-file-name-handler(file-directory-p "[scp-path]")
  apply(tramp-sh-file-name-handler file-directory-p "[scp-path]")
  tramp-file-name-handler(file-directory-p "[scp-path]")
  file-directory-p("[scp-path]")
  apply(file-directory-p "[scp-path]")
  tramp-archive-run-real-handler(file-directory-p ("[scp-path]"))
  tramp-archive-file-name-handler(file-name-case-insensitive-p "[scp-path]")
  file-name-case-insensitive-p("[scp-path]")
  abbreviate-file-name("[scp-path]")
  read-file-name-default("Change default directory: " "[scp-path]" "[scp-path]" 
t nil file-directory-p)
  read-file-name("Change default directory: " "[scp-path]" "[scp-path]" t nil 
file-directory-p)
  read-directory-name("Change default directory: " "[scp-path]" "[scp-path]" t)
  byte-code(... [cd-path default-directory #f(compiled-function () #<bytecode 
0x1fd5f0c2e0f3>) nil make-byte-code 0 ... vconcat vector [remove-hook 
minibuffer-setup-hook] 3 ... [remove-hook minibuffer-setup-hook] add-hook 
minibuffer-setup-hook parse-colon-path getenv "CDPATH" "./" read-directory-name 
"Change default directory: " t] 10)
  call-interactively(cd record nil)
  command-execute(cd record)
  execute-extended-command(nil "cd" "cd")
  funcall-interactively(execute-extended-command nil "cd" "cd")
  call-interactively(execute-extended-command nil nil)
  command-execute(execute-extended-command)



In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2019-11-18 built on crash
Repository revision: ed52618e707682e812002c98b99efe4baf43e1ca
Repository branch: master
Windowing system distributor 'Fedora Project', version 11.0.12006000
System Description: Fedora 31 (Workstation Edition)

Recent messages:
Timeout reached, see buffer ‘*tramp/scp 
root@thompson-01.permabit.lab.eng.bos.redhat.com*’ for details
Tramp: Waiting for prompts from remote shell...failed
Tramp: Opening connection for root@thompson-01.permabit.lab.eng.bos.redhat.com 
using scp...failed
Entering debugger...
Mark set [4 times]
Back to top level
Entering debugger...
Back to top level
uncompressing files.el.gz...done
GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars) of 
2019-11-18
Quit
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