ensure world-readability for trusted.gpg in postinst

apt-key creates trusted.gpg if it needs it with 644 nowadays, but before
it ensured this, it was gpg creating it, which gives it by default 600.
Not a problem as long as our gpgv is run as root, but now that we drop
privileges we have to ensure that we can also read trusted.gpg files
created by earlier apt-key versions.

Closes: 647001

debian/apt.postinst

@@ -22,6 +22,10 @@ case "$1" in
 		    rm -f "$keyring"
 		fi
 	    done
+	    # apt-key before 0.9.8.2 could create 0600 trusted.gpg file
+	    if test -e /etc/apt/trusted.gpg ; then
+	        chmod -f 0644 /etc/apt/trusted.gpg || true
+	    fi
 	fi
 
 	if dpkg --compare-versions "$2" lt-nl 0.9.9.5; then

test/integration/test-apt-key

@@ -41,7 +41,14 @@ gpg:              unchanged: 1' aptkey --fakeroot update
 
 	testaptkeys 'pub   2048R/DBAC8DAE 2010-08-18'
 
+	testsuccess test ! -e rootdir/etc/apt/trusted.gpg
 	testsuccess aptkey --fakeroot add ./keys/rexexpired.pub
+	msgtest 'Check if trusted.gpg is created with permissions set to' '0644'
+	if [ "$(stat -c '%a' rootdir/etc/apt/trusted.gpg )" = '644' ]; then
+		msgpass
+	else
+		msgfail
+	fi
 
 	testaptkeys 'pub   2048R/27CE74F9 2013-07-12 [expired: 2013-07-13]
 pub   2048R/DBAC8DAE 2010-08-18'