perf(ci): skip redundant test collection in calculate-shards

When selective testing isn't active (push to master/branch), the
calculate-shards job now outputs static defaults (22 shards) without
checkout, setup-sentry, or pytest --collect-only. Saves ~3 min on
the critical path. Full collection only runs for PRs with selective
testing.

Author: mchen-sentry

.github/workflows/backend-xdist.yml

@@ -201,15 +201,24 @@ jobs:
     needs: [files-changed, prepare-selective-tests]
     name: calculate test shards
     runs-on: ubuntu-24.04
-    timeout-minutes: 5
+    timeout-minutes: ${{ needs.prepare-selective-tests.outputs.has-selected-tests == 'true' && 5 || 1 }}
     outputs:
-      shard-count: ${{ steps.calculate-shards.outputs.shard-count }}
-      shard-indices: ${{ steps.calculate-shards.outputs.shard-indices }}
+      shard-count: ${{ steps.static-shards.outputs.shard-count || steps.calculate-shards.outputs.shard-count }}
+      shard-indices: ${{ steps.static-shards.outputs.shard-indices || steps.calculate-shards.outputs.shard-indices }}
 
     steps:
+      - name: Use default shards (no selective testing)
+        id: static-shards
+        if: needs.prepare-selective-tests.outputs.has-selected-tests != 'true'
+        run: |
+          echo "shard-count=22" >> "$GITHUB_OUTPUT"
+          echo "shard-indices=[0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]" >> "$GITHUB_OUTPUT"
+
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+        if: needs.prepare-selective-tests.outputs.has-selected-tests == 'true'
 
       - name: Setup sentry env
+        if: needs.prepare-selective-tests.outputs.has-selected-tests == 'true'
         uses: ./.github/actions/setup-sentry
         id: setup
         with:
@@ -225,8 +234,9 @@ jobs:
 
       - name: Calculate test shards
         id: calculate-shards
+        if: needs.prepare-selective-tests.outputs.has-selected-tests == 'true'
         env:
-          SELECTED_TESTS_FILE: ${{ needs.prepare-selective-tests.outputs.has-selected-tests == 'true' && '.artifacts/selected-tests.txt' || '' }}
+          SELECTED_TESTS_FILE: '.artifacts/selected-tests.txt'
           SELECTED_TEST_COUNT: ${{ needs.prepare-selective-tests.outputs.test-count || '' }}
         run: |
           python3 .github/workflows/scripts/calculate-backend-test-shards.py

.github/workflows/backend.yml

@@ -195,23 +195,31 @@ jobs:
           retention-days: 1
 
   calculate-shards:
-    # Use always() so this job runs even when prepare-selective-tests is skipped (master)
     if: >-
       always() &&
       !cancelled() &&
       needs.files-changed.outputs.backend == 'true'
     needs: [files-changed, prepare-selective-tests]
     name: calculate test shards
     runs-on: ubuntu-24.04
-    timeout-minutes: 5
+    timeout-minutes: ${{ needs.prepare-selective-tests.outputs.has-selected-tests == 'true' && 5 || 1 }}
     outputs:
-      shard-count: ${{ steps.calculate-shards.outputs.shard-count }}
-      shard-indices: ${{ steps.calculate-shards.outputs.shard-indices }}
+      shard-count: ${{ steps.static-shards.outputs.shard-count || steps.calculate-shards.outputs.shard-count }}
+      shard-indices: ${{ steps.static-shards.outputs.shard-indices || steps.calculate-shards.outputs.shard-indices }}
 
     steps:
+      - name: Use default shards (no selective testing)
+        id: static-shards
+        if: needs.prepare-selective-tests.outputs.has-selected-tests != 'true'
+        run: |
+          echo "shard-count=22" >> "$GITHUB_OUTPUT"
+          echo "shard-indices=[0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]" >> "$GITHUB_OUTPUT"
+
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+        if: needs.prepare-selective-tests.outputs.has-selected-tests == 'true'
 
       - name: Setup sentry env
+        if: needs.prepare-selective-tests.outputs.has-selected-tests == 'true'
         uses: ./.github/actions/setup-sentry
         id: setup
         with:
@@ -227,8 +235,9 @@ jobs:
 
       - name: Calculate test shards
         id: calculate-shards
+        if: needs.prepare-selective-tests.outputs.has-selected-tests == 'true'
         env:
-          SELECTED_TESTS_FILE: ${{ needs.prepare-selective-tests.outputs.has-selected-tests == 'true' && '.artifacts/selected-tests.txt' || '' }}
+          SELECTED_TESTS_FILE: '.artifacts/selected-tests.txt'
           SELECTED_TEST_COUNT: ${{ needs.prepare-selective-tests.outputs.test-count || '' }}
         run: |
           python3 .github/workflows/scripts/calculate-backend-test-shards.py

docs/tiered-xdist-changes.md

@@ -83,8 +83,16 @@ Sets `pytest_rerunfailures.HAS_PYTEST_HANDLECRASHITEM = False`.
 
 Under xdist, the server spawns a `run_connection` thread per worker. Each thread blocks on `conn.recv(1)` waiting for the next message. If no message arrives within 5 seconds (e.g., during heavy Django/plugin initialization or between test batches), `recv` raises `TimeoutError`, the thread dies, and crash recovery for that worker is lost. With 3 workers, all 3 threads die during startup, producing the `Exception in thread Thread-N (run_connection)` errors.
 
+The 5s timeout is hit because the `ClientStatusDB` connects during `pytest_configure`, but doesn't send any data until a test actually runs. Between connection and first message, the worker does Django initialization (~10s) and test collection (~100s). The server's `run_connection` thread is waiting on `recv(1)` that entire time and dies after 5s. The client still has its socket open but the server side is gone.
+
 Normal `--reruns` is unaffected — each worker retries failed tests locally via `StatusDB` (in-memory, no sockets). Only segfault crash recovery (reassigning a dead worker's test to another worker) is lost, which is a rare edge case.
 
+## 3. Skip Redundant Test Collection in calculate-shards
+
+**Modified:** `.github/workflows/backend.yml`, `.github/workflows/backend-xdist.yml`
+
+The `calculate-shards` job now has a fast path: when selective testing isn't active (push to master/branch), it outputs static defaults (22 shards) without checkout, setup-sentry, or `pytest --collect-only`. Saves ~3 min on the critical path. When selective testing IS active (PR), the full collection pipeline still runs to compute the right shard count.
+
 ### 2g. Snowflake test fix
 
 **Modified:** `tests/sentry/utils/test_snowflake.py`