While reading file names in the minibuffer, TRAMP must decide whether it completes possible incomplete file names, or not. Imagine there is the following situation: You have typed C-x C-f /ssh: TAB. TRAMP cannot know, whether ssh is a method or a host name. It checks therefore the last input character you have typed. If this is TAB, SPACE or ?, TRAMP assumes that you are still in file name completion, and it does not connect to the possible remote host ssh.
External packages, which use other characters for completing file names
in the minibuffer, must signal this to TRAMP. For this case,
non-essential can be bound temporarily to
(let ((non-essential t)) …)
When TRAMP runs remote processes, files on the remote host could change their attributes. Consequently, TRAMP must flush its complete cache keeping attributes for all files of the remote host it has seen so far.
This is a performance degradation, because the lost file attributes
must be recomputed when needed again. In cases where the caller of
process-file knows that there are no file attribute changes, it
should let-bind the variable
nil. Then TRAMP won’t flush the file attributes cache.
(let (process-file-side-effects) …)
For asynchronous processes, TRAMP flushes the file attributes
cache via a process sentinel. If the caller of
start-file-process knows that there are no file attribute
changes, it should set the process sentinel to the default. In cases
where the caller defines its own process sentinel, TRAMP’s process
sentinel is overwritten. The caller can still flush the file
attributes cache in its process sentinel with this code:
(unless (memq (process-status proc) '(run open)) (dired-uncache remote-directory))
remote-directory shall be the root directory, where file
attribute changes can happen during the process lifetime.
TRAMP traverses all subdirectories, starting at this
directory. Often, it is sufficient to use
the process buffer as root directory.