Remove DB reindex during a read operation (#26601)

The DB should be what is trusted for certain operations.
If it is not present when read we should assume the
corresponding store is empty, rather than trying a
write operation during a read.

* Add a unit test
* Document what needs to be there in tests

lib/spack/spack/database.py

@@ -1024,12 +1024,7 @@ def _write(self, type, value, traceback):
             raise
 
     def _read(self):
-        """Re-read Database from the data in the set location.
-
-        This does no locking, with one exception: it will automatically
-        try to regenerate a missing DB if local. This requires taking a
-        write lock.
-        """
+        """Re-read Database from the data in the set location. This does no locking."""
         if os.path.isfile(self._index_path):
             current_verifier = ''
             if _use_uuid:
@@ -1049,12 +1044,6 @@ def _read(self):
                 "No database index file is present, and upstream"
                 " databases cannot generate an index file")
 
-        # The file doesn't exist, try to traverse the directory.
-        # reindex() takes its own write lock, so no lock here.
-        with lk.WriteTransaction(self.lock):
-            self._write(None, None, None)
-        self.reindex(spack.store.layout)
-
     def _add(
             self,
             spec,

lib/spack/spack/test/database.py

@@ -894,3 +894,18 @@ def _raise(db, spec):
     with pytest.raises(Exception):
         with spack.store.db.prefix_write_lock(s):
             assert False
+
+
+@pytest.mark.regression('26600')
+def test_database_works_with_empty_dir(tmpdir):
+    # Create the lockfile and failures directory otherwise
+    # we'll get a permission error on Database creation
+    db_dir = tmpdir.ensure_dir('.spack-db')
+    db_dir.ensure('lock')
+    db_dir.ensure_dir('failures')
+    tmpdir.chmod(mode=0o555, rec=1)
+    db = spack.database.Database(str(tmpdir))
+    with db.read_transaction():
+        db.query()
+    # Check that reading an empty directory didn't create a new index.json
+    assert not os.path.exists(db._index_path)