.gitlab-ci: deploy via copy and symlink switcheroo

During analysis on Maxwell, for one of the jobs we saw error messages
indicating that "libmsgpack.so" and some xgandalf precomputed files
weren't found. The cause must have been a partial update that was
being made by the deployment script.

To fix it, we now copy and rename, instead of renaming and then
copying.

.gitlab-ci.yml

@@ -150,14 +150,7 @@ deploy-remote-maxwell:
       fi
 
   script:
-    - ssh   $MAXWELL_USER_NAME@$HOST
-      "if [ -d /software/crystfel/devel ]; then
-           mv -f /software/crystfel/devel /software/crystfel/devel-`date +%F-%H:%M:%S`;
-       else if [ ! -d /software/crystfel/ ]; then
-               mkdir -p /software/crystfel/;
-            fi;
-       fi"
-    - scp -r $PREFIX/crystfel/devel $MAXWELL_USER_NAME@$HOST:/software/crystfel/
+    - deployment/deploy-to-maxwell.sh "$MAXWELL_USER_NAME" "$HOST" "$PREFIX"
 
   rules:
     - if: $CI_COMMIT_BRANCH == "master"

deployment/deploy-to-maxwell.sh

+#!/usr/bin/env bash
+
+set -euo pipefail
+
+MAXWELL_USER_NAME="$1"
+MAXWELL_HOST="$2"
+PREFIX="$3"
+
+BASE_REMOTE_DIR="/software/crystfel"
+CURRENT_TARGET_DIR="$BASE_REMOTE_DIR/devel-$(date +%F-%H:%M:%S)"
+
+# To avoid people starting CrystFEL devel processes while those are
+# being redeployed (leading to weird error messages), we copy first,
+# and then atomically switch a symlink (see "mv -T" below) so we have
+# minimal overlap time.
+
+scp -r "$PREFIX/crystfel/devel" "${MAXWELL_USER_NAME}@${MAXWELL_HOST}:$CURRENT_TARGET_DIR"
+
+ssh "${MAXWELL_USER_NAME}@${MAXWELL_HOST}" <<EOF
+set -euo pipefail
+
+rm -f "$BASE_REMOTE_DIR/crystfel-deployment-temp"
+ln -s "$CURRENT_TARGET_DIR" "$BASE_REMOTE_DIR/crystfel-deployment-temp"
+mv -T "$BASE_REMOTE_DIR/crystfel-deployment-temp" "$BASE_REMOTE_DIR/devel"
+
+EOF