Cirrus: Fix gate task + make lint|validate

A recent Makefile change (4ec893a748) removed a side-effect necessary
for 'make validation' to pass under automation.  Making things worse,
change 12bd7e927c was found upon investigation to always point at
the latest upstream HEAD.  However, this is rarely a fork-point for
pull-requests.  Further investigation showed the built-in Cirrus-CI,
golang-based git does not obtain sufficient data for the Makefile
command `git merge-base HEAD $${DEST_BRANCH:-master}` to function
properly (in the context of the gate container).

Fix this by customizing the clone operation and slightly adjust the
Makefile command to function as intended in the gate container.  Also
add checks to the validate and lint targets which validate the
variable EPOCH_TEST_COMMIT value is never an empty string or whitespace.

Signed-off-by: Chris Evich <cevich@redhat.com>

.cirrus.yml

@@ -93,7 +93,6 @@ gating_task:
         CIRRUS_WORKING_DIR: "/usr/src/libpod"
         GOPATH: "/go"
         GOSRC: "/go/src/github.com/containers/libpod"
-        EPOCH_TEST_COMMIT: "${CIRRUS_BASE_SHA}"
 
     # Runs within Cirrus's "community cluster"
     container:
@@ -103,6 +102,16 @@ gating_task:
 
     timeout_in: 20m
 
+    # Custom cloning is required to satisfy lint/validation needs
+    clone_script: |
+        git clone --recursive --branch=$DEST_BRANCH https://x-access-token:${CIRRUS_REPO_CLONE_TOKEN}@github.com/${CIRRUS_REPO_FULL_NAME}.git $CIRRUS_WORKING_DIR
+        git remote update origin
+        if [[ -n "$CIRRUS_PR" ]]; then # running for a PR
+            git fetch origin pull/$CIRRUS_PR/head:pull/$CIRRUS_PR
+            git checkout pull/$CIRRUS_PR
+        fi
+        git reset --hard $CIRRUS_CHANGE_IN_REPO
+
     # Don't bother going further if something is down
     networking_script: '${CIRRUS_WORKING_DIR}/${SCRIPT_BASE}/networking.sh'
 

Makefile

@@ -3,7 +3,7 @@ export GOPROXY=https://proxy.golang.org
 
 GO ?= go
 DESTDIR ?=
-EPOCH_TEST_COMMIT ?= $(shell git merge-base HEAD $${DEST_BRANCH:-master})
+EPOCH_TEST_COMMIT ?= $(shell git merge-base $${DEST_BRANCH:-master} HEAD)
 HEAD ?= HEAD
 CHANGELOG_BASE ?= HEAD~
 CHANGELOG_TARGET ?= HEAD
@@ -132,6 +132,9 @@ for cmd in sorted(cmds):
 endef
 export PRINT_HELP_PYSCRIPT
 
+# Dereference variable $(1), return value if non-empty, otherwise raise an error.
+err_if_empty = $(if $(strip $($(1))),$(strip $($(1))),$(error Required variable $(1) value is undefined, whitespace, or empty))
+
 .PHONY: help
 help:
 	@$(PYTHON) -c "$$PRINT_HELP_PYSCRIPT" < $(MAKEFILE_LIST)
@@ -146,6 +149,7 @@ endif
 
 .PHONY: lint
 lint: golangci-lint
+	@echo "Linting vs commit '$(call err_if_empty,EPOCH_TEST_COMMIT)'"
 ifeq ($(PRE_COMMIT),)
 	@echo "FATAL: pre-commit was not found, check https://pre-commit.com/ about installing it." >&2
 	@exit 2
@@ -531,6 +535,7 @@ uninstall:
 
 .PHONY: .gitvalidation
 .gitvalidation: .gopathok
+	@echo "Validating vs commit '$(call err_if_empty,EPOCH_TEST_COMMIT)'"
 	GIT_CHECK_EXCLUDE="./vendor:docs/make.bat" $(GOBIN)/git-validation -run DCO,short-subject,dangling-whitespace -range $(EPOCH_TEST_COMMIT)..$(HEAD)
 
 .PHONY: install.tools