Change how QDebug escapes QStrings in the output

[ChangeLog][Important Behavior Changes] QDebug output for QStrings
changed compared to Qt 5.5.0 to more closely match the output of
previous Qt versions. Like Qt 5.5.0, QDebug will escape non-printable
characters, the backslash and quote characters, but will no longer
escape the printable characters.

Task-number: QTBUG-47316
Change-Id: I52dd43c12685407bb9a6ffff13f62ef68cbc80c5
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>

src/corelib/io/qdebug.cpp

@@ -174,6 +174,16 @@ void QDebug::putUcs4(uint ucs4)
     maybeQuote('\'');
 }
 
+// These two functions return true if the character should be printed by QDebug.
+// For QByteArray, this is technically identical to US-ASCII isprint();
+// for QString, we use QChar::isPrint, which requires a full UCS-4 decode.
+static inline bool isPrintable(uint ucs4)
+{ return QChar::isPrint(ucs4); }
+static inline bool isPrintable(ushort uc)
+{ return QChar::isPrint(uc); }
+static inline bool isPrintable(uchar c)
+{ return c >= ' ' && c < 0x7f; }
+
 template <typename Char>
 static inline void putEscapedString(QTextStreamPrivate *d, const Char *begin, int length, bool isUnicode = true)
 {
@@ -194,22 +204,23 @@ static inline void putEscapedString(QTextStreamPrivate *d, const Char *begin, in
         }
 
         if (sizeof(Char) == sizeof(QChar)) {
+            // Surrogate characters are category Cs (Other_Surrogate), so isPrintable = false for them
             int runLength = 0;
             while (p + runLength != end &&
-                   p[runLength] < 0x7f && p[runLength] >= 0x20 && p[runLength] != '\\' && p[runLength] != '"')
+                   isPrintable(p[runLength]) && p[runLength] != '\\' && p[runLength] != '"')
                 ++runLength;
             if (runLength) {
                 d->write(reinterpret_cast<const QChar *>(p), runLength);
                 p += runLength - 1;
                 continue;
             }
-        } else if (*p < 0x7f && *p >= 0x20 && *p != '\\' && *p != '"') {
+        } else if (isPrintable(*p) && *p != '\\' && *p != '"') {
             QChar c = QLatin1Char(*p);
             d->write(&c, 1);
             continue;
         }
 
-        // print as an escape sequence
+        // print as an escape sequence (maybe, see below for surrogate pairs)
         int buflen = 2;
         ushort buf[sizeof "\\U12345678" - 1];
         buf[0] = '\\';
@@ -248,7 +259,11 @@ static inline void putEscapedString(QTextStreamPrivate *d, const Char *begin, in
                 if ((p + 1) != end && QChar::isLowSurrogate(p[1])) {
                     // properly-paired surrogates
                     uint ucs4 = QChar::surrogateToUcs4(*p, p[1]);
-                    ++p;
+                    if (isPrintable(ucs4)) {
+                        buf[0] = *p;
+                        buf[1] = p[1];
+                        buflen = 2;
+                    } else {
                         buf[1] = 'U';
                         buf[2] = '0'; // toHexUpper(ucs4 >> 32);
                         buf[3] = '0'; // toHexUpper(ucs4 >> 28);
@@ -259,6 +274,8 @@ static inline void putEscapedString(QTextStreamPrivate *d, const Char *begin, in
                         buf[8] = toHexUpper(ucs4 >> 4);
                         buf[9] = toHexUpper(ucs4);
                         buflen = 10;
+                    }
+                    ++p;
                     break;
                 }
                 // improperly-paired surrogates, fall through

tests/auto/corelib/io/qdebug/tst_qdebug.cpp

@@ -379,18 +379,48 @@ void tst_QDebug::qDebugQString() const
     qDebug().noquote().nospace() << qSetFieldWidth(8) << string;
     QCOMPARE(s_msg, "   " + string);
 
-    string = QLatin1String("\nSm\xF8rg\xE5sbord\\");
+    string = "Sm\xc3\xb8rg\xc3\xa5sbord "                               // Latin script
+             "\xce\x91\xce\xb8\xce\xae\xce\xbd\xce\xb1 "                // Greek script
+             "\xd0\x9c\xd0\xbe\xd1\x81\xd0\xba\xd0\xb2\xd0\xb0";        // Cyrillic script
     qDebug().noquote().nospace() << string;
     QCOMPARE(s_msg, string);
 
