hooks/maybe-merged-usr: prepare for the time when usr-is-merged exists only as a virtual package

hooks/maybe-merged-usr/essential00.sh

@@ -15,6 +15,10 @@ case "$ver" in
 		echo "usr-is-merged package from src:usrmerge installed -- not running merged-usr essential hook" >&2
 		exit 0
 		;;
+	'not-installed  ')
+		echo "usr-is-merged was not installed in a previous hook -- not running merged-usr essential hook" >&2
+		exit 0
+		;;
 	*)
 		echo "unexpected situation for package usr-is-merged: $ver" >&2
 		exit 1

hooks/maybe-merged-usr/extract00.sh

@@ -4,12 +4,22 @@ set -eu
 
 env --chdir="$1" APT_CONFIG="$MMDEBSTRAP_APT_CONFIG" apt-get update --error-on=any
 
-# if the usr-is-merged package cannot be installed with apt, do nothing
-if ! env --chdir="$1" APT_CONFIG="$MMDEBSTRAP_APT_CONFIG" apt-cache show --no-all-versions usr-is-merged > /dev/null 2>&1; then
-	echo "no package called usr-is-merged found -- not running merged-usr extract hook" >&2
-	exit 0
+if env --chdir="$1" APT_CONFIG="$MMDEBSTRAP_APT_CONFIG" apt-cache show --no-all-versions usr-is-merged > /dev/null 2>&1; then
+	# if apt-cache exited successfully, then usr-is-merged exists either as
+	# a real or virtual package
+	if env --chdir="$1" APT_CONFIG="$MMDEBSTRAP_APT_CONFIG" apt-cache show --no-all-versions usr-is-merged 2>/dev/null | grep -q "Package: usr-is-merged"; then
+		echo "usr-is-merged found -- running merged-usr extract hook" >&2
+	else
+		# The usr-is-merged must be virtual, so assume that nothing
+		# has to be done. This is the case with Debian Trixie or later
+		# or with Ubuntu Lunar or later
+		echo "usr-is-merged found but not real -- not running merged-usr extract hook" >&2
+		exit 0
+	fi
 else
-	echo "package usr-is-merged found -- running merged-usr extract hook" >&2
+	# if the usr-is-merged package cannot be installed with apt, do nothing
+	echo "no package providing usr-is-merged found -- not running merged-usr extract hook" >&2
+	exit 0
 fi
 
 # resolve the script path using several methods in order:

hooks/maybe-merged-usr/setup00.sh

@@ -4,12 +4,22 @@ set -eu
 
 env --chdir="$1" APT_CONFIG="$MMDEBSTRAP_APT_CONFIG" apt-get update --error-on=any
 
-# if the usr-is-merged package cannot be installed with apt, do nothing
-if ! env --chdir="$1" APT_CONFIG="$MMDEBSTRAP_APT_CONFIG" apt-cache show --no-all-versions usr-is-merged > /dev/null 2>&1; then
-	echo "no package called usr-is-merged found -- not running merged-usr setup hook" >&2
-	exit 0
+if env --chdir="$1" APT_CONFIG="$MMDEBSTRAP_APT_CONFIG" apt-cache show --no-all-versions usr-is-merged > /dev/null 2>&1; then
+	# if apt-cache exited successfully, then usr-is-merged exists either as
+	# a real or virtual package
+	if env --chdir="$1" APT_CONFIG="$MMDEBSTRAP_APT_CONFIG" apt-cache show --no-all-versions usr-is-merged 2>/dev/null | grep -q "Package: usr-is-merged"; then
+		echo "usr-is-merged found -- running merged-usr setup hook" >&2
+	else
+		# The usr-is-merged must be virtual, so assume that nothing
+		# has to be done. This is the case with Debian Trixie or later
+		# or with Ubuntu Lunar or later
+		echo "usr-is-merged found but not real -- not running merged-usr setup hook" >&2
+		exit 0
+	fi
 else
-	echo "package usr-is-merged found -- running merged-usr setup hook" >&2
+	# if the usr-is-merged package cannot be installed with apt, do nothing
+	echo "no package providing usr-is-merged found -- not running merged-usr setup hook" >&2
+	exit 0
 fi
 
 # resolve the script path using several methods in order: