Bug#917478: popularity-contest: Improve performance (4x faster)

[Benoît <bdejean@gmail.com>]

Package: popularity-contest
Version: 1.67
Severity: minor
Tags: patch

Dear Maintainer,

For each installed packages, popcon globs the complete list of files
in /var/lib/dpkg/info.
This is very slow as I noticed that popcon takes more than a minute of CPU
time on my modest laptop, which is enough to start the fan.

I'm attaching a patch that lists only once /var/lib/dpkg/info and associates
each .list file with a package.

I don't see any difference in /usr/sbin/popularity-contest output.
And the CPU time goes from 1min08s to 0min14s.

-- System Information:
Debian Release: buster/sid
  APT prefers unstable-debug
  APT policy: (500, 'unstable-debug'), (500, 'unstable'), (500, 'testing'), (1, 'experimental-debug'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.17.0-3-amd64 (SMP w/2 CPU cores)
Locale: LANG=fr_FR.UTF-8, LC_CTYPE=fr_FR.UTF-8 (charmap=UTF-8), LANGUAGE=fr_FR.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages popularity-contest depends on:
ii  debconf [debconf-2.0]  1.5.69
ii  dpkg                   1.19.2

Versions of packages popularity-contest recommends:
ii  cron [cron-daemon]                         3.0pl1-130
ii  exim4-daemon-light [mail-transport-agent]  4.91-8+b1
ii  gnupg                                      2.2.10-3

Versions of packages popularity-contest suggests:
ii  anacron   2.3-27
pn  tor       <none>
pn  torsocks  <none>

-- debconf-show failed

--- /usr/sbin/popularity-contest	2018-08-09 20:41:19.000000000 +0200
+++ popularity-contest	2018-12-27 22:51:27.710362640 +0100
@@ -119,6 +119,19 @@
   close DIVERSIONS;
 }
 
+my %pkgs_files = ();
+
+if (opendir(my $DPKG_DB, $dpkg_db))
+{
+    for my $e (readdir($DPKG_DB)) {
+	if ($e =~ m/^([^:]+) .*? \. list$/x) {
+	    $pkgs_files{$1} ||= [];
+	    push @{$pkgs_files{$1}}, "$dpkg_db/$e";
+	}
+    }
+    closedir($DPKG_DB);
+}
+
 # Read dpkg database of installed packages
 open PACKAGES, "dpkg-query --show --showformat='\${status} \${package}\\n'|";
 while (<PACKAGES>)
@@ -128,7 +141,7 @@
   my $bestatime = undef;
   my $list;
   $popcon{$pkg}=[0,0,$pkg,"<NOFILES>"];
-  foreach ("$dpkg_db/$pkg.list", glob("$dpkg_db/$pkg:*.list"))
+  foreach (@{$pkgs_files{$pkg}})
   {
     open FILES, $_ or next;
     while (<FILES>)