ci: Check the API visibility

Check we don't accidentally add private headers to public ones, and that
all public headers are included in the public handy.h. We do so by
running the newly added lint/api-visibility.sh script during the build
stage. This script can be run locally too.

.gitlab-ci.yml

@@ -20,6 +20,12 @@ before_script:
     meson ${BUILD_OPTS} . _build  &&
     ninja -C _build
 
+api-visibility:
+  <<: *tags
+  stage: build
+  script:
+    - lint/api-visibility.sh
+
 build-debian-gcc:
   image: debian:buster
   <<: *tags

lint/api-visibility.sh

+#!/bin/bash
+
+# Check that private headers aren't included in public ones.
+if grep "include.*private.h" $(find src -name '*\.h' -not -regex '.*private.h');
+then
+  echo "Private headers shouldn't be included in public ones."
+  exit 1
+fi
+
+# Check that handy.h contains all the public headers.
+for header in $(find src -name '*.h' -not -regex '.*private.h' -not -regex '.*/handy.h');
+do
+  if ! grep -q "$(basename $header)" src/handy.h;
+  then
+    echo "The public header" $(basename $header) "should be included in handy.h."
+    exit 1
+  fi
+done