point users at the log file on error, try to show a package name too

When installation or configuration of a package fails, output a message
that points the user to the log file[1]. Attempt to grep out the first
package that dpkg failed on and show its name too. Closes: #472704

Sample:

W: Failure while configuring base packages.  This will be re-attempted up to five times.
W: See sid/debootstrap.log for details (possibly the package libept1 is at fault)

This relies on the LANG=C currently set for the debootstrap run.
Changes to the "dpkg: error processing $PACKAGE" message would break
the package name extractor, but it'd fall back to not showing a
package name.

This adds cut to the set of utilities debootstrap uses, which shouldn't
be a problem.

[1] In d-i, debootstrap doesn't know where the log file is, but then
this message goes to the same syslog file, so I didn't add any special
handling for this case.

1 files changed, 12 insertions, 0 deletions

diff --git a/functions b/functions
index edc6332..ad7b211 100644
--- a/functions
+++ b/functions
@@ -877,6 +877,18 @@ in_target_failmsg () {
 	shift; shift; shift
 	if ! $CHROOT_CMD "$@"; then
 		warning "$code" "$msg" "$arg"
+		# Try to point user at actual failing package.
+		msg="See %s for details"
+		if [ -e "$TARGET/debootstrap/debootstrap.log" ]; then
+			arg="$TARGET/debootstrap/debootstrap.log"
+			local pkg="$(grep '^dpkg: error processing ' "$TARGET/debootstrap/debootstrap.log" | head -n 1 | cut -d ' ' -f 4)"
+			if [ -n "$pkg" ]; then
+				msg="$msg (possibly the package $pkg is at fault)"
+			fi
+		else
+			arg="the log"
+		fi
+		warning "$code" "$msg" "$arg"
 		return 1
 	fi
 	return 0