QHash: Initialize the hash seed as soon as QtCore loads

Instead of lazily initializing (with some platform exceptions, see the
equivalent qsimd.cpp commit). The rationale is the same as for
qCpuFeatures(): this the qHashBits() function is hot and the current
code generation needs to save a lot of state because of the possible
call to the C++ runtime functions to enable the thread-safe
initialization of the hash seed.

[ChangeLog][Important Behavior Changes] QtCore now initializes the QHash
global seed before the main() function is run, so it is no longer
possible to use qputenv() to affect the seed value for the current
process. Disabling the random global seed for the current process should
be done programmatically with by calling either the 6.2 function
QHashSeed::setDeterministicGlobalSeed() or, if compatibility with Qt 5
is required, by calling qSetGlobalQHashSeed() with value 0.

Change-Id: I54f205f6b7314351b078fffd16cf7eae93f9e27e
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 31 insertions, 11 deletions

diff --git a/src/corelib/tools/qhash.cpp b/src/corelib/tools/qhash.cpp
index 5bea0bf9f0..9264ae53a3 100644
--- a/src/corelib/tools/qhash.cpp
+++ b/src/corelib/tools/qhash.cpp
@@ -66,11 +66,17 @@
 #ifndef QT_BOOTSTRAPPED
 #include <qcoreapplication.h>
 #include <qrandom.h>
+#include <private/qlocale_tools_p.h>
 #endif // QT_BOOTSTRAPPED
 
 #include <array>
 #include <limits.h>
 
+#if defined(QT_NO_DEBUG) && !defined(NDEBUG)
+#  define NDEBUG
+#endif
+#include <assert.h>
+
 #ifdef Q_CC_GNU
 #  define Q_DECL_HOT_FUNCTION       __attribute__((hot))
 #else
@@ -89,7 +95,11 @@ struct HashSeedStorage
     static constexpr int SeedCount = 2;
     QBasicAtomicInteger<quintptr> seeds[SeedCount] = { Q_BASIC_ATOMIC_INITIALIZER(0), Q_BASIC_ATOMIC_INITIALIZER(0) };
 
+#if !QT_SUPPORTS_INIT_PRIORITY || defined(QT_BOOTSTRAPPED)
     constexpr HashSeedStorage() = default;
+#else
+    HashSeedStorage() { initialize(0); }
+#endif
 
     enum State {
         OverriddenByEnvironment = -1,
@@ -126,19 +136,24 @@ struct HashSeedStorage
 
 private:
     Q_DECL_COLD_FUNCTION Q_NEVER_INLINE StateResult initialize(int which) noexcept;
-    [[maybe_unused]] static void ensureConstexprConstructibility()
-    {
-        static_assert(std::is_trivially_destructible_v<HashSeedStorage>);
-        static constexpr HashSeedStorage dummy {};
-        Q_UNUSED(dummy);
-    }
 };
 
 [[maybe_unused]] HashSeedStorage::StateResult HashSeedStorage::initialize(int which) noexcept
 {
     StateResult result = { 0, OverriddenByEnvironment };
-    bool ok;
-    int seed = qEnvironmentVariableIntValue("QT_HASH_SEED", &ok);
+#ifdef QT_BOOTSTRAPPED
+    Q_UNUSED(which);
+    Q_UNREACHABLE();
+#else
+    // can't use qEnvironmentVariableIntValue (reentrancy)
+    const char *seedstr = getenv("QT_HASH_SEED");
+    const char *endptr = nullptr;
+    bool ok = false;
+    int seed;
+    if (seedstr)
+        seed = qstrntoll(seedstr, strlen(seedstr), &endptr, 10, &ok);
+    if (ok && endptr != seedstr + strlen(seedstr))
+        ok = false;
     if (ok) {
         if (seed) {
             // can't use qWarning here (reentrancy)
@@ -158,6 +173,7 @@ private:
             result.requestedSeed = x.data[i];
     }
     result.state = JustInitialized;
+#endif
     return result;
 }
 
@@ -166,14 +182,15 @@ inline HashSeedStorage::StateResult HashSeedStorage::state(int which)
     constexpr quintptr BadSeed = quintptr(Q_UINT64_C(0x5555'5555'5555'5555));
     StateResult result = { BadSeed, AlreadyInitialized };
 
-#ifndef QT_BOOTSTRAPPED
+#if defined(QT_BOOTSTRAPPED)
+    result = { 0, OverriddenByEnvironment };
+#elif !QT_SUPPORTS_INIT_PRIORITY
+    // dynamic initialization
     static auto once = [&]() {
         result = initialize(which);
         return true;
     }();
     Q_UNUSED(once);
-#else
-    result = { 0, OverriddenByEnvironment };
 #endif
 
     if (result.state == AlreadyInitialized && which >= 0)
@@ -185,6 +202,9 @@ inline HashSeedStorage::StateResult HashSeedStorage::state(int which)
 /*
     The QHash seed itself.
 */
+#ifdef Q_DECL_INIT_PRIORITY
+Q_DECL_INIT_PRIORITY(05)
+#endif
 static HashSeedStorage qt_qhash_seed;
 
 /*