prepopulate CARGO_HOME caches (#5505)

* test (ci): prepopulate caches with debug

* test (ci): error in path

* optimization (ci): CARGO_HOME cache prepopulation

substrate/.gitlab-ci.yml

@@ -63,6 +63,24 @@ variables:
     - rustup show
     - cargo --version
     - sccache -s
+    # if there is no directory for this $CI_COMMIT_REF_NAME/$CI_JOB_NAME
+    # create such directory and
+    # copy recursively all the files from the newest dir which has $CI_JOB_NAME, if it exists
+    - |
+      if [[ ! -d $CARGO_HOME ]]; then
+        mkdir -p /ci-cache/${CI_PROJECT_NAME}/cargo/${CI_COMMIT_REF_NAME};
+        FRESH_CACHE=$(find /ci-cache/${CI_PROJECT_NAME}/cargo -mindepth 2 -maxdepth 2 \
+          -type d -name ${CI_JOB_NAME}  -exec stat --printf="%Y\t%n\n" {} \; |sort -n -r |head -1 |cut -f2);
+        if [[ -d $FRESH_CACHE ]]; then
+          echo "____Using" "$FRESH_CACHE" "to prepopulate the cache____";
+          cp -r "${FRESH_CACHE}" "${CARGO_HOME}";
+          touch ${CARGO_HOME}/config;
+        else
+          echo "_____No such cargo dir, proceeding from scratch_____";
+        fi
+      else
+        echo "____No need to prepopulate the cache____";
+      fi
   only:
     - master
     - /^v[0-9]+\.[0-9]+.*$/        # i.e. v1.0, v2.1rc1
@@ -78,7 +96,7 @@ variables:
   dependencies:                    []
   interruptible:                   true
   tags:
-    - linux-docker
+    - ci2
 
 .build-only:                       &build-only
   only: