database.py: stream of json objects forward compat (#43598)

In the future we may transform the database from a single JSON object to
a stream of JSON objects.

This paves the way for constant time writes and constant time rereads
when only O(1) changes are made. Currently both are linear time.

This commit gives just enough forward compat for Spack to produce a
friendly error when we would move to a stream of json objects, and a db
would look like this:

```json
{"database": {"version": "<something newer>"}}
```

2 files changed, 43 insertions, 16 deletions

diff --git a/lib/spack/spack/database.py b/lib/spack/spack/database.py
index e2f9e57e82..3c7dff96db 100644
--- a/lib/spack/spack/database.py
+++ b/lib/spack/spack/database.py
@@ -25,6 +25,7 @@ import pathlib
 import socket
 import sys
 import time
+from json import JSONDecoder
 from typing import (
     Any,
     Callable,
@@ -818,7 +819,8 @@ class Database:
         """
         try:
             with open(filename, "r") as f:
-                fdata = sjson.load(f)
+                # In the future we may use a stream of JSON objects, hence `raw_decode` for compat.
+                fdata, _ = JSONDecoder().raw_decode(f.read())
         except Exception as e:
             raise CorruptDatabaseError("error parsing database:", str(e)) from e
 
@@ -833,27 +835,24 @@ class Database:
 
         # High-level file checks
         db = fdata["database"]
-        check("installs" in db, "no 'installs' in JSON DB.")
         check("version" in db, "no 'version' in JSON DB.")
 
-        installs = db["installs"]
-
         # TODO: better version checking semantics.
         version = vn.Version(db["version"])
         if version > _DB_VERSION:
             raise InvalidDatabaseVersionError(self, _DB_VERSION, version)
-        elif version < _DB_VERSION:
-            if not any(old == version and new == _DB_VERSION for old, new in _SKIP_REINDEX):
-                tty.warn(
-                    "Spack database version changed from %s to %s. Upgrading."
-                    % (version, _DB_VERSION)
-                )
-
-                self.reindex(spack.store.STORE.layout)
-                installs = dict(
-                    (k, v.to_dict(include_fields=self._record_fields))
-                    for k, v in self._data.items()
-                )
+        elif version < _DB_VERSION and not any(
+            old == version and new == _DB_VERSION for old, new in _SKIP_REINDEX
+        ):
+            tty.warn(f"Spack database version changed from {version} to {_DB_VERSION}. Upgrading.")
+
+            self.reindex(spack.store.STORE.layout)
+            installs = dict(
+                (k, v.to_dict(include_fields=self._record_fields)) for k, v in self._data.items()
+            )
+        else:
+            check("installs" in db, "no 'installs' in JSON DB.")
+            installs = db["installs"]
 
         spec_reader = reader(version)
 
diff --git a/lib/spack/spack/test/database.py b/lib/spack/spack/test/database.py
index 0b611e93e3..b981191676 100644
--- a/lib/spack/spack/test/database.py
+++ b/lib/spack/spack/test/database.py
@@ -1106,3 +1106,31 @@ def test_database_construction_doesnt_use_globals(tmpdir, config, nullify_global
     lock_cfg = lock_cfg or spack.database.lock_configuration(config)
     db = spack.database.Database(str(tmpdir), lock_cfg=lock_cfg)
     assert os.path.exists(db.database_directory)
+
+
+def test_database_read_works_with_trailing_data(tmp_path, default_mock_concretization):
+    # Populate a database
+    root = str(tmp_path)
+    db = spack.database.Database(root)
+    spec = default_mock_concretization("a")
+    db.add(spec, directory_layout=None)
+    specs_in_db = db.query_local()
+    assert spec in specs_in_db
+
+    # Append anything to the end of the database file
+    with open(db._index_path, "a") as f:
+        f.write(json.dumps({"hello": "world"}))
+
+    # Read the database and check that it ignores the trailing data
+    assert spack.database.Database(root).query_local() == specs_in_db
+
+
+def test_database_errors_with_just_a_version_key(tmp_path):
+    root = str(tmp_path)
+    db = spack.database.Database(root)
+    next_version = f"{spack.database._DB_VERSION}.next"
+    with open(db._index_path, "w") as f:
+        f.write(json.dumps({"database": {"version": next_version}}))
+
+    with pytest.raises(spack.database.InvalidDatabaseVersionError):
+        spack.database.Database(root).query_local()