warn if apt-key is used in scripts/its output parsed

apt-key needs gnupg for most of its operations, but depending on it
isn't very efficient as apt-key is hardly used by users – and scripts
shouldn't use it to begin with as it is just a silly wrapper. To draw
more attention on the fact that e.g. 'apt-key add' should not be used in
favor of "just" dropping a keyring file into the trusted.gpg.d
directory this commit implements the display of warnings.

cmdline/apt-key.in

@@ -494,7 +494,7 @@ prepare_gpg_home() {
     # package depends on gnupg or not. We accept recommends here as
     # well as the script hopefully uses apt-key optionally then like e.g.
     # debian-archive-keyring for (upgrade) cleanup did
-    if [ -n "$DPKG_MAINTSCRIPT_PACKAGE" ]; then
+    if [ -n "$DPKG_MAINTSCRIPT_PACKAGE" ] && [ -z "$APT_KEY_DONT_WARN_ON_DANGEROUS_USAGE" ]; then
 	if ! dpkg-query --show --showformat '${Pre-Depends}${Depends}${Recommends}\n' "$DPKG_MAINTSCRIPT_PACKAGE" 2>/dev/null | grep -q gnupg; then
 	    cat >&2 <<EOF
 Warning: The $DPKG_MAINTSCRIPT_NAME maintainerscript of the package $DPKG_MAINTSCRIPT_PACKAGE
@@ -554,12 +554,25 @@ exec '$(escape_shell "${GPG_EXE}")' --ignore-time-conflict --no-options --no-def
     fi
 }
 
+warn_on_script_usage() {
+    if [ -n "$APT_KEY_DONT_WARN_ON_DANGEROUS_USAGE" ]; then
+	return
+    fi
+    # (Maintainer) scripts should not be using apt-key
+    if [ -n "$DPKG_MAINTSCRIPT_PACKAGE" ]; then
+	echo >&2 "Warning: apt-key should not be used in scripts (called from $DPKG_MAINTSCRIPT_NAME maintainerscript of the package ${DPKG_MAINTSCRIPT_PACKAGE})"
+    elif [ ! -t 1 ]; then
+	echo >&2 "Warning: apt-key output should not be parsed (stdout is not a terminal)"
+    fi
+}
+
 if [ "$command" != 'help' ] && [ "$command" != 'verify' ]; then
     prepare_gpg_home
 fi
 
 case "$command" in
     add)
+	warn_on_script_usage
 	requires_root
 	setup_merged_keyring
 	aptkey_execute "$GPG" --quiet --batch --import "$@"
@@ -567,11 +580,13 @@ case "$command" in
 	aptkey_echo "OK"
         ;;
     del|rm|remove)
+	# no script warning here as removing 'add' usage needs 'del' for cleanup
 	requires_root
 	foreach_keyring_do 'remove_key_from_keyring' "$@"
 	aptkey_echo "OK"
         ;;
     update)
+	warn_on_script_usage
 	requires_root
 	setup_merged_keyring
 	update
@@ -584,13 +599,16 @@ case "$command" in
 	merge_back_changes
 	;;
     list|finger*)
+	warn_on_script_usage
 	foreach_keyring_do 'run_cmd_on_keyring' --fingerprint "$@"
 	;;
     export|exportall)
+	warn_on_script_usage
 	merge_all_trusted_keyrings_into_pubring
 	aptkey_execute "$GPG_SH" --keyring "${GPGHOMEDIR}/pubring.gpg" --armor --export "$@"
 	;;
     adv*)
+	warn_on_script_usage
 	setup_merged_keyring
 	aptkey_echo "Executing: $GPG $*"
 	aptkey_execute "$GPG" "$@"

doc/apt-key.8.xml

@@ -13,7 +13,7 @@
    &apt-email;
    &apt-product;
    <!-- The last update date -->
-   <date>2015-10-15T00:00:00Z</date>
+   <date>2016-07-01T00:00:00Z</date>
  </refentryinfo>
 
