Don't linkcheck external web links in PR CI

Author: jieyouxu

ci/linkcheck.sh

@@ -18,6 +18,7 @@ fi
 if [ "$GITHUB_EVENT_NAME" = "schedule" ] ; then # running in scheduled job
   FLAGS=""
   USE_TOKEN=1
+  FOLLOW_WEB_LINKS="true"
 
   echo "Doing full link check."
 elif [ "$GITHUB_EVENT_NAME" = "pull_request" ] ; then # running in PR CI build
@@ -26,13 +27,18 @@ elif [ "$GITHUB_EVENT_NAME" = "pull_request" ] ; then # running in PR CI build
     exit 1
   fi
 
+  # External links should not block PR CI because they can fail due to network
+  # conditions or server conditions of the external links, which is beyond the
+  # control of contributors.
+  FOLLOW_WEB_LINKS="false"
   CHANGED_FILES=$(git diff --name-only $BASE_SHA... | sed 's#^src/##' | tr '\n' ' ')
   FLAGS="--no-cache -f $CHANGED_FILES"
   USE_TOKEN=1
 
   echo "Checking files changed since $BASE_SHA: $CHANGED_FILES"
 else # running locally
   COMMIT_RANGE=master...
+  FOLLOW_WEB_LINKS="true"
   CHANGED_FILES=$(git diff --name-only $COMMIT_RANGE | sed 's#^src/##' | tr '\n' ' ')
   FLAGS="-f $CHANGED_FILES"
 
@@ -42,7 +48,7 @@ fi
 echo "exec $LINKCHECK_BINARY $FLAGS"
 if [ "$USE_TOKEN" = 1 ]; then
   config=$(set_github_token)
-  exec $LINKCHECK_BINARY $FLAGS <<<"$config"
+  exec env MDBOOK_OUTPUT__LINKCHECK__FOLLOW_WEB_LINKS=$FOLLOW_WEB_LINKS $LINKCHECK_BINARY $FLAGS <<<"$config"
 else
-  exec $LINKCHECK_BINARY $FLAGS
+  exec env MDBOOK_OUTPUT__LINKCHECK__FOLLOW_WEB_LINKS=$FOLLOW_WEB_LINKS $LINKCHECK_BINARY $FLAGS
 fi