QtGlobal: (new) qEnvironmentVariableIs{Set,Empty}()

These functions are a faster version of {,!}qgetenv().is{Null,Empty}(),
a common pattern in Qt code.

Their main advantage is that they don't need to allocate memory, so
they can be used in noexcept functions, or dynamic initialisation of
namespace-scope statics, because throwing in these contexts invokes
std::terminate().

Change-Id: I651c5bd72f450b5d7df76590f8791572fe992af5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qglobal.cpp

@@ -2083,6 +2083,62 @@ QByteArray qgetenv(const char *varName)
 #endif
 }
 
+/*!
+    \relates <QtGlobal>
+    \internal
+
+    This function checks whether the environment variable \a varName
+    is empty.
+
+    Equivalent to
+    \code
+    qgetenv(varName).isEmpty()
+    \endcode
+    except that it's potentially much faster, and can't throw exceptions.
+
+    \sa qgetenv(), qEnvironmentVariableIsSet()
+*/
+bool qEnvironmentVariableIsEmpty(const char *varName) Q_DECL_NOEXCEPT
+{
+#if defined(_MSC_VER) && _MSC_VER >= 1400
+    // we provide a buffer that can only hold the empty string, so
+    // when the env.var isn't empty, we'll get an ERANGE error (buffer
+    // too small):
+    size_t dummy;
+    char buffer = '\0';
+    return getenv_s(&dummy, &buffer, 1, varName) != ERANGE;
+#else
+    const char * const value = ::getenv(varName);
+    return !value || !*value;
+#endif
+}
+
+/*!
+    \relates <QtGlobal>
+    \internal
+
+    This function checks whether the environment variable \a varName
+    is set.
+
+    Equivalent to
+    \code
+    !qgetenv(varName).isNull()
+    \endcode
+    except that it's potentially much faster, and can't throw exceptions.
+
+    \sa qgetenv(), qEnvironmentVariableIsEmpty()
+*/
+bool qEnvironmentVariableIsSet(const char *varName) Q_DECL_NOEXCEPT
+{
+#if defined(_MSC_VER) && _MSC_VER >= 1400
+    size_t requiredSize = 0;
+    (void)getenv_s(&requiredSize, 0, 0, varName);
+    return requiredSize != 0;
+#else
+    return ::getenv(varName) != 0;
+#endif
+}
+
 /*!
     \relates <QtGlobal>
 

src/corelib/global/qglobal.h

@@ -1055,6 +1055,9 @@ class QByteArray;
 Q_CORE_EXPORT QByteArray qgetenv(const char *varName);
 Q_CORE_EXPORT bool qputenv(const char *varName, const QByteArray& value);
 
+Q_CORE_EXPORT bool qEnvironmentVariableIsEmpty(const char *varName) Q_DECL_NOEXCEPT;
+Q_CORE_EXPORT bool qEnvironmentVariableIsSet(const char *varName) Q_DECL_NOEXCEPT;
+
 inline int qIntCast(double f) { return int(f); }
 inline int qIntCast(float f) { return int(f); }
 

tests/auto/corelib/global/qgetputenv/tst_qgetputenv.cpp

@@ -53,12 +53,27 @@ private slots:
 
 void tst_QGetPutEnv::getSetCheck()
 {
-    const char* varName = "should_not_exist";
+    const char varName[] = "should_not_exist";
+
+    QVERIFY(!qEnvironmentVariableIsSet(varName));
+    QVERIFY(qEnvironmentVariableIsEmpty(varName));
     QByteArray result = qgetenv(varName);
     QCOMPARE(result, QByteArray());
+
+#ifndef Q_OS_WIN
+    QVERIFY(qputenv(varName, "")); // deletes varName instead of making it empty, on Windows
+
+    QVERIFY(qEnvironmentVariableIsSet(varName));
+    QVERIFY(qEnvironmentVariableIsEmpty(varName));
+#endif
+
     QVERIFY(qputenv(varName, QByteArray("supervalue")));
+
+    QVERIFY(qEnvironmentVariableIsSet(varName));
+    QVERIFY(!qEnvironmentVariableIsEmpty(varName));
     result = qgetenv(varName);
     QVERIFY(result == "supervalue");
+
     qputenv(varName,QByteArray());
 }