A test to prove that @fromfile invalidation works now.

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/init/options_initializer.py

@@ -135,7 +135,6 @@ class OptionsInitializer:
     TODO: We would eventually like to use the bootstrap Scheduler to construct the
     OptionsBootstrapper as well, but for now we do the opposite thing, and the Scheduler is
     used only to resolve plugins.
-      see: https://github.com/pantsbuild/pants/issues/10360
     """
 
     def __init__(

src/python/pants/option/options_integration_test.py

@@ -2,12 +2,14 @@
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
 import os
+import re
 from pathlib import Path
 from textwrap import dedent
 
 from pants.fs.fs import safe_filename_from_path
 from pants.testutil.pants_integration_test import (
     ensure_daemon,
+    read_pants_log,
     run_pants,
     run_pants_with_workdir,
     setup_tmpdir,
@@ -108,3 +110,48 @@ 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()
+    pid = None
+
+    def assert_same_daemon():
+        nonlocal pid
+        logs = "\n".join(read_pants_log(workdir))
+        pids = [m.group("pid") for m in re.finditer(r"running with PID: (?P<pid>\d+)", logs)]
+        assert len(pids) == 1
+        if pid is None:
+            pid = pids[0]
+        else:
+            assert pids[0] == pid
+
+    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
+    assert_same_daemon()
+
+    pants_run = run_pants_with_workdir(
+        [f"--pants-distdir=@{fromfile_path}"],
+        use_pantsd=True,
+        workdir=workdir,
+    )
+    # Same pantsd process, same scheduler.
+    assert_same_daemon()
+    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,
+    )
+    # Same pantsd process, new scheduler.
+    assert_same_daemon()
+    assert "Initialization options changed: reinitializing scheduler..." in pants_run.stderr
+    assert "Scheduler initialized." in pants_run.stderr