  <refmeta>
@@ -36,6 +36,15 @@
    by apt to authenticate packages.  Packages which have been
    authenticated using these keys will be considered trusted.
    </para>
+   <para>
+   Note that if usage of <command>apt-key</command> is desired the additional
+   installation of the GNU Privacy Guard suite (packaged in
+   <package>gnupg</package>) is required. For this reason alone the programatic
+   usage (especially in package maintainerscripts!) is strongly discouraged.
+   Further more the output format of all commands is undefined and can and does
+   change whenever the underlying commands change. <command>apt-key</command> will
+   try to detect such usage and generates warnings on stderr in these cases.
+   </para>
 </refsect1>
 
 <refsect1><title>Commands</title>
@@ -53,6 +62,12 @@
      verified to belong to the owner of the repositories they claim to be for
      otherwise the &apt-secure; infrastructure is completely undermined.
      </para>
+     <para>
+       Instead of using this command a keyring can be placed directly in the
+       <filename>/etc/apt/trusted.gpg.d/</filename> directory with a descriptive name
+       (same rules for filename apply as for &apt-conf; files) and "<literal>gpg</literal>"
+       as file extension.
+     </para>
      </listitem>
      </varlistentry>
 

test/integration/framework

@@ -403,6 +403,7 @@ EOF
 	fi
 	echo "Acquire::https::CaInfo \"${TMPWORKINGDIRECTORY}/rootdir/etc/webserver.pem\";" > rootdir/etc/apt/apt.conf.d/99https
 	echo "Apt::Cmd::Disable-Script-Warning \"1\";" > rootdir/etc/apt/apt.conf.d/apt-binary
+	export APT_KEY_DONT_WARN_ON_DANGEROUS_USAGE=no
 	echo 'Acquire::Connect::AddrConfig "false";' > rootdir/etc/apt/apt.conf.d/connect-addrconfig
 
 	configcompression '.' 'gz' #'bz2' 'lzma' 'xz'

test/integration/test-apt-key-used-in-maintainerscript

@@ -5,6 +5,7 @@ TESTDIR="$(readlink -f "$(dirname "$0")")"
 . "$TESTDIR/framework"
 
 setupenvironment
+unset APT_KEY_DONT_WARN_ON_DANGEROUS_USAGE
 configarchitecture 'native'
 configdpkgnoopchroot
 
@@ -14,7 +15,7 @@ buildingpkg() {
 	setupsimplenativepackage "$PKG" 'native' '1' 'unstable' "$@"
 	BUILDDIR="incoming/${PKG}-1"
 	echo '#!/bin/sh
-apt-key list' > "${BUILDDIR}/debian/postinst"
+apt-key list >/dev/null' > "${BUILDDIR}/debian/postinst"
 	buildpackage "$BUILDDIR" 'unstable' 'main' 'native'
 	rm -rf "$BUILDDIR"
 }
@@ -27,8 +28,15 @@ insertinstalledpackage 'unrelated' 'native' '1'
 insertinstalledpackage 'gnupg' 'native' '1'
 
 testsuccess apt install aptkeyuser-depends -y
-testfailure grep '^Warning: This will BREAK' rootdir/tmp/testsuccess.output
+cp rootdir/tmp/testsuccess.output apt.output
+testfailure grep '^Warning: This will BREAK' apt.output
+testsuccess grep '^Warning: apt-key' apt.output
 
 testsuccess apt install aptkeyuser-nodepends -y
 cp rootdir/tmp/testsuccess.output apt.output
 testsuccess grep '^Warning: This will BREAK' apt.output
+testsuccess grep '^Warning: apt-key' apt.output
+
+testsuccess aptkey list
+cp rootdir/tmp/testsuccess.output aptkey.list
+testsuccess grep '^Warning: apt-key' aptkey.list