diff --git a/bin/ebuild.sh b/bin/ebuild.sh
index 35f4b91..8e702d0 100755
--- a/bin/ebuild.sh
+++ b/bin/ebuild.sh
@@ -90,7 +90,7 @@ __qa_source() {
 __qa_call() {
 	local shopts=$(shopt) OLDIFS="$IFS"
 	local retval
-	"$@"
+	__call-ebuildshell "$@"
 	retval=$?
 	set +e
 	[[ $shopts != $(shopt) ]] &&
@@ -467,6 +467,58 @@ if [[ -n ${QA_INTERCEPTORS} ]] ; then
 	unset BIN_PATH BIN BODY FUNC_SRC
 fi
 
+__call-ebuildshell() {
+	if ! has ebuildshell ${FEATURES}; then
+		"$@"
+		return $?
+	fi
+	(
+		umask 002
+		(
+			(
+				set
+				declare -p
+				declare -fp
+				shopt -p
+				if [[ ${BASH_VERSINFO[0]} == 3 ]]; then
+					export
+				fi
+			) | __filter_readonly_variables --filter-for-subshell
+			echo 'declare -r $PORTAGE_READONLY_METADATA $PORTAGE_READONLY_VARS'
+			echo "export PS1='EBUILD ${PN} $1 \$ '" 
+			echo 'unset ENV'
+			echo "type $1"
+			echo "echo WANTED: $@"
+			echo "trap '"
+			echo "umask 002"
+			echo "("
+			echo "	set"
+			echo "	declare -p"
+			echo "	declare -fp"
+			echo "	shopt -p | grep -v extdebug$"
+			if [[ ${BASH_VERSINFO[0]} == 3 ]]; then
+				echo "	export"
+			fi
+			echo -n ")"
+			echo -n " | __filter_readonly_variables --filter-metadata"
+			echo " > ${T}/ebuildshell-environment-outcome' 0"
+			echo "shopt -u extdebug"
+			echo "trap - DEBUG"
+		) > ${T}/ebuildshell-environment
+		chown ${PORTAGE_USER:-portage}:${PORTAGE_GROUP:-portage} "${T}/ebuildshell-environment" &>/dev/null
+		chmod g+w "${T}/ebuildshell-environment" &>/dev/null
+	)
+
+	rm -f "${T}/ebuildshell-environment-outcome" >&/dev/null
+
+	unset BASH_ENV
+	env -i ${BASH} --rcfile "${T}/ebuildshell-environment"  -i || die
+
+	chown ${PORTAGE_USER:-portage}:${PORTAGE_GROUP:-portage} "${T}/ebuildshell-environment-outcome" &>/dev/null
+	chmod g+w "${T}/ebuildshell-environment-outcome" &>/dev/null
+	source "${T}/ebuildshell-environment-outcome" &>/dev/null
+}
+
 # Subshell/helper die support (must export for the die helper).
 export EBUILD_MASTER_PID=$BASHPID
 trap 'exit 1' SIGTERM
diff --git a/bin/phase-functions.sh b/bin/phase-functions.sh
index 711b721..5f3a8a2 100644
--- a/bin/phase-functions.sh
+++ b/bin/phase-functions.sh
@@ -87,7 +87,7 @@ PORTAGE_MUTABLE_FILTERED_VARS="AA HOSTNAME"
 __filter_readonly_variables() {
 	local x filtered_vars
 	local readonly_bash_vars="BASHOPTS BASHPID DIRSTACK EUID
-		FUNCNAME GROUPS PIPESTATUS PPID SHELLOPTS UID"
+		FUNCNAME GROUPS PIPESTATUS PPID SHELLOPTS UID BASH_VERSINFO"
 	local bash_misc_vars="BASH BASH_.* COLUMNS COMP_WORDBREAKS HISTCMD
 		HISTFILE HOSTNAME HOSTTYPE IFS LINENO MACHTYPE OLDPWD
 		OPTERR OPTIND OSTYPE POSIXLY_CORRECT PS4 PWD RANDOM
@@ -98,12 +98,14 @@ __filter_readonly_variables() {
 	# Untrusted due to possible application of package renames to binpkgs
 	local binpkg_untrusted_vars="CATEGORY P PF PN PR PV PVR"
 	local misc_garbage_vars="_portage_filter_opts"
-	filtered_vars="$readonly_bash_vars $bash_misc_vars
-		$PORTAGE_READONLY_VARS $misc_garbage_vars"
+	filtered_vars="$readonly_bash_vars"
+	if ! has --filter-for-subshell $* ; then
+		filtered_vars="$filtered_vars $bash_misc_vars $PORTAGE_READONLY_VARS $misc_garbage_vars"
+	fi
 
 	# Don't filter/interfere with prefix variables unless they are
 	# supported by the current EAPI.
-	if ___eapi_has_prefix_variables; then
+	if ___eapi_has_prefix_variables && ! has --filter-for-subshell $* ; then
 		filtered_vars+=" ED EPREFIX EROOT"
 	fi
 
@@ -123,7 +125,11 @@ __filter_readonly_variables() {
 			LC_CTYPE LC_MESSAGES LC_MONETARY
 			LC_NUMERIC LC_PAPER LC_TIME"
 	fi
-	if ! has --allow-extra-vars $* ; then
+	if has --filter-metadata $* ; then
+		filtered_vars="$filtered_vars $PORTAGE_READONLY_METADATA"
+	fi
+	if ! has --allow-extra-vars $* &&
+	   ! has --filter-for-subshell $* ; then
 		if [ "${EMERGE_FROM}" = binary ] ; then
 			# preserve additional variables from build time,
 			# while excluding untrusted variables
diff --git a/man/make.conf.5 b/man/make.conf.5
index ab9b44e..45903e3 100644
--- a/man/make.conf.5
+++ b/man/make.conf.5
@@ -345,6 +345,12 @@ exist). Also see the related \fIunmerge\-backup\fR feature.
 Use locks to ensure that unsandboxed ebuild phases never execute
 concurrently. Also see \fIparallel\-install\fR.
 .TP
+.B ebuildshell
+Drop into an interactive shell for each phase function, meant for
+debugging.  Because the shell would normally be used to execute the
+phase function, commands like src_unpack or epatch are available in the
+interactive shell.  Use `exit 1` to terminate the merge.
+.TP
 .B fakeroot
 Enable fakeroot for the install and package phases when a non-root user runs
 the \fBebuild\fR(1) command.
diff --git a/pym/_emerge/AbstractEbuildProcess.py b/pym/_emerge/AbstractEbuildProcess.py
index 31127f4..73031d0 100644
--- a/pym/_emerge/AbstractEbuildProcess.py
+++ b/pym/_emerge/AbstractEbuildProcess.py
@@ -136,6 +136,7 @@ class AbstractEbuildProcess(SpawnProcess):
 			self.fd_pipes = {}
 		null_fd = None
 		if 0 not in self.fd_pipes and \
+			"ebuildshell" not in self.settings.features and \
 			self.phase not in self._phases_interactive_whitelist and \
 			"interactive" not in self.settings.get("PROPERTIES", "").split():
 			null_fd = os.open('/dev/null', os.O_RDONLY)
diff --git a/pym/portage/const.py b/pym/portage/const.py
index 214ede4..725a7d0 100644
--- a/pym/portage/const.py
+++ b/pym/portage/const.py
@@ -159,6 +159,7 @@
 	"distlocks",
 	"downgrade-backup",
 	"ebuild-locks",
+	"ebuildshell",
 	"fail-clean",
 	"fakeroot",
 	"fixlafiles",