Make sure QDir::absoluteFilePath("/dir") includes a drive on MS

QDir::isAbsolutePath(name) thinks any path starting with a slash is
absolute; however, to return a valid absolute path, we need to put a
drive prefix onto such a name. So use QFileSystemEntry::isAbsolute()
for that check (it believes in the need for a drive, or UNC prefix)
and handle the absolute-but-for-drive case when it arises.

Add a regression test and make related changes to existing tests.

Task-number: QTBUG-50839
Change-Id: Id5d2b2586bb1423fa2d9375a298a4bb5241cffe0
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 47 insertions, 7 deletions

diff --git a/src/corelib/io/qdir.cpp b/src/corelib/io/qdir.cpp
index 520f98b18f..e4d384e4ca 100644
--- a/src/corelib/io/qdir.cpp
+++ b/src/corelib/io/qdir.cpp
@@ -750,13 +750,47 @@ QString QDir::filePath(const QString &fileName) const
 QString QDir::absoluteFilePath(const QString &fileName) const
 {
     const QDirPrivate* d = d_ptr.constData();
-    if (isAbsolutePath(fileName))
+    // Don't trust our own isAbsolutePath(); Q_OS_WIN needs a drive.
+    if (QFileSystemEntry(fileName).isAbsolute())
         return fileName;
 
     d->resolveAbsoluteEntry();
     const QString absoluteDirPath = d->absoluteDirEntry.filePath();
     if (fileName.isEmpty())
         return absoluteDirPath;
+#ifdef Q_OS_WIN
+    // Handle the "absolute except for drive" case (i.e. \blah not c:\blah):
+    int size = absoluteDirPath.length();
+    if ((fileName.startsWith(QLatin1Char('/'))
+         || fileName.startsWith(QLatin1Char('\\')))
+        && size > 1) {
+        // Combine absoluteDirPath's drive with fileName
+        int drive = 2; // length of drive prefix
+        if (Q_UNLIKELY(absoluteDirPath.at(1).unicode() != ':')) {
+            // Presumably, absoluteDirPath is an UNC path; use its //server/share
+            // part as "drive" - it's as sane a thing as we can do.
+            for (int i = 2; i-- > 0; ) { // Scan two "path fragments":
+                while (drive < size && absoluteDirPath.at(drive).unicode() == '/')
+                    drive++;
+                if (drive >= size) {
+                    qWarning("Base directory starts with neither a drive nor a UNC share: %s",
+                             qPrintable(QDir::toNativeSeparators(absoluteDirPath)));
+                    return QString();
+                }
+                while (drive < size && absoluteDirPath.at(drive).unicode() != '/')
+                    drive++;
+            }
+            // We'll append fileName, which starts with a slash; so omit trailing slash:
+            if (absoluteDirPath.at(drive).unicode() == '/')
+                drive--;
+        } else if (!absoluteDirPath.at(0).isLetter()) {
+            qWarning("Base directory's drive is not a letter: %s",
+                     qPrintable(QDir::toNativeSeparators(absoluteDirPath)));
+            return QString();
+        }
+        return absoluteDirPath.leftRef(drive) % fileName;
+    }
+#endif // Q_OS_WIN
     if (!absoluteDirPath.endsWith(QLatin1Char('/')))
         return absoluteDirPath % QLatin1Char('/') % fileName;
     return absoluteDirPath % fileName;
diff --git a/tests/auto/corelib/io/qdir/tst_qdir.cpp b/tests/auto/corelib/io/qdir/tst_qdir.cpp
index 9d47bb2884..734b26cd65 100644
--- a/tests/auto/corelib/io/qdir/tst_qdir.cpp
+++ b/tests/auto/corelib/io/qdir/tst_qdir.cpp
@@ -1384,16 +1384,22 @@ void tst_QDir::absoluteFilePath_data()
     QTest::addColumn<QString>("fileName");
     QTest::addColumn<QString>("expectedFilePath");
 
-    QTest::newRow("0") << "/etc" << "/passwd" << "/passwd";
-    QTest::newRow("1") << "/etc" << "passwd" << "/etc/passwd";
-    QTest::newRow("2") << "/" << "passwd" << "/passwd";
-    QTest::newRow("3") << "relative" << "path" << QDir::currentPath() + "/relative/path";
-    QTest::newRow("4") << "" << "" << QDir::currentPath();
 #if defined(Q_OS_WIN) && !defined(Q_OS_WINRT)
-    QTest::newRow("5") << "//machine" << "share" << "//machine/share";
+    QTest::newRow("UNC") << "//machine" << "share" << "//machine/share";
+    QTest::newRow("Drive") << "c:/side/town" << "/my/way/home" << "c:/my/way/home";
+#define DRIVE "Q:"
+#else
+#define DRIVE
 #endif
 
+    QTest::newRow("0") << DRIVE "/etc" << "/passwd" << DRIVE "/passwd";
+    QTest::newRow("1") << DRIVE "/etc" << "passwd" << DRIVE "/etc/passwd";
+    QTest::newRow("2") << DRIVE "/" << "passwd" << DRIVE "/passwd";
+    QTest::newRow("3") << "relative" << "path" << QDir::currentPath() + "/relative/path";
+    QTest::newRow("4") << "" << "" << QDir::currentPath();
+
     QTest::newRow("resource") << ":/prefix" << "foo.bar" << ":/prefix/foo.bar";
+#undef DRIVE
 }
 
 void tst_QDir::absoluteFilePath()