> This is a sanity check. It avoids to hang in a blocked connection,
> because this special command is surrounded by a timeout of 10 sec. Other
> commands w/o this protection could hang forever. See also the comment in
> tramp-maybe-open-connection.
Maybe you could then single out commands that are supposed to return result nearly-instantly anyway, e.g. `stat' or `test' (though the latter I have suggested to replace with `stat' before). If they fail to produce the result in 10 seconds, the connection can be assumed dead, just as with `echo are you awake'. Commands that involve transmitting (potentially) large amount of data this or that way can work as now. So, you would usually avoid one extra command (which with a high ping means sth. like 0.2 s, already pretty noticeable in UI) and achieve the same result.
Sanity checks are good. but not if they visibly slow down normal operation.
> If a caller can live w/o a valid result of file-exists-p, it shouldn't
> call it. Everything else is too sophisticated and good for trouble, I believe.
It might be easier to convince the rest of Emacs developers to use an extra variable to possibly skip `file-exists-p' on only remote files than to remove the call altogether. Though in either case it is required that `file-missing' error is handled properly which is, incidentally, easier to test if the call to `file-exists-p' is dropped. A target here would be `insert-file-contents' and whatever it calls, and this is a part of Emacs, not some external library.
Yet another idea: removing temporary should be done asynchronously (I haven't checked, maybe it's already the case, but likely not). The caller doesn't really care about call result and even if it has succeeded. Again, not sure if this is easy to achieve interface-wise, maybe it's from a higher level. But I suspect it's something like `with-temp-file' and can be optimized: one-time optimization are worth it even if that costs readability.
Paul