qstrncpy: NUL-terminate even when src is nullptr

The goal of this function is to ensure that dst is _always_
NUL-terminated. The only exception is if there's no space to write
even one NUL byte, of course, but not when src is nullptr but dst
would have space.

Update the docs to the new behavior and make them more precise.

Fix a test that assumed qstrncpy() would not write to dst for
(dst, nullptr, 10).

[ChangeLog][QtCore][qstrncpy()] Now NUL-terminates the target
buffer even when the source pointer is nullptr, provided the
target buffer has space for at least one byte.

Pick-to: 6.5 6.4 6.2 5.15
Change-Id: I7806d8c71e260f8f02b79af7b6ce94f23599dd69
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp b/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
index 76b3cc1aca..6ff8cc088e 100644
--- a/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
+++ b/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
@@ -740,7 +740,10 @@ void tst_QByteArray::qstrncpy()
 
     // src == nullptr
     QCOMPARE(::qstrncpy(dst.data(), 0,  0), (char*)0);
+    QCOMPARE(*dst.data(), 'b'); // must not have written to dst
     QCOMPARE(::qstrncpy(dst.data(), 0, 10), (char*)0);
+    QCOMPARE(*dst.data(), '\0'); // must have written to dst
+    *dst.data() = 'b'; // restore
 
     // valid pointers, but len == 0
     QCOMPARE(::qstrncpy(dst.data(), src.data(), 0), dst.data());