fix(test): remove /dev/shm reset scratch left by killed browser servers (#384)

A server killed outright never runs its cleanup, and tmpfs keeps the
scratch folder in memory until reboot: the Python suite's server tests
left one per killed server. Scratch folders now carry the owner's pid,
and each new one first removes those whose process is gone (POSIX only).

tests/browser/fast_reset.py

@@ -43,6 +43,8 @@ MEMORY_ROOT = Path("/dev/shm")
 # the browsers and the other workers; with less free than this, the scratch
 # copy goes on disk instead.
 MEMORY_FREE_BYTES = 32 * 1024 * 1024
+# Scratch folders are named <prefix><owner pid>-<random>.
+SCRATCH_PREFIX = "pm-reset-"
 
 
 def _quote(name: str) -> str:
@@ -94,6 +96,31 @@ def table_digests(conn: sqlite3.Connection, schema: str = "main") -> dict[str, s
     return result
 
 
+def _process_alive(pid: int) -> bool:
+    try:
+        os.kill(pid, 0)
+    except ProcessLookupError:
+        return False
+    except OSError:
+        return True  # it exists but belongs to someone else
+    return True
+
+
+def remove_orphaned_scratch(root: Path) -> None:
+    """Remove scratch folders of servers that died without cleaning up.
+
+    A server killed outright (SIGKILL, a crash) never runs ``close``, and
+    tmpfs keeps its folder in memory until reboot. The folder name carries
+    the owner's process id; a folder whose process is gone is removed.
+    """
+    if os.name != "posix":
+        return  # os.kill(pid, 0) would terminate the process on Windows
+    for folder in root.glob(f"{SCRATCH_PREFIX}*-*"):
+        pid = folder.name[len(SCRATCH_PREFIX) :].split("-", 1)[0]
+        if pid.isdigit() and not _process_alive(int(pid)):
+            shutil.rmtree(folder, ignore_errors=True)
+
+
 class _SeedApi:
     """The live API, except that ``db`` is the scratch database.
 
@@ -247,12 +274,10 @@ class ScratchSeeder:
         kernel is writing back the pages they rewrite.
         """
         root = self._memory_root
-        if (
-            root is not None
-            and root.is_dir()
-            and os.access(root, os.W_OK)
-            and shutil.disk_usage(root).free >= MEMORY_FREE_BYTES
-        ):
-            return Path(tempfile.mkdtemp(prefix="pm-reset-", dir=root))
+        if root is not None and root.is_dir() and os.access(root, os.W_OK):
+            remove_orphaned_scratch(root)
+            if shutil.disk_usage(root).free >= MEMORY_FREE_BYTES:
+                prefix = f"{SCRATCH_PREFIX}{os.getpid()}-"
+                return Path(tempfile.mkdtemp(prefix=prefix, dir=root))
         self._fallback.mkdir(parents=True, exist_ok=True)
         return self._fallback

tests/test_browser_fast_reset.py

@@ -1,7 +1,10 @@
 """Browser-test resets seed a scratch copy and publish it in one commit (#384)."""
 
 import hashlib
+import os
 import sqlite3
+import subprocess
+import sys
 import tempfile
 import threading
 import time
@@ -300,6 +303,27 @@ class TestScratchSeeder(unittest.TestCase):
         self.assertEqual(list(memory.iterdir()), [])
         self.assertTrue((self.root / "fallback" / "seed.db").exists())
 
+    @unittest.skipUnless(os.name == "posix", "process probe is POSIX only")
+    def test_scratch_folders_of_dead_servers_are_removed(self):
+        memory = self.root / "memory"
+        memory.mkdir()
+        dead = subprocess.Popen([sys.executable, "-c", "pass"])
+        dead.wait()
+        orphan = memory / f"pm-reset-{dead.pid}-abc"
+        alive = memory / f"pm-reset-{os.getpid()}-def"
+        other = memory / "pm-reset-notapid"
+        for folder in (orphan, alive, other):
+            folder.mkdir()
+            (folder / "seed.db").write_bytes(b"x")
+        seeder = ScratchSeeder(self.api, self.root / "fallback", memory_root=memory)
+        self.addCleanup(seeder.close)
+        seeder.run(reseed)
+        self.assertFalse(orphan.exists())
+        self.assertTrue(alive.exists())
+        self.assertTrue(other.exists())
+        own = [p for p in memory.glob(f"pm-reset-{os.getpid()}-*") if p != alive]
+        self.assertEqual(len(own), 1)
+
     def test_close_releases_the_scratch_files(self):
         self.seeder.run(reseed)
         self.seeder.close()