QFSFileEngine: make rename() on Unix not overwrite

The rename(2) system call overwrites, so instead of using it, we try to
use the link/unlink pair. This works for regular cases, but can fail if
trying to change case in case-insensitive filesystems, if we're
operating on a non-Unix filesystem (FAT) or, on Linux, if the file
doesn't belong to the calling user (BSDs permit this). For those cases,
we fall back to rename(2).

That means there's a race condition if a new file is created there. But
we at least reduce the likelihood of that happening for regular files.

Change-Id: I1eba2b016de74620bfc8fffd14ccb38fd929e5aa
Reviewed-by: David Faure <david.faure@kdab.com>

1 files changed, 44 insertions, 0 deletions

diff --git a/src/corelib/io/qfilesystemengine_unix.cpp b/src/corelib/io/qfilesystemengine_unix.cpp
index b0c23e3f82..b77cea3f80 100644
--- a/src/corelib/io/qfilesystemengine_unix.cpp
+++ b/src/corelib/io/qfilesystemengine_unix.cpp
@@ -651,6 +651,50 @@ bool QFileSystemEngine::copyFile(const QFileSystemEntry &source, const QFileSyst
 //static
 bool QFileSystemEngine::renameFile(const QFileSystemEntry &source, const QFileSystemEntry &target, QSystemError &error)
 {
+    QFileSystemEntry::NativePath srcPath = source.nativeFilePath();
+    QFileSystemEntry::NativePath