Do not build when no Dockerfiles have changed

.github/workflows/ci.yaml

@@ -57,6 +57,7 @@ jobs:
     permissions: write-all
     runs-on: ubuntu-22.04
     needs: [check_templated_files, create_matrix]
+    if: ${{ needs.create_matrix.outputs.matrix != '' && needs.create_matrix.outputs.matrix != '{"include":[]}' }}
     strategy:
       fail-fast: false
       matrix: ${{ fromJson(needs.create_matrix.outputs.matrix) }}

matrix.py

@@ -46,14 +46,14 @@ def main():
     parser.add_argument(
         "--dockerfiles",
         "--filepaths",
+        type=argparse.FileType("r"),
         help="Path to file containing list of Dockerfiles to filter on, e.g. `--dockerfiles files.txt`",
-        type=str,
     )
     parser.add_argument(
         "--platforms",
-        help="List of platforms to filter on, e.g. `--platforms n64 ps2`",
         type=str,
         nargs="+",
+        help="List of platforms to filter on, e.g. `--platforms n64 ps2`",
     )
     args = parser.parse_args()
 
@@ -69,26 +69,28 @@ def main():
             ]
         )
 
-    allowed_dockerfiles = []
     if args.dockerfiles:
-        with open(args.dockerfiles, "r") as f:
-            lines = [x.strip() for x in f.readlines()]
-            for line in lines:
-                path = Path(line)
-                res = parse_dockerfile_path(path)
-                if res is not None:
-                    allowed_dockerfiles.append(res)
+        allowed_dockerfiles = []
+        lines = [x.strip() for x in args.dockerfiles.readlines()]
+        for line in lines:
+            path = Path(line)
+            res = parse_dockerfile_path(path)
+            if res is not None:
+                allowed_dockerfiles.append(res)
+
+        if len(allowed_dockerfiles) > 0:
+            local_dockerfiles = filter(
+                lambda x: x in allowed_dockerfiles, local_dockerfiles
+            )
+        else:
+            # no Dockerfiles were modified
+            local_dockerfiles = []
 
     if args.platforms:
         local_dockerfiles = filter(
             lambda x: x[0] in args.platforms, local_dockerfiles
         )
 
-    if len(allowed_dockerfiles) > 0:
-        local_dockerfiles = filter(
-            lambda x: x in allowed_dockerfiles, local_dockerfiles
-        )
-
     includes = [process_dockerfile(p, c, a) for (p, c, a) in local_dockerfiles]
     matrix = dict(include=includes)
     print(json.dumps(matrix, separators=(",", ":")))