Make Qt aware of NTFS Junctions on Windows

On NTFS, a junction point can be created and deleted by the mklink and
rmdir commands, respectively. If a directory is not identified
correctly as a junction, then applications will likely try to remove
it using recursive methods, leading to fatal data loss.

With this change, Qt can identify file system entries as junctions,
allowing applications to use the correct file system operation to
remove it.

The test needs to delay the cleaning up of junctions and files it
creates until the checks are complete; since they might fail and make
the test function return prematurely, use a scope guard.

[ChangeLog][QtCore][QFileInfo] Add QFileInfo::isJunction so that
applications can recognize NTFS file system entries as junctions

Task-number: QTBUG-75869
Change-Id: I3c208245afbd9fb7555515fb776ff63b133ca858
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

3 files changed, 33 insertions, 3 deletions

diff --git a/src/corelib/io/qfileinfo.cpp b/src/corelib/io/qfileinfo.cpp
index 89834de29f..93696c1320 100644
--- a/src/corelib/io/qfileinfo.cpp
+++ b/src/corelib/io/qfileinfo.cpp
@@ -1145,6 +1145,25 @@ bool QFileInfo::isShortcut() const
             [d]() { return d->getFileFlags(QAbstractFileEngine::LinkType); });
 }
 
+
+/*!
+    Returns \c true if the object points to a junction;
+    otherwise returns \c false.
+
+    Junctions only exist on Windows' NTFS file system, and are typically
+    created by the \c{mklink} command. They can be thought of as symlinks for
+    directories, and can only be created for absolute paths on the local
+    volume.
+*/
+bool QFileInfo::isJunction() const
+{
+    Q_D(const QFileInfo);
+    return d->checkAttribute<bool>(
+            QFileSystemMetaData::LegacyLinkType,
+            [d]() { return d->metaData.isJunction(); },
+            [d]() { return d->getFileFlags(QAbstractFileEngine::LinkType); });
+}
+
 /*!
     Returns \c true if the object points to a directory or to a symbolic
     link to a directory, and that directory is the root directory; otherwise
diff --git a/src/corelib/io/qfileinfo.h b/src/corelib/io/qfileinfo.h
index 3ac028085a..7c7ff56ae4 100644
--- a/src/corelib/io/qfileinfo.h
+++ b/src/corelib/io/qfileinfo.h
@@ -113,6 +113,7 @@ public:
     bool isSymLink() const;
     bool isSymbolicLink() const;
     bool isShortcut() const;
+    bool isJunction() const;
     bool isRoot() const;
     bool isBundle() const;
 
diff --git a/src/corelib/io/qfilesystemmetadata_p.h b/src/corelib/io/qfilesystemmetadata_p.h
index 81f4b3ba13..275a4bf8d0 100644
--- a/src/corelib/io/qfilesystemmetadata_p.h
+++ b/src/corelib/io/qfilesystemmetadata_p.h
@@ -111,8 +111,10 @@ public:
         AliasType           =        0x0,
 #endif
 #if defined(Q_OS_WIN)
+        JunctionType        = 0x04000000,
         WinLnkType          = 0x08000000,   // Note: Uses the same position for AliasType on Mac
 #else
+        JunctionType        =        0x0,
         WinLnkType          =        0x0,
 #endif
         SequentialType      = 0x00800000,   // Note: overlaps with QAbstractFileEngine::RootFlag
@@ -205,8 +207,10 @@ public:
     bool wasDeleted() const                 { return (entryFlags & WasDeletedAttribute); }
 #if defined(Q_OS_WIN)
     bool isLnkFile() const                  { return (entryFlags & WinLnkType); }
+    bool isJunction() const                 { return (entryFlags & JunctionType); }
 #else
     bool isLnkFile() const                  { return false; }
+    bool isJunction() const                 { return false; }
 #endif
 
     qint64 size() const                     { return size_; }
@@ -356,9 +360,15 @@ inline void QFileSystemMetaData::fillFromFindData(WIN32_FIND_DATA &findData, boo
     if (setLinkType) {
         knownFlagsMask |=  LinkType;
         entryFlags &= ~LinkType;
-        if ((fileAttribute_ & FILE_ATTRIBUTE_REPARSE_POINT)
-            && (findData.dwReserved0 == IO_REPARSE_TAG_SYMLINK)) {
-            entryFlags |= LinkType;
+        if (fileAttribute_ & FILE_ATTRIBUTE_REPARSE_POINT) {
+            if (findData.dwReserved0 == IO_REPARSE_TAG_SYMLINK) {
+                entryFlags |= LinkType;
+#if defined(IO_REPARSE_TAG_MOUNT_POINT)
+            } else if ((fileAttribute_ & FILE_ATTRIBUTE_DIRECTORY)
+                    && (findData.dwReserved0 == IO_REPARSE_TAG_MOUNT_POINT)) {
+                entryFlags |= JunctionType;
+#endif
+            }
         }
     }
 }