Re: [PATCH] git-difftool-helper.sh: learn a new way skip to save point

Sorry, but a not-yet-written reply went out by accident; please discard it.

Funny capitalization "In"?

It makes it hard to lack SP after punctuation like '.', ',', and ':'.

I think the idea sounds good. Admittedly I do not use difftool myself, so I do not even know if and how the current end user experience is so bad to require a patch like this (e.g. I do not know how "unexpected exit" is "unexpected"---isn't it the end user initiated action to "quit", or does the tool crash or something?).

So I won't be the best qualified person to judge if the solution presented is the best one for the problem.

    $ git shortlog --no-merges git-diff-helper.sh

might be a good way to find whom to ask for review and help.

Having said that, I do have one opinion on the "skip-to" filename. I do not think it is wise to call it after the purpose you want to use it for (i.e. "I want to use it to skip to the recorded position"). Instead, if the file records "the last visited position", it is better to name it after that (e.g. "difftool-last-position". If it records "the next file to be visited", then "difftool-next-file" may be a good name).

The reason is because your first design may be to visit the file the user was visiting before the "crash" happened, but you may later want to revise the design to allow the user to say "start at one file before the file I was visiting" etc. The location recorded in the file may still be used to decide where the code skips to when restarting, but no longer exactly where the code "skips to". If you name it after what it is, not what it is (currently) used for, the design would become clearer.

You can avoid the TOCTTOU race by

		if SAVE_POINT=$(cat 2>/dev/null "$GIT_DIFFTOOL_SKIP_TO_FILE")
		then

but that wouldn't probably matter in this application.

Think what happens if the file is corrupt and SAVE_POINT_NUM has (1) an empty string, (2) garbage that has $IFS whitespace, (3) non number, in it. At least, quoting the variable inside double-quotes, i.e. "$SAVE_POINT_NUM", would help an error condition reported correctly at the runtime.

A bit funny language.  Isn't the code clear enough without this comment?

No need to quote the constant "1"; quoting the variable side may be a good practice, even though I think in this codepath we know GIT_DIFF_PATH_COUNTER is a well-formatted number.

"Skip" is probably an implementation detail that the user does not have to know. "Do you want to start from the last file you were viewing?", perhaps?

Have this code been tested? I think "test" is missing after the "||", and I am not quite sure what you are trying to check with "test !$SAVE_POINT_NUM", either. The "test" utility, when given a non-operator string (like "!23" this one is checking when the last visited path was the 23rd one), returns true if the string is not an empty string, and by definition a string made by appending anything after "!" would not be empty, so the entire "|| $SAVE_POINT_NUM ..." have been skipped in your test, I think.

Is writing the current position to the file unconditionally good enough? After all, we are about to go interactive with the user, so the body of this "if" statement won't be performance critical in any sense, no? Or is there something more subtle going on and correctness of the code depends on this condition? I cannot quite tell.

		echo "$GIT_DIFF_PATH_COUNTER" >"$GIT_DIFFTOOL_SKIP_TO_FILE"

cf. Documentation/CodingGuidelines

 - Redirection operators should be written with space before, but no
   space after them.  In other words, write 'echo test >"$file"'
   instead of 'echo test> $file' or 'echo test > $file'.  Note that
   even though it is not required by POSIX to double-quote the
   redirection target in a variable (as shown above), our code does so
   because some versions of bash issue a warning without the quotes.

	(incorrect)
	cat hello > world < universe
	echo hello >$world

	(correct)
	cat hello >world <universe
	echo hello >"$world"

Wouldn't it be simpler to clear when we have reached at the end, i.e.

	if test "$GIT_DIFF_PATH_COUNTER" -eq "$GIT_DIFF_PATH_TOTAL"
	then
		rm -f "$GIT_DIFFTOOL_SKIP_TO_FILE"
	fi

Thanks.