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[bug #51495] Notice when a rule changed, so target needs rebuilding

From: Nadav Har'El
Subject: [bug #51495] Notice when a rule changed, so target needs rebuilding
Date: Tue, 18 Jul 2017 03:07:32 -0400 (EDT)
User-agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:53.0) Gecko/20100101 Firefox/53.0


                 Summary: Notice when a rule changed, so target needs
                 Project: make
            Submitted by: nyh
            Submitted on: Tue 18 Jul 2017 07:07:31 AM UTC
                Severity: 3 - Normal
              Item Group: Enhancement
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
       Component Version: None
        Operating System: Any
           Fixed Release: None
           Triage Status: None



It's no secret that in recent years, many alternatives to the classic "make"
have come up. Most of them differ from "make" in non-interesting ways, which
reflects more on their author's preferences than they are actual improvements.
However, there are cases where such alternative build systems actually came up
with new ideas which can be adopted by "make" without rewriting it. The idea I
propose here comes from the "ninja" build system:

The problem this idea is trying to solve is that sometimes you modify the
Makefile and change one or many of the rules (either directly or indirectly by
modifying some variable), and running "make" does not rebuild anything because
none of the dependency files changed. Sure, one can do "make clean; make". But
it's very easy to forget and end up with a broken build.

What ninja does is to keep a file (.ninja_log in the build directory) which
lists each of the targets that were built, with a hash of the rule that was
used to build it. When building again, we consider a target as outdated not
only if one of its dependencies is changed, but also if the target's rule no
longer matches the saved hash.


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