Watch host_key.py so a module upgrade reaches existing output bases

MODULE.bazel

@@ -1,6 +1,6 @@
 module(
     name = "rules_host_cache_key",
-    version = "0.0.3",
+    version = "0.0.4",
 )
 
 bazel_dep(name = "platforms", version = "1.1.0")

README.md

@@ -23,7 +23,7 @@ key, so:
 Add to `MODULE.bazel`:
 
 ```starlark
-bazel_dep(name = "rules_host_cache_key", version = "0.0.3")
+bazel_dep(name = "rules_host_cache_key", version = "0.0.4")
 
 host_cache_key = use_repo_rule("@rules_host_cache_key//:defs.bzl", "host_cache_key")
 host_cache_key(name = "host_cache_key")
@@ -80,12 +80,13 @@ The selected directory remains in the key because host tools can record its path
 
 ## When the key changes
 
-The rule asks Bazel to watch everything it read, except search path entries
-and listings on the macOS system volume: those change only with the OS build,
-which Bazel watches instead. When any of it changes, Bazel recomputes the key at
-its next command, including in a server that is already running, and every
-action whose key changed runs again or comes from a cache entry made under the
-same key.
+The rule asks Bazel to watch everything it read, except search path entries and
+listings on the macOS system volume: those change only with the OS build, which
+Bazel watches instead. It also watches its own `host_key.py`, so upgrading this
+module brings an existing output base onto the new release's key and watches.
+When any of it changes, Bazel recomputes the key at its next command, including
+in a server that is already running, and every action whose key changed runs
+again or comes from a cache entry made under the same key.
 
 Bazel reads each watched file in full when a new server first uses the
 repository. Leaving the system volume out spares that check a read of every

defs.bzl

@@ -18,7 +18,13 @@ def _host_cache_key_impl(rctx):
     interpreter = rctx.path(python) if python.startswith("/") else rctx.which(python)
     if interpreter == None or not interpreter.exists:
         fail("host_cache_key: no Python interpreter at %r; set the `python` attribute" % python)
-    command = [interpreter, rctx.path(Label("//:host_key.py"))]
+    script = Label("//:host_key.py")
+
+    # Bazel records nothing for a file reached through rctx.path. Unwatched, an
+    # upgrade that changes only host_key.py would leave an existing output base
+    # on the old script's key and watches until some other watched path changed.
+    rctx.watch(script)
+    command = [interpreter, rctx.path(script)]
     for directory in rctx.attr.search_path:
         command.extend(["--search-path", directory])
     for prefix in rctx.attr.homebrew_prefixes:

tests/check_platform.py

@@ -33,6 +33,14 @@
 )
 """
 
+# Stands in for a release that changed only host_key.py: a report under the
+# rule's interface naming another key.
+UPGRADED_SCRIPT = """\
+import json
+
+print(json.dumps({"key": "upgraded", "fields": {}, "watch_files": [], "watch_directories": []}))
+"""
+
 
 def consumer_module(
     module_root: Path, tools: Path, prefix: Path | None, developer_dir: str = ""
@@ -154,6 +162,12 @@ def expect(step: str, stderr: str, wanted: str) -> None:
 
 
 def check_watched_changes(bazel: str, module_root: Path, scratch: Path) -> None:
+    # A copy, so a step can upgrade the module's own script. Convenience links
+    # into an output base would be followed and copied whole.
+    module = scratch / "module"
+    shutil.copytree(
+        module_root, module, ignore=shutil.ignore_patterns(".git", "bazel-*")
+    )
     # The search path names the tools through a link to their directory.
     tools = scratch / "tools"
     (scratch / "tools-one").mkdir()
@@ -166,9 +180,9 @@ def check_watched_changes(bazel: str, module_root: Path, scratch: Path) -> None:
     write_sbom(keg, BOTTLE)
     workspace = scratch / "consumer"
     workspace.mkdir()
-    shutil.copy(module_root / ".bazelversion", workspace / ".bazelversion")
+    shutil.copy(module / ".bazelversion", workspace / ".bazelversion")
     (workspace / "BUILD.bazel").write_text(CONSUMER_BUILD)
-    (workspace / "MODULE.bazel").write_text(consumer_module(module_root, tools, prefix))
+    (workspace / "MODULE.bazel").write_text(consumer_module(module, tools, prefix))
 
     consumer = Consumer(bazel, workspace, scratch / "disk-cache")
     first, second = scratch / "first-output-base", scratch / "second-output-base"
@@ -209,6 +223,13 @@ def check_watched_changes(bazel: str, module_root: Path, scratch: Path) -> None:
             consumer.build(second),
             "executed",
         )
+
+        # Upgrading this module can change host_key.py and nothing else; the
+        # key follows only because the rule watches its own script.
+        (module / "host_key.py").write_text(UPGRADED_SCRIPT)
+        expect(
+            "host_key.py upgraded, running server", consumer.build(second), "executed"
+        )
     finally:
         consumer.shutdown()