A test to prove that @fromfile invaldiation works now.

See #10360

docs/docs/using-pants/key-concepts/options.mdx

@@ -355,13 +355,6 @@ PANTS_SCOPE_OPTION=@path/to/file.json
 pants --scope-option="@path/to/file.json"
 ```
 
-:::caution Gotcha: If you modify the value file, you must manually restart pantsd
-Until we resolve [this issue](https://github.com/pantsbuild/pants/issues/10360), changing
-the value in a file used with the `@` syntax as described above will not invalidate the build.
-For now, if such a file changes you will have to stop pantsd so that it will be restarted on
-the next invocation of pants. To do so, run `rm -r .pants.d/pids/` in the build root.
-:::
-
 ## `.pants.rc` file
 
 You can set up personal Pants config files, using the same TOML syntax as `pants.toml`. By default, Pants looks for the paths `/etc/pantsrc`, `~/.pants.rc`, and `.pants.rc` in the repository root.

src/python/pants/option/options_integration_test.py

@@ -108,3 +108,32 @@ def test_pants_symlink_workdirs(tmp_path: Path) -> None:
     pants_run.assert_success()
     # Make sure symlink workdir is pointing to physical workdir
     assert Path(os.readlink(symlink_workdir.as_posix())) == physical_workdir
+
+
+def test_fromfile_invalidation(tmp_path: Path) -> None:
+    workdir = (tmp_path / "workdir").as_posix()
+
+    fromfile_path = tmp_path / "fromfile.txt"
+    fromfile_path.write_text("dist1")
+    pants_run = run_pants_with_workdir(
+        [f"--pants-distdir=@{fromfile_path}"],
+        use_pantsd=True,
+        workdir=workdir,
+    )
+    assert "Scheduler initialized." in pants_run.stderr
+
+    pants_run = run_pants_with_workdir(
+        [f"--pants-distdir=@{fromfile_path}"],
+        use_pantsd=True,
+        workdir=workdir,
+    )
+    assert "Scheduler initialized." not in pants_run.stderr
+
+    fromfile_path.write_text("dist2")
+    pants_run = run_pants_with_workdir(
+        [f"--pants-distdir=@{fromfile_path}"],
+        use_pantsd=True,
+        workdir=workdir,
+    )
+    assert "Initialization options changed: reinitializing scheduler..." in pants_run.stderr
+    assert "Scheduler initialized." in pants_run.stderr