Unify behavior for long path or UNC prefix removal

Split the code out of QDir::fromNativeSeparator into a separate
reusable function to remove the above-mentioned prefixes. Fixes
and unifies behavior if the prefix was given with slashes instead
of backslashes. Add a couple more test cases.

Fixes: QTBUG-93868
Pick-to: 5.15 6.0 6.1
Change-Id: Ibd94ae283e2fb113f9c2db97475fbc7d89522bbf
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

4 files changed, 34 insertions, 40 deletions

diff --git a/src/corelib/io/qdir.cpp b/src/corelib/io/qdir.cpp
index c4dbd27aa1..e92d2f9ca7 100644
--- a/src/corelib/io/qdir.cpp
+++ b/src/corelib/io/qdir.cpp
@@ -953,35 +953,10 @@ QString QDir::toNativeSeparators(const QString &pathName)
 QString QDir::fromNativeSeparators(const QString &pathName)
 {
 #if defined(Q_OS_WIN)
-    const QChar nativeSeparator = u'\\';
-    int i = pathName.indexOf(nativeSeparator);
-    if (i != -1) {
-        QString n(pathName);
-        const QStringView uncPrefix(uR"(\\?\UNC\)");
-        const QStringView extendedLengthPathPrefix(uR"(\\?\)");
-        if (n.startsWith(uncPrefix)) {
-            // Keep the initial double-slash, chop out the rest of the prefix.
-            n = n.remove(2, uncPrefix.size() - 2);
-            if ((i = n.indexOf(nativeSeparator)) == -1)
-                return n;
-        } else if (n.startsWith(extendedLengthPathPrefix)) {
-            n = n.sliced(extendedLengthPathPrefix.size());
-            if ((i = n.indexOf(nativeSeparator)) == -1)
-                return n;
-        }
-
-        QChar * const data = n.data();
-        data[i++] = u'/';
-
-        for (; i < n.length(); ++i) {
-            if (data[i] == nativeSeparator)
-                data[i] = u'/';
-        }
-
-        return n;
-    }
-#endif
+    return QFileSystemEntry::removeUncOrLongPathPrefix(pathName).replace(u'\\', u'/');
+#else
     return pathName;
+#endif
 }
 
 static QString qt_cleanPath(const QString &path, bool *ok = nullptr);
diff --git a/src/corelib/io/qfilesystemengine_win.cpp b/src/corelib/io/qfilesystemengine_win.cpp
index 6c4418a83b..d587e2bfd0 100644
--- a/src/corelib/io/qfilesystemengine_win.cpp
+++ b/src/corelib/io/qfilesystemengine_win.cpp
@@ -297,18 +297,8 @@ static QString readSymLink(const QFileSystemEntry &link)
                 const wchar_t* PathBuffer = &rdb->SymbolicLinkReparseBuffer.PathBuffer[offset];
                 result = QString::fromWCharArray(PathBuffer, length);
             }
-            // cut-off "\\?\" and "\??\"
-            if (result.size() > 4
-                    && result.at(0) == QLatin1Char('\\')
-                    && result.at(2) == QLatin1Char('?')
-                    && result.at(3) == QLatin1Char('\\')) {
-                result = result.mid(4);
-                // cut off UNC in addition when the link points at a UNC share
-                // in which case we need to prepend another backslash to get \\server\share
-                if (QStringView{result}.left(3) == QLatin1String("UNC")) {
-                    result.replace(0, 3, QLatin1Char('\\'));
-                }
-            }
+            // remove "\\?\", "\??\" or "\\?\UNC\"
+            result = QFileSystemEntry::removeUncOrLongPathPrefix(result);
         }
         free(rdb);
         CloseHandle(handle);
diff --git a/src/corelib/io/qfilesystementry.cpp b/src/corelib/io/qfilesystementry.cpp
index c061f58e87..bb39a46642 100644
--- a/src/corelib/io/qfilesystementry.cpp
+++ b/src/corelib/io/qfilesystementry.cpp
@@ -288,6 +288,34 @@ bool QFileSystemEntry::isDriveRootPath(const QString &path)
            && path.at(0).isLetter() && path.at(1) == QLatin1Char(':')
            && path.at(2) == QLatin1Char('/'));
 }
+
+QString QFileSystemEntry::removeUncOrLongPathPrefix(QString path)
+{
+    constexpr qsizetype minPrefixSize = 4;
+    if (path.size() < minPrefixSize)
+        return path;
+
+    auto data = path.data();
+    const auto slash = path[0];
+    if (slash != u'\\' && slash != u'/')
+        return path;
+
+    // check for "//?/" or "/??/"
+    if (data[2] == u'?' && data[3] == slash && (data[1] == slash || data[1] == u'?')) {
+        path = path.sliced(minPrefixSize);
+
+        // check for a possible "UNC/" prefix left-over
+        if (path.size() >= 4) {
+            data = path.data();
+            if (data[0] == u'U' && data[1] == u'N' && data[2] == u'C' && data[3] == slash) {
+                data[2] = slash;
+                return path.sliced(2);
+            }
+        }
+    }
+
+    return path;
+}
 #endif // Q_OS_WIN
 
 bool QFileSystemEntry::isRootPath(const QString &path)
diff --git a/src/corelib/io/qfilesystementry_p.h b/src/corelib/io/qfilesystementry_p.h
index d6b1abf498..ba0c7812e9 100644
--- a/src/corelib/io/qfilesystementry_p.h
+++ b/src/corelib/io/qfilesystementry_p.h
@@ -91,6 +91,7 @@ public:
 #if defined(Q_OS_WIN)
     bool isDriveRoot() const;
     static bool isDriveRootPath(const QString &path);
+    static QString removeUncOrLongPathPrefix(QString path);
 #endif
     bool isRoot() const;