Provide an option that allows the user to turn off comments.

  - Add an input variable, comment, to allow the user to stop the
    workflow from posting a comment.

  - The comment is still shown in the log.

README.md

@@ -104,3 +104,31 @@ This workflow is only run when a comment that includes `/fast-forward`
 is added to the pull request.  The workflow is careful to check that
 the user who triggered the workflow is actually authorized to push to
 the repository.
+
+## Disabling Comments
+
+If you prefer to disable comments, you can set the `comment` input
+variable to `false` like so:
+
+```yaml
+name: pull-request
+on:
+  pull_request:
+    types: [opened, reopened, synchronize]
+jobs:
+  check-fast-forward:
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: read
+      # We appear to need write permission for both pull-requests and
+      # issues in order to post a comment to a pull request.
+      pull-requests: write
+      issues: write
+
+    steps:
+      - name: Checking if fast forwarding is possible
+        uses: sequoia-pgp/fast-forward@main
+        with:
+          comment: false
+```

action.yml

@@ -15,6 +15,18 @@ inputs:
       indicating whether it is possible to fast forward the target
       branch.
     default: false
+  comment:
+    description: >
+      Whether to post a comment.
+
+      If set to true, this posts a comment to the pull request
+      indicating whether it is possible to fast-forward the target
+      branch, and, if merge is true, whether fast forwarding
+      succeeded.
+
+      If false, the comment is still available via the comment output
+      variable.
+    default: true
   debug:
     description: >
       Enables debugging output.
@@ -26,10 +38,12 @@ inputs:
 runs:
   using: "composite"
   steps:
-    # github.action_path is set to $REPO.
     - run: |
         export GITHUB_TOKEN=${{ inputs.github_token }}
         export DEBUG=${{ inputs.debug }}
+        export COMMENT=${{ inputs.comment }}
+
+        # github.action_path is set to $REPO.
         if test "x${{ inputs.merge }}" = xtrue
         then
             ${{ github.action_path }}/src/fast-forward.sh --merge

src/fast-forward.sh

@@ -34,6 +34,16 @@ case "${DEBUG:-0}" in
         ;;
 esac
 
+# Set to true to post a comment to the issue.
+case "${COMMENT:-true}" in
+    0 | false | FALSE) COMMENT=0;;
+    1 | true | TRUE) COMMENT=1;;
+    *)
+        echo "Warning: Invalid value ('$COMMENT') for COMMENT." >&2;
+        COMMENT=1
+        ;;
+esac
+
 if test "x$GITHUB_EVENT_PATH" = x
 then
     echo "GITHUB_EVENT_PATH environment variable must be set." >&2
@@ -292,22 +302,27 @@ LOG=$(mktemp)
     fi
 } 2>&1 | tee -a $GITHUB_STEP_SUMMARY "$LOG"
 
-COMMENT=$(mktemp)
-jq -n --rawfile log "$LOG" '{ "body": $log }' >"$COMMENT"
+COMMENT_CONTENT=$(mktemp)
+jq -n --rawfile log "$LOG" '{ "body": $log }' >"$COMMENT_CONTENT"
 
-COMMENTS_URL="$(github_pull_request .comments_url)"
-if test "x$COMMENTS_URL" != x
+if test $COMMENT -gt 0
 then
-    echo "Posting comment to $COMMENTS_URL."
-    curl --silent --show-error --location \
-         -X POST \
-         -H "Accept: application/vnd.github+json" \
-         -H "Authorization: Bearer $GITHUB_TOKEN" \
-         -H "X-GitHub-Api-Version: 2022-11-28" \
-         "$COMMENTS_URL" \
-         -d "@$COMMENT"
+    COMMENTS_URL="$(github_pull_request .comments_url)"
+    if test "x$COMMENTS_URL" != x
+    then
+        echo "Posting comment to $COMMENTS_URL."
+        curl --silent --show-error --location \
+             -X POST \
+             -H "Accept: application/vnd.github+json" \
+             -H "Authorization: Bearer $GITHUB_TOKEN" \
+             -H "X-GitHub-Api-Version: 2022-11-28" \
+             "$COMMENTS_URL" \
+             -d "@$COMMENT_CONTENT"
+    else
+        echo "Can't post a comment: github.event.pull_request.comments_url is not set." | tee -a $GITHUB_STEP_SUMMARY
+    fi
 else
-    echo "Can't post a comment: github.event.pull_request.comments_url is not set." | tee -a $GITHUB_STEP_SUMMARY
+    echo "Not posting comment, disabled by user."
 fi
 
 exit $(cat $EXIT_CODE)