add --keyring option as a shorthand for --aptopt='Dir::Etc::Trusted...

coverage.sh

@@ -52,7 +52,7 @@ if [ ! -e shared/mmdebstrap ] || [ mmdebstrap -nt shared/mmdebstrap ]; then
 fi
 
 starttime=
-total=108
+total=109
 i=1
 
 print_header() {
@@ -910,6 +910,25 @@ else
 	./run_null.sh SUDO
 fi
 
+print_header "mode=root,variant=apt: test --keyring"
+cat << END > shared/test.sh
+#!/bin/sh
+set -eu
+export LC_ALL=C.UTF-8
+echo 'Acquire::Languages "none";' > config
+$CMD --mode=root --variant=apt --aptopt='Acquire::Check-Valid-Until "false"' --keyring=/usr/share/keyrings/debian-archive-keyring.gpg --keyring=/usr/share/keyrings/ --aptopt=config $DEFAULT_DIST /tmp/debian-chroot $mirror
+cat /tmp/debian-chroot/etc/apt/apt.conf.d/99mmdebstrap
+printf 'Acquire::Check-Valid-Until "false";\nDir::Etc::Trusted "/usr/share/keyrings/debian-archive-keyring.gpg";\nDir::Etc::TrustedParts "/usr/share/keyrings/";\nAcquire::Languages "none";\n' | cmp /tmp/debian-chroot/etc/apt/apt.conf.d/99mmdebstrap -
+rm /tmp/debian-chroot/etc/apt/apt.conf.d/99mmdebstrap
+tar -C /tmp/debian-chroot --one-file-system -c . | tar -t | sort | diff -u tar1.txt -
+rm -r /tmp/debian-chroot
+END
+if [ "$HAVE_QEMU" = "yes" ]; then
+	./run_qemu.sh
+else
+	./run_null.sh SUDO
+fi
+
 print_header "mode=root,variant=apt: test --dpkgopt"
 cat << END > shared/test.sh
 #!/bin/sh

mmdebstrap

@@ -1819,6 +1819,20 @@ sub main() {
 	'mode=s' => \$options->{mode},
 	'dpkgopt=s@' => \$options->{dpkgopts},
 	'aptopt=s@' => \$options->{aptopts},
+	'keyring=s' => sub {
+	    my ($opt_name, $opt_value) = @_;
+	    if ($opt_value =~ /"/) {
+		error "apt cannot handle paths with double quotes";
+	    }
+	    if (! -e $opt_value) {
+		error "keyring \"$opt_value\" does not exist";
+	    }
+	    if (-d $opt_value) {
+		push @{$options->{aptopts}}, "Dir::Etc::TrustedParts \"$opt_value\"";
+	    } else {
+		push @{$options->{aptopts}}, "Dir::Etc::Trusted \"$opt_value\"";
+	    }
+	},
 	's|silent' => sub { $verbosity_level = 0; },
 	'q|quiet' => sub { $verbosity_level = 0; },
 	'v|verbose' => sub { $verbosity_level = 2; },
@@ -2745,6 +2759,19 @@ Example: Minimizing the number of packages installed from experimental
     --aptopt='APT::Solver "aspcud"'
     --aptopt='APT::Solver::aspcud::Preferences "-count(solution,APT-Release:=/a=experimental/),-removed,-changed,-new"'
 
+=item B<--keyring>=I<file>|I<directory>
+
+A shorthand for using C<--aptopt='Dir::Etc::Trusted "file"'> or
+C<-aptopt='Dir::Etc::TrustedParts "directory"'> when passing a file or
+directory to the B<--keyring> option, respectively. B<mmdebstrap> will add the
+right keyring for the given I<SUITE> if it knows about the distribution and if
+the keyring is installed in a path known by B<mmdebstrap>, usually
+F</usr/share/keyrings>. If B<mmdebstrap> does not know or cannot find the
+right keyring for the given I<SUITE> it will only know about the keys that apt
+on the host system knows about. If you want to prevent B<mmdebstrap> from
+choosing the right keyring for you for known values of I<SUITE>, choose an
+arbitrary value for I<SUITE> and specify the right apt line manually.
+
 =item B<--dpkgopt>=I<option>|I<file>
 
 Pass arbitrary I<option>s to dpkg. Will be added to