Properly detect when already using the current protobuf tag. (#1986)

There were two problems to address -

- The shallow checkout github actions do by default didn't have any tags
in the submodule, and we weren't fetching them until doing the update.
So fetch the tags for the protobuf module upfront so the steps can
properly inspect them.
- Since protobuf does language specific tags, most of the time a
language specific tag will be what was collected for the current
checkout, which would still result in all the work being done to then
decide nothing was needed. So change things to collect _all_ tags for
the current comment, and filter them down to the general protobuf tag
instead. This also now errors if anything goes wrong instead of doing
lots of work to then not make a PR.

Fixes #1904

Author: thomasvl

.github/workflows/update_protobuf.yml

@@ -25,6 +25,14 @@ jobs:
         submodules: true
         token: ${{ secrets.GITHUB_TOKEN }}
 
+    - name: Fetch tags in protobuf submodule
+      run: |
+        # The checkout doesn't fetch the tags in submodules, we'll need them. so
+        # rather then forcing a full history checkout ('fetch-depth: 0 modules: true')
+        # of the main and all submodules, just do this one place ourselves.
+        cd Sources/protobuf/protobuf
+        git fetch --tags
+
     - name: Update and install dependencies
       # This step is run before check_release because we need curl and jq
       run: apt-get update && apt-get install -y curl jq
@@ -36,9 +44,27 @@ jobs:
         LATEST_TAG=$(curl -s https://api.github.com/repos/protocolbuffers/protobuf/releases/latest | jq -r '.tag_name')
         echo "Latest protobuf release: $LATEST_TAG"
 
-        # Get current protobuf submodule commit
+        # Get current protobuf submodule tag
+        #
+        # It would be nice to just use `git describe --tags --exact-match`, but
+        # the protobuf repo also cuts langugage specific tags. So odds are one
+        # of those is "closer" than the main protobuf tag, so look for all tags
+        # for the current commit and then filter down to the right style.
         cd Sources/protobuf/protobuf
-        CURRENT_TAG=$(git describe --tags --exact-match 2>/dev/null || echo "none")
+        TAG_REGEX='^v[0-9]+\.[0-9]+$'
+        TAG_MATCHES=$(git tag --points-at HEAD | grep -E "$TAG_REGEX")
+        if [ -z "$TAG_MATCHES" ]; then
+          echo "Error: Failed to figure out any current tag."
+          exit 1
+        fi
+        NUM_TAG_MATCHES=$(echo "$TAG_MATCHES" | wc -l)
+        if [ "$NUM_TAG_MATCHES" -gt 1 ]; then
+          echo "Error: Found $NUM_TAG_MATCHES lines that could be the current tag:" >&2
+          echo "$TAG_MATCHES" >&2
+          exit 1
+        fi
+        # At this point, only one match, so good to use it as the current.
+        CURRENT_TAG="$TAG_MATCHES"
         echo "Current protobuf version: $CURRENT_TAG"
         cd ../../..
 
@@ -56,7 +82,6 @@ jobs:
       if: steps.check_release.outputs.needs_update == 'true'
       run: |
         cd Sources/protobuf/protobuf
-        git fetch --tags
         git checkout ${{ steps.check_release.outputs.latest_tag }}
         cd ../../..