+    // This string only contains printable characters
     qDebug() << string;
-    QCOMPARE(s_msg, QString("\"\\nSm\\u00F8rg\\u00E5sbord\\\\\""));
+    QCOMPARE(s_msg, '"' + string + '"');
 
-    // surrogate pairs (including broken pairings)
+    string = "\n\t\\\"";
-    ushort utf16[] = { 0xDC00, 0xD800, 0xDC00, 'x', 0xD800, 0xDC00, 0xD800, 0 };
+    qDebug().noquote().nospace() << string;
+    QCOMPARE(s_msg, string);
+
+    // This string only contains characters that must be escaped
+    qDebug() << string;
+    QCOMPARE(s_msg, QString("\"\\n\\t\\\\\\\"\""));
+
+    // Unicode escapes, BMP
+    string = "\1"                           // U+0001: START OF HEADING (category Cc)
+             "\x7f"                         // U+007F: DELETE (category Cc)
+             "\xc2\xad"                     // U+00AD: SOFT HYPHEN (category Cf)
+             "\xef\xbb\xbf";                // U+FEFF: ZERO WIDTH NO-BREAK SPACE / BOM (category Cf)
+    qDebug() << string;
+    QCOMPARE(s_msg, QString("\"\\u0001\\u007F\\u00AD\\uFEFF\""));
+
+    // Unicode printable non-BMP
+    string = "\xf0\x90\x80\x80";            // U+10000: LINEAR B SYLLABLE B008 A (category Lo)
+    qDebug() << string;
+    QCOMPARE(s_msg, '"' + string + '"');
+
+    // non-BMP and non-printable
+    string = "\xf3\xa0\x80\x81 "            // U+E0001: LANGUAGE TAG (category Cf)
+             "\xf4\x80\x80\x80";            // U+100000: Plane 16 Private Use (category Co)
+    qDebug() << string;
+    QCOMPARE(s_msg, QString("\"\\U000E0001 \\U00100000\""));
+
+    // broken surrogate pairs
+    ushort utf16[] = { 0xDC00, 0xD800, 'x', 0xD800, 0 };
     string = QString::fromUtf16(utf16);
     qDebug() << string;
-    QCOMPARE(s_msg, QString("\"\\uDC00\\U00010000x\\U00010000\\uD800\""));
+    QCOMPARE(s_msg, QString("\"\\uDC00\\uD800x\\uD800\""));
 }
 
 void tst_QDebug::qDebugQStringRef() const

tests/auto/testlib/selftests/expected_badxml.lightxml

@@ -113,7 +113,7 @@
 </TestFunction>
 <TestFunction name="encoding">
 <Message type="qdebug" file="" line="0">
-    <Description><![CDATA["\u00DClrich \u00DCml\u00E4ut"]]></Description>
+    <Description><![CDATA["Ülrich Ümläut"]]></Description>
 </Message>
 <Incident type="pass" file="" line="0" />
     <Duration msecs="0"/>

tests/auto/testlib/selftests/expected_badxml.xml

@@ -115,7 +115,7 @@
 </TestFunction>
 <TestFunction name="encoding">
 <Message type="qdebug" file="" line="0">
-    <Description><![CDATA["\u00DClrich \u00DCml\u00E4ut"]]></Description>
+    <Description><![CDATA["Ülrich Ümläut"]]></Description>
 </Message>
 <Incident type="pass" file="" line="0" />
     <Duration msecs="0"/>

tests/auto/testlib/selftests/expected_badxml.xunitxml

@@ -30,7 +30,7 @@
     <failure message="failure message" result="fail"/>
   </testcase>
   <testcase result="pass" name="encoding">
-    <!-- message="&quot;\u00DClrich \u00DCml\u00E4ut&quot;" type="qdebug" -->
+    <!-- message="&quot;Ülrich Ümläut&quot;" type="qdebug" -->
   </testcase>
   <testcase result="pass" name="cleanupTestCase"/>
   <system-err>
@@ -46,6 +46,6 @@
 <![CDATA[quotes " text" more text]]>
 <![CDATA[xml close > open < tags < text]]>
 <![CDATA[all > " mixed ]]]><![CDATA[]> up > " in < the ]]]><![CDATA[]> hopes < of triggering "< ]]]><![CDATA[]> bugs]]>
-<![CDATA["\u00DClrich \u00DCml\u00E4ut"]]>
+<![CDATA["Ülrich Ümläut"]]>
   </system-err>
 </testsuite>