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Comment by JdeBP | Hacker News Reader

Those arguments are not enough. For dependencies to be fully reliable, the compiler needs to emit information about where headers were not found, in addition to where they were found. Otherwise headers introduced earlier into search paths after a build do not properly trigger a rebuild.

* http://jdebp.eu./FGA/introduction-to-redo.html#CompilerDefic...

* http://cr.yp.to/redo/honest-nonfile.html

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The following sentence about my redo implementation is wrong:

> In 2014, Nils Dagsson Moskopp re-implemented Pennarun redo, retargetting it at the Bourne Again shell and BusyBox.

I targeted the Bourne Shell (sh), not the Bourne Again Shell (bash). Also, my redo implementation contains redo-dot that paints a dependency-tree – I have not seen this otherwere.

You only put /bin/sh as the interpreter. You still used quite a number of Bourne Again shell constructs in the scripts themselves. Some are not in POSIX sh and utilities, some are not in the Bourne shell. (The Bourne shell is not conformant with POSIX sh, so targetting POSIX sh is not the same as targetting the Bourne shell.)

These include amongst others local, $(), >&-, the -a and -o operators to test, and command. (You also failed to guard against variables expanding to operators in the test command, but that is a common general error rather than a Bashism.)

Beware of testing against /bin/sh, even the BusyBox one, and assuming that that means POSIX compliance, let alone Bourne shell compatibility. Even the OpenBSD Korn shell running as /bin/sh or the Debian Almquist shell running as /bin/sh will silently admit some non-POSIX constructs.

While you are right about POSIX problems (like using “local”) I actually targeted Dash and older versions of BusyBox – not Bash.

I plan to work on POSIX compatibility for my redo implementation.

Honestly, that's just intractable. That can't be solved for the same reason that changes to the makefile itself can't be tracked. Changes to the structure of the build system aren't "dependencies" as commonly understood.

Almost all other systems have this flaw in some way as well. You can fool everything, because a system capable of not being fooled would literally have to reparse and rebuild everything from scratch.

Almost all other systems have this flaw because they require dependencies before a build. If recording dependencies after the build, the entire problem becomes very simple. See here:

http://news.dieweltistgarnichtso.net/posts/redo-gcc-automati...

The example above was of a change to the build structure (include path, specifically) which changes the dependency tree in a way that doesn't involve changes to the files themselves and will be invisible to make or other tools that use file timestamps. I pointed out that this was just one of a whole class of intractable problems with dependency tracking that we all choose to ignore. It can't be fixed, even in principle. The only truly sure way to know you're building correctly is to build from scratch. Everything else is just a heuristic.

Why do you think the problem can not be fixed?

Also, have you looked at the redo tool redo-ifcreate? http://news.dieweltistgarnichtso.net/bin/redo-sh.html

I would like to boast that I have clearly solved an intractable problem, since (as I said) I have tools that happily track this very thing. But the truth is that the idea that it is intractable is rubbish, rather than that I can achieve impossible things before breakfast. (-:

You've switched tack between "not such a big deal" and "can't be solved" in the space of two messages. Both of your extremes are wrong. The truth is that this is simply difficult and needs either improvements to compilers or, as in my case, add-on tools that replicate the compiler's pre-processing and emit both the redo-ifchange and the redo-ifcreate information.

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