fix: give each download run its own staging dir

scripts/download.py

@@ -20,6 +20,7 @@ import os
 import re
 import shutil
 import sys
+import tempfile
 import urllib.error
 import urllib.parse
 import urllib.request
@@ -33,7 +34,7 @@ DEFAULT_API = "https://api.github.com"
 REPO = "Abdess/retrobios"
 PACK_SUFFIX = "_BIOS_Pack.zip"
 CHECKSUMS_ASSET = "SHA256SUMS.txt"
-STAGING_DIR = ".retrobios-download"
+STAGING_PREFIX = ".retrobios-download-"
 MAX_CHECKSUMS_BYTES = 1 << 20
 CHUNK = 1 << 20
 
@@ -199,6 +200,19 @@ def join_volumes(parts: list[Path], dest: Path) -> None:
                 shutil.copyfileobj(src, out, CHUNK)
 
 
+def make_staging(dest: Path) -> Path:
+    """Return a staging directory of this run's own, on the destination volume.
+
+    A pack is gigabytes and that is the filesystem the user picked for them:
+    the system temp directory is a RAM disk on the appliances these packs
+    target. The name is unique per run because two downloads into one BIOS
+    folder used to share a fixed path, so whichever finished first removed the
+    other's archive mid-transfer and the second died on the missing file with
+    nothing installed.
+    """
+    return Path(tempfile.mkdtemp(prefix=STAGING_PREFIX, dir=dest))
+
+
 def fetch_pack(pack: Pack, staging: Path, checksums: dict[str, str]) -> Path:
     """Download every volume of a pack and return the assembled archive."""
     staging.mkdir(parents=True, exist_ok=True)
@@ -317,10 +331,7 @@ To check files already in place, use: python install.py --check
     dest = Path(os.path.expanduser(args.dest))
     dest.mkdir(parents=True, exist_ok=True)
 
-    # Staged inside the destination: a pack is gigabytes, and that is the
-    # filesystem the user picked for them. The system temp directory is a RAM
-    # disk on the appliances these packs target.
-    staging = dest / STAGING_DIR
+    staging = make_staging(dest)
     try:
         archive = fetch_pack(pack, staging, fetch_checksums(release))
         print(f"Extracting to {dest}/...")

tests/test_download.py

@@ -182,6 +182,35 @@ class TestJoinVolumes(unittest.TestCase):
             self.assertEqual(zf.testzip(), None)
 
 
+class TestStagingIsolation(unittest.TestCase):
+    """Two downloads into one BIOS folder must not share a staging directory."""
+
+    def setUp(self):
+        self.dest = Path(tempfile.mkdtemp())
+        self.addCleanup(shutil.rmtree, self.dest, True)
+
+    def test_two_runs_stage_in_separate_directories(self):
+        first = download.make_staging(self.dest)
+        second = download.make_staging(self.dest)
+        self.assertNotEqual(first, second)
+        for staging in (first, second):
+            self.assertEqual(staging.parent, self.dest)
+            self.assertTrue(staging.name.startswith(download.STAGING_PREFIX))
+
+    def test_one_run_cleaning_up_leaves_the_other_transfer_alone(self):
+        finished = download.make_staging(self.dest)
+        in_flight = download.make_staging(self.dest)
+        (in_flight / "Batocera_BIOS_Pack.zip").write_bytes(b"still downloading")
+
+        # What main() does in its finally clause once a pack is extracted.
+        shutil.rmtree(finished, ignore_errors=True)
+
+        self.assertTrue(
+            (in_flight / "Batocera_BIOS_Pack.zip").is_file(),
+            "a finished download removed an archive another run was still writing",
+        )
+
+
 class ReleaseServer:
     """Serves a release index and its assets over loopback."""