Move check for --extra and --all-extras outside loop

Previously, an error would always be thrown when running `compile` with
`--all-extras` on multiple source files containing extras. The reason was
that the first iteration over the first source file would fill the
`extras` variable, which in the second iteration would trigger an error
since both `all_extras` and `extras` would be set.

This change moves the check outside the loop and early in the script to
avoid unnecessary processing before throwing the error.

piptools/scripts/compile.py

@@ -192,6 +192,10 @@ def cli(
                 ).format(", ".join(DEFAULT_REQUIREMENTS_FILES))
             )
 
+    if all_extras and extras:
+        msg = "--extra has no effect when used with --all-extras"
+        raise click.BadParameter(msg)
+
     if not output_file:
         # An output file must be provided for stdin
         if src_files == ("-",):
@@ -359,10 +363,7 @@ def cli(
             if not only_build_deps:
                 constraints.extend(metadata.requirements)
                 if all_extras:
-                    if extras:
-                        msg = "--extra has no effect when used with --all-extras"
-                        raise click.BadParameter(msg)
-                    extras = metadata.extras
+                    extras += metadata.extras
             if build_deps_targets:
                 constraints.extend(metadata.build_requirements)
         else:

tests/test_cli_compile.py

@@ -3001,6 +3001,38 @@ def test_cli_compile_strip_extras(runner, make_package, make_sdist, tmpdir):
     assert "[more]" not in out.stderr
 
 
+def test_cli_compile_all_extras_with_multiple_packages(
+    runner, make_package, make_sdist, tmpdir
+):
+    """
+    Assures that ``--all-extras`` works when multiple sources are specified.
+    """
+    test_package_1 = make_package(
+        "test_package_1",
+        version="0.1",
+        extras_require={"more": []},
+    )
+    test_package_2 = make_package(
+        "test_package_2",
+        version="0.1",
+        extras_require={"more": []},
+    )
+    dists_dir = tmpdir / "dists"
+
+    out = runner.invoke(
+        cli,
+        [
+            "--all-extras",
+            "--output-file",
+            "requirements.txt",
+            str(test_package_1 / "setup.py"),
+            str(test_package_2 / "setup.py"),
+        ],
+    )
+
+    assert out.exit_code == 0, out
+
+
 @pytest.mark.parametrize(
     ("package_specs", "constraints", "existing_reqs", "expected_reqs"),
     (
@@ -3133,7 +3165,6 @@ def test_resolver_drops_existing_conflicting_constraint(
         req_txt_content = req_txt.read()
         assert expected_requirements.issubset(req_txt_content.splitlines())
 
-
 def test_resolution_failure(runner):
     """Test resolution impossible for unknown package."""
     with open("requirements.in", "w") as reqs_out: