Skip build and test on OSes that have no changes

Many of the PRs on SWT are OS specific, this change only runs the build
on OSes if there are changes that can affect that OS.

This is going to be more important with #2714 because potentially a number
of new jobs will be run per PR and avoiding some of the extra time
waiting will be a benefit to all.

The minor negative of skipping tests conditionally is that the **Test
Results** summary comment will report less runs that base commit.

Part of #2714

Author: jonahgraham

.github/workflows/maven.yml

@@ -34,8 +34,56 @@ jobs:
         name: Event File
         path: ${{ github.event_path }}
 
+  changes:
+    name: Detect changed paths
+    runs-on: ubuntu-latest
+    outputs:
+      # Lint warnings are expected in this block
+      # https://github.com/dorny/paths-filter/issues/223#issuecomment-2463309889
+      win32: ${{ steps.filter.outputs.win32  || 'false' }}
+      gtk:   ${{ steps.filter.outputs.gtk    || 'false' }}
+      cocoa: ${{ steps.filter.outputs.cocoa  || 'false' }}
+      md:    ${{ steps.filter.outputs.md     || 'false' }}
+      other: ${{ steps.filter.outputs.other  || 'false' }}
+    steps:
+      - uses: actions/checkout@ff7abcd0c3c05ccf6adc123a8cd1fd4fb30fb493 # v5.0.0
+        with:
+          fetch-depth: 0  # so base/ref SHAs are diffable
+      - name: Paths filter (PR/Push only)
+        if: github.event.pull_request.base
+        # dorney/paths-filter is out of date, so use this fork with
+        # a specific fix for missing predicate-quantifier in action's
+        # inputs
+        # https://github.com/dorny/paths-filter/pull/226#issuecomment-2805358761
+        uses: petermetz/paths-filter@5ee2f5d4cf5d7bdd998a314a42da307e2ae1639d
+        id: filter
+        with:
+          predicate-quantifier: 'every'
+          filters: |
+            win32:
+              - '**/win32/**'
+            gtk:
+              - '**/gtk/**'
+            cocoa:
+              - '**/cocoa/**'
+            md:
+              - '**/*.md'
+            jenkins:
+              - 'Jenkinsfile'
+            # "other" = anything not matching the above buckets
+            other:
+              - '**'
+              - '!**/win32/**'
+              - '!**/gtk/**'
+              - '!**/cocoa/**'
+              - '!**/*.md'
+              - '!Jenkinsfile'
+
   build-linux:
     name: Build (Linux)
+    needs: changes
+    # push => always; PR => run if gtk changed OR "other" changed (run-everything case)
+    if: ${{ github.event_name != 'pull_request' || needs.changes.outputs.gtk == 'true' || needs.changes.outputs.other == 'true' }}
     strategy:
       fail-fast: false
       matrix:
@@ -49,6 +97,9 @@ jobs:
 
   build-windows:
     name: Build (Windows)
+    needs: changes
+    # push => always; PR => run if win32 changed OR "other" changed (run-everything case)
+    if: ${{ github.event_name != 'pull_request' || needs.changes.outputs.win32 == 'true' || needs.changes.outputs.other == 'true' }}
     strategy:
       fail-fast: false
       matrix:
@@ -61,6 +112,9 @@ jobs:
 
   build-macos:
     name: Build (macOS)
+    needs: changes
+    # push => always; PR => run if cocoa changed OR "other" changed (run-everything case)
+    if: ${{ github.event_name != 'pull_request' || needs.changes.outputs.cocoa == 'true' || needs.changes.outputs.other == 'true' }}
     strategy:
       fail-fast: false
       matrix: