pageserver: handle empty get vectored queries (#11652)

## Problem

If all batched requests are excluded from the query by
`Timeine::get_rel_page_at_lsn_batched` (e.g. because they are past the
end of the relation), the read path would panic since it doesn't expect
empty queries. This is a change in behaviour that was introduced with
the scattered query implementation.

## Summary of Changes

Handle empty queries explicitly.

.github/workflows/_meta.yml

@@ -165,5 +165,5 @@ jobs:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           CURRENT_SHA: ${{ github.sha }}
         run: |
-          RELEASE_PR_RUN_ID=$(gh api "/repos/${GITHUB_REPOSITORY}/actions/runs?head_sha=$CURRENT_SHA" | jq '[.workflow_runs[] | select(.name == "Build and Test") | select(.head_branch | test("^rc/release(-(proxy|compute))?/[0-9]{4}-[0-9]{2}-[0-9]{2}$"; "s"))] | first | .id // ("Failed to find Build and Test run from  RC PR!" | halt_error(1))')
+          RELEASE_PR_RUN_ID=$(gh api "/repos/${GITHUB_REPOSITORY}/actions/runs?head_sha=$CURRENT_SHA" | jq '[.workflow_runs[] | select(.name == "Build and Test") | select(.head_branch | test("^rc/release.*$"; "s"))] | first | .id // ("Failed to find Build and Test run from  RC PR!" | halt_error(1))')
           echo "release-pr-run-id=$RELEASE_PR_RUN_ID" | tee -a $GITHUB_OUTPUT

.github/workflows/fast-forward.yml

@@ -27,15 +27,17 @@ jobs:
       - name: Fast forwarding
         uses: sequoia-pgp/fast-forward@ea7628bedcb0b0b96e94383ada458d812fca4979
         # See https://docs.github.com/en/graphql/reference/enums#mergestatestatus
-        if: ${{ github.event.pull_request.mergeable_state  == 'clean' }}
+        if: ${{ contains(fromJSON('["clean", "unstable"]'), github.event.pull_request.mergeable_state) }}
         with:
           merge: true
           comment: on-error
           github_token: ${{ secrets.CI_ACCESS_TOKEN }}
 
       - name: Comment if mergeable_state is not clean
-        if: ${{ github.event.pull_request.mergeable_state  != 'clean' }}
+        if: ${{ !contains(fromJSON('["clean", "unstable"]'), github.event.pull_request.mergeable_state) }}
+        env:
+          GH_TOKEN: ${{ secrets.CI_ACCESS_TOKEN }}
         run: |
           gh pr comment ${{ github.event.pull_request.number }} \
             --repo "${GITHUB_REPOSITORY}" \
-            --body "Not trying to forward pull-request, because \`mergeable_state\` is \`${{ github.event.pull_request.mergeable_state }}\`, not \`clean\`."
+            --body "Not trying to forward pull-request, because \`mergeable_state\` is \`${{ github.event.pull_request.mergeable_state }}\`, not \`clean\` or \`unstable\`."

pageserver/src/tenant/timeline.rs

@@ -1285,6 +1285,10 @@ impl Timeline {
         reconstruct_state: &mut ValuesReconstructState,
         ctx: &RequestContext,
     ) -> Result<BTreeMap<Key, Result<Bytes, PageReconstructError>>, GetVectoredError> {
+        if query.is_empty() {
+            return Ok(BTreeMap::default());
+        }
+
         let read_path = if self.conf.enable_read_path_debugging || ctx.read_path_debug() {
             Some(ReadPath::new(
                 query.total_keyspace(),