QHash: only fetch qt_qhash_seed when detaching from a null QHash

The old code fetched QHashData::seed from qt_qhash_seed on every detach.

That is both unnecessary and wrong.

It is uneccessary, because if the detached-from QHashData isn't shared_null,
the seed has already been populated from qt_qhash_seed. It thus suffices to
fetch the seed from qt_qhash_seed only when we detach from shared_null.

It is wrong, because if qt_qhash_seed was changed between the detach from
shared_null and a following detach, d->seed is now different from this->seed,
but detach_helper simply clones the buckets 1:1 from this to d, leaving d
in a corrupt state.

By doing this change, we make QHash robust against on-the-fly changes
to qt_qhash_seed (e.g. for testing, or added security). It also opens up the
option to have API for changing the seed of a given QHash instance after it
has been created (detach, set new seed, rehash).

Change-Id: Ib251fc9a6204b42036e97a2fc66f644b379ab841
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/corelib/tools/qhash.cpp b/src/corelib/tools/qhash.cpp
index 7696e1cf6d..cd198743bf 100644
--- a/src/corelib/tools/qhash.cpp
+++ b/src/corelib/tools/qhash.cpp
@@ -418,7 +418,7 @@ QHashData *QHashData::detach_helper(void (*node_duplicate)(Node *, void *),
         Node *e;
     };
     if (this == &shared_null)
-        qt_initialize_qhash_seed();
+        qt_initialize_qhash_seed(); // may throw
     d = new QHashData;
     d->fakeNext = 0;
     d->buckets = 0;
@@ -428,7 +428,7 @@ QHashData *QHashData::detach_helper(void (*node_duplicate)(Node *, void *),
     d->userNumBits = userNumBits;
     d->numBits = numBits;
     d->numBuckets = numBuckets;
-    d->seed = uint(qt_qhash_seed.load());
+    d->seed = (this == &shared_null) ? uint(qt_qhash_seed.load()) : seed;
     d->sharable = true;
     d->strictAlignment = nodeAlign > 8;
     d->reserved = 0;