Use more appropriate return type for ThreadPool::getPreferredConcurrency

All call sites already effectively asked for an unsigned return type, including:
* The ThreadPool ctor took an nWorkers argument of type sal_Int32, but
internally stores that as std::size_t mnMaxWorkers.
* ZipOutputStream::reduceScheduledThreadTasksToGivenNumberOrLess apparently
benefits from an unsigned nThreadTasks parameter, getting rid of various casts
in its implementation that were necessary to silence signed vs. unsigned
comparison warnings.

The only drawback is that

comphelper::ThreadPool::getPreferredConcurrency() * 4

in package/source/zippackage/ZipPackageStream.cxx would now silently wrap around
instead of causing UB on overflow (which could be detected with appropriate
tools).  Ideally, it would use some o3tl::saturating_mul if we had that, so add
a TODO comment for now.

While std::thread::hardware_concurrency returns unsigned, it looked more natural
to go with std::size_t here, as some call sites already used that (see above),
so the implementation of ThreadPool::getPreferredConcurrency is a natural place
to hide clamping std::thread::hardware_concurrency() to std::size_t (in the
unlikely case that std::size_t is of smaller rank than unsigned).

This required addition of o3tl::clamp_to_unsigned in o3tl/safeint.hxx.

Change-Id: I0a04a8b32e63ebfeb39f924c4b38520455a6fb38
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/135309
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

2 files changed, 14 insertions, 10 deletions

diff --git a/comphelper/qa/unit/threadpooltest.cxx b/comphelper/qa/unit/threadpooltest.cxx
index 14da59988ef4..13eaf210a18b 100644
--- a/comphelper/qa/unit/threadpooltest.cxx
+++ b/comphelper/qa/unit/threadpooltest.cxx
@@ -17,6 +17,7 @@
 
 #include <stdlib.h>
 #include <atomic>
+#include <cstddef>
 #include <thread>
 #include <mutex>
 
@@ -42,7 +43,7 @@ void ThreadPoolTest::testPreferredConcurrency()
 {
     // Check default.
     auto nThreads = comphelper::ThreadPool::getPreferredConcurrency();
-    sal_Int32 nExpected = 4; // UTs are capped to 4.
+    std::size_t nExpected = 4; // UTs are capped to 4.
     CPPUNIT_ASSERT_MESSAGE("Expected no more than 4 threads", nExpected >= nThreads);
 
 #ifndef _WIN32
@@ -51,7 +52,7 @@ void ThreadPoolTest::testPreferredConcurrency()
     setenv("MAX_CONCURRENCY", std::to_string(nThreads).c_str(), true);
     nThreads = comphelper::ThreadPool::getPreferredConcurrency();
     CPPUNIT_ASSERT_MESSAGE("Expected no more than hardware threads",
-                           nThreads <= static_cast<sal_Int32>(std::thread::hardware_concurrency()));
+                           nThreads <= std::thread::hardware_concurrency());
 
     // Revert and check. Again, nothing should change.
     unsetenv("MAX_CONCURRENCY");
diff --git a/comphelper/source/misc/threadpool.cxx b/comphelper/source/misc/threadpool.cxx
index 00fee7d8eb59..f0a71eb05168 100644
--- a/comphelper/source/misc/threadpool.cxx
+++ b/comphelper/source/misc/threadpool.cxx
@@ -11,6 +11,7 @@
 
 #include <com/sun/star/uno/Exception.hpp>
 #include <config_options.h>
+#include <o3tl/safeint.hxx>
 #include <sal/config.h>
 #include <sal/log.hxx>
 #include <salhelper/thread.hxx>
@@ -18,6 +19,7 @@
 #include <memory>
 #include <thread>
 #include <chrono>
+#include <cstddef>
 #include <comphelper/debuggerinfo.hxx>
 #include <utility>
 
@@ -91,7 +93,7 @@ public:
     }
 };
 
-ThreadPool::ThreadPool(sal_Int32 nWorkers)
+ThreadPool::ThreadPool(std::size_t nWorkers)
     : mbTerminate(true)
     , mnMaxWorkers(nWorkers)
     , mnBusyWorkers(0)
@@ -116,7 +118,7 @@ std::shared_ptr< ThreadPool >& GetStaticThreadPool()
     static std::shared_ptr< ThreadPool > POOL =
     []()
     {
-        const sal_Int32 nThreads = ThreadPool::getPreferredConcurrency();
+        const std::size_t nThreads = ThreadPool::getPreferredConcurrency();
         return std::make_shared< ThreadPool >( nThreads );
     }();
     return POOL;
@@ -129,21 +131,22 @@ ThreadPool& ThreadPool::getSharedOptimalPool()
     return *GetStaticThreadPool();
 }
 
-sal_Int32 ThreadPool::getPreferredConcurrency()
+std::size_t ThreadPool::getPreferredConcurrency()
 {
-    static sal_Int32 ThreadCount = []()
+    static std::size_t ThreadCount = []()
     {
-        const sal_Int32 nHardThreads = std::max(std::thread::hardware_concurrency(), 1U);
-        sal_Int32 nThreads = nHardThreads;
+        const std::size_t nHardThreads = o3tl::clamp_to_unsigned<std::size_t>(
+            std::max(std::thread::hardware_concurrency(), 1U));
+        std::size_t nThreads = nHardThreads;
         const char *pEnv = getenv("MAX_CONCURRENCY");
         if (pEnv != nullptr)
         {
             // Override with user/admin preference.
-            nThreads = rtl_str_toInt32(pEnv, 10);
+            nThreads = o3tl::clamp_to_unsigned<std::size_t>(rtl_str_toInt32(pEnv, 10));
         }
 
         nThreads = std::min(nHardThreads, nThreads);
-        return std::max<sal_Int32>(nThreads, 1);
+        return std::max<std::size_t>(nThreads, 1);
     }();
 
     return ThreadCount;