since copy() is using syswrite(), we must flush the filehandle before calling it

coverage.sh

@@ -419,8 +419,9 @@ cat << END > shared/test.sh
 #!/bin/sh
 set -eu
 export LC_ALL=C.UTF-8
-$CMD --mode=root --variant=apt --aptopt="Acquire::Check-Valid-Until false" unstable /tmp/debian-unstable $mirror
-echo "Acquire::Check-Valid-Until false;" | cmp /tmp/debian-unstable/etc/apt/apt.conf.d/99mmdebstrap -
+echo 'Acquire::Languages "none";' > config
+$CMD --mode=root --variant=apt --aptopt='Acquire::Check-Valid-Until "false"' --aptopt=config unstable /tmp/debian-unstable $mirror
+printf 'Acquire::Check-Valid-Until "false";\nAcquire::Languages "none";\n' | cmp /tmp/debian-unstable/etc/apt/apt.conf.d/99mmdebstrap -
 rm /tmp/debian-unstable/etc/apt/apt.conf.d/99mmdebstrap
 tar -C /tmp/debian-unstable --one-file-system -c . | tar -t | sort > tar2.txt
 diff -u tar1.txt tar2.txt
@@ -437,8 +438,9 @@ cat << END > shared/test.sh
 #!/bin/sh
 set -eu
 export LC_ALL=C.UTF-8
-$CMD --mode=root --variant=apt --dpkgopt="path-exclude=/usr/share/doc/*" unstable /tmp/debian-unstable $mirror
-echo "path-exclude=/usr/share/doc/*" | cmp /tmp/debian-unstable/etc/dpkg/dpkg.cfg.d/99mmdebstrap -
+echo no-pager > config
+$CMD --mode=root --variant=apt --dpkgopt="path-exclude=/usr/share/doc/*" --dpkgopt=config unstable /tmp/debian-unstable $mirror
+printf 'path-exclude=/usr/share/doc/*\nno-pager\n' | cmp /tmp/debian-unstable/etc/dpkg/dpkg.cfg.d/99mmdebstrap -
 rm /tmp/debian-unstable/etc/dpkg/dpkg.cfg.d/99mmdebstrap
 tar -C /tmp/debian-unstable --one-file-system -c . | tar -t | sort > tar2.txt
 grep -v '^./usr/share/doc/.' tar1.txt | diff -u - tar2.txt

mmdebstrap

@@ -963,6 +963,9 @@ sub setup {
 	open my $fh, '>', "$options->{root}/etc/apt/apt.conf.d/99mmdebstrap" or error "cannot open /etc/apt/apt.conf.d/99mmdebstrap: $!";
 	foreach my $opt (@{$options->{aptopts}}) {
 	    if (-r $opt) {
+		# flush handle because copy() uses syswrite() which bypasses
+		# buffered IO
+		$fh->flush();
 		copy $opt, $fh or error "cannot copy $opt: $!";
 	    } else {
 		print $fh $opt;
@@ -983,6 +986,9 @@ sub setup {
 	open my $fh, '>', "$options->{root}/etc/dpkg/dpkg.cfg.d/99mmdebstrap" or error "cannot open /etc/dpkg/dpkg.cfg.d/99mmdebstrap: $!";
 	foreach my $opt (@{$options->{dpkgopts}}) {
 	    if (-r $opt) {
+		# flush handle because copy() uses syswrite() which bypasses
+		# buffered IO
+		$fh->flush();
 		copy $opt, $fh or error "cannot copy $opt: $!";
 	    } else {
 		print $fh $opt;