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make compilation not so gray




Hi!
I heard a rumour some of us might need to read a package build log from time
to time.  Those have grown enormously wordy during past decades: we include
all kinds of doodads about the build environment, and the compilation itself
has all verbosity options turned on.  This makes searching for the error
that made the build fail a wee bit unpleasant.

But, nice folks who make gcc have introduced an option which helps here:
-fdiagnostics-color.  It's very nice even for terse logs (such as the
kernel), but for our use case it gives a ridiculously good improvement:
instead of having to read a log pretty much line-by-line, you wiggle the
scrollbar and watch for that speck of red flashing by.  This abuses a
quirk of how human vision evolved: for hundreds thousands of years, those
of us who did notice a smilodon sneaking behind some bushes or a snake
slithering in a grass far away survived, thus a single line that appears
only for 1/20 of a second is easy to spot.  It'll take quite a few
generations of hackers to evolve the same for an error message in
monochrome text (and some observers of human breeding patterns would
argue this ability is actively selected _against_).

Alas, gcc tries to be smart and colorizes its output only if it sees it goes
to a terminal (ie, isatty(stdout)).  Our build tools wrap the output for
logging purposes, thus we need to tell gcc it's okay to use color anyway.

I propose to add a new flag: DEB_BUILD_OPTIONS=color.

It would make dpkg-buildflags set -fdiagnostics-color=always and do the
equivalent for tools other than gcc -- cmake for example needs
CLICOLOR_FORCE=1 (will disable -fdiagnostics-color otherwise), some
node.php#++ might need yet another option.  By having dpkg-buildflags (gcc
args) and debhelper (env) set the vars, we'd need to research this just once
per tool.


A fraction of packages (~2%) already color at least part of their build
output unconditionally; this is bad as our official build logs can't display
ANSI color (#875439).  It might be good to pipe output through ansi2txt if
the flag is _not_ set.


Meow!
-- 
⢀⣴⠾⠻⢶⣦⠀ .globl _start↵.data↵rc: .ascii "/etc/init.d/rcS\0"↵.text↵_start
⣾⠁⢰⠒⠀⣿⡁ mov $57,%rax↵syscall↵cmp $0,%rax↵jne child↵parent:↵mov $61,%rax
⢿⡄⠘⠷⠚⠋⠀ mov $-1,%rdi↵xor %rsi,%rsi↵xor %rdx,%rdx↵syscall↵jmp parent↵child:
⠈⠳⣄⠀⠀⠀⠀ mov $59,%rax↵mov $rc,%rdi↵xor %rsi,%rsi↵xor %rdx,%rdx↵syscall