QVarLengthArray: make reallocation strongly exception safe

The old code had several bugs:

- it immediately clobbered *this with new state, before having copied
  over the elements from the old to the new buffer

- when buffer relocation threw, it would keep the new (partially-filled)
  buffer and throw away the old

- it unconditionally used std::move() for non-relocatable types, making
  it impossible to restore the original buffer when a move throws

Instead of clobbering *this with new state, do all the work on the
side and change *this only once the reallocation has happened
successfully.

Also use q_uninitialized_relocate_n() and unique_ptr in the
implementation to simplify the code. The former got the necessary
update to use std::move_if_noexcept() instead of an unconditional
std::move() for the non-relocatable case.

[ChangeLog][QtCore][QVarLengthArray] The append()-like functions are
now strongly exception safe. This means reallocation will now use
copies instead of moves, unless the value_type has a noexcept move
constructor.

Fixes: QTBUG-99039
Change-Id: I031251b8d14ac045592d01caed59d4638c3d9892
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

1 files changed, 22 insertions, 2 deletions

diff --git a/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp b/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp
index 4944da71a9..8f2871a54f 100644
--- a/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp
+++ b/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp
@@ -429,8 +429,6 @@ void tst_QVarLengthArray::appendIsStronglyExceptionSafe()
     };
 
     {
-        // ### TODO: QVLA isn't exception-safe when throwing during reallocation,
-        // ### so check with size() < capacity() for now
         QVarLengthArray<Thrower, 2> vla(1);
         {
             Thrower t;
@@ -443,6 +441,28 @@ void tst_QVarLengthArray::appendIsStronglyExceptionSafe()
             QVERIFY_THROWS_EXCEPTION(int, vla.push_back({}));
             QCOMPARE(vla.size(), 1);
         }
+        vla.push_back({});
+        QCOMPARE(vla.size(), 2);
+        {
+            Thrower t;
+            {
+                // tests the copy inside append()
+                const QScopedValueRollback rb(throwOnCopyNow, true);
+                QVERIFY_THROWS_EXCEPTION(int, vla.push_back(t));
+                QCOMPARE(vla.size(), 2);
+            }
+            {
+                // tests the move inside reallocate()
+                const QScopedValueRollback rb(throwOnMoveNow, true);
+                QVERIFY_THROWS_EXCEPTION(int, vla.push_back(t));
+                QCOMPARE(vla.size(), 2);
+            }
+        }
+        {
+            const QScopedValueRollback rb(throwOnMoveNow, true);
+            QVERIFY_THROWS_EXCEPTION(int, vla.push_back({}));
+            QCOMPARE(vla.size(), 2);
+        }
     }
 #endif
 }