snap: make future major version upgrades easier (iterative#4038)

Related: iterative#3999
Related: iterative#4037

.travis.yml

@@ -107,7 +107,6 @@ jobs:
     - sudo /snap/bin/lxd.migrate -yes
     - sudo /snap/bin/lxd waitready
     - sudo /snap/bin/lxd init --auto
-    - git fetch --tags
     script:
     - ./scripts/build_snap.sh
     after_failure:
@@ -158,6 +157,14 @@ deploy:
     script: "(echo $SNAP_TOKEN | snapcraft login --with -) && timeout 600 snapcraft push dvc_*.snap --release $SNAP_CHANNEL || echo timed out"
     on:
       all_branches: true
-      condition: "$(./scripts/ci/deploy_condition.sh dvc_*.snap) && ($TRAVIS_BRANCH = master || -n $TRAVIS_TAG)"
+      condition: "$(./scripts/ci/deploy_condition.sh dvc_*.snap) && ($TRAVIS_BRANCH = master || -n $TRAVIS_TAG) && -n $SNAP_CHANNEL"
+      repo: iterative/dvc
+      stage: build
+  - provider: script
+    skip_cleanup: true
+    script: "(echo $SNAP_TOKEN | snapcraft login --with -) && timeout 600 snapcraft push dvc_*.snap --release $SNAP_CHANNEL_MAJOR || echo timed out"
+    on:
+      all_branches: true
+      condition: "$(./scripts/ci/deploy_condition.sh dvc_*.snap) && ($TRAVIS_BRANCH = master || -n $TRAVIS_TAG) && -n $SNAP_CHANNEL_MAJOR"
       repo: iterative/dvc
       stage: build

scripts/ci/before_install.sh

@@ -51,12 +51,27 @@ elif [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
   ln -s -f /usr/local/bin/pip3 /usr/local/bin/pip
 fi
 
-if [[ -n "$TRAVIS_TAG" ]]; then
-  if [[ $(echo "$TRAVIS_TAG" | grep -E '^[0-9]+\.[0-9]+\.[0-9]+$') ]]; then
-    echo "export SNAP_CHANNEL=stable,v1/stable" >>env.sh
+if [ "$TRAVIS_OS_NAME" == "linux" ]; then
+  # fetch tags for `git-describe`, since
+  # - can't rely on $TRAVIS_TAG for snapcraft `edge` (master) releases, and
+  # - `snapcraft` also uses `git-describe` for version detection
+  git fetch --tags
+  TAG_MAJOR="$(git describe --tags | sed -r 's/^v?([0-9]+)\.[0-9]+\.[0-9]+.*/\1/')"
+  [[ -n "$TAG_MAJOR" ]] || exit 1  # failed to detect major version
+
+  if [[ -n "$TRAVIS_TAG" ]]; then
+    if [[ $(echo "$TRAVIS_TAG" | grep -E '^[0-9]+\.[0-9]+\.[0-9]+$') ]]; then
+      echo "export SNAP_CHANNEL=stable" >>env.sh
+      echo "export SNAP_CHANNEL_MAJOR=v$TAG_MAJOR/stable" >>env.sh
+    else
+      echo "export SNAP_CHANNEL=beta" >>env.sh
+      echo "export SNAP_CHANNEL_MAJOR=v$TAG_MAJOR/beta" >>env.sh
+    fi
   else
-    echo "export SNAP_CHANNEL=beta,v1/beta" >>env.sh
+    echo "export SNAP_CHANNEL=edge" >>env.sh
+    echo "export SNAP_CHANNEL_MAJOR=v$TAG_MAJOR/edge" >>env.sh
   fi
-else
-  echo "export SNAP_CHANNEL=edge,v1/edge" >>env.sh
+
+  # NOTE: after deprecating this major version branch, uncomment this line
+  # echo "unset SNAP_CHANNEL" >>env.sh
 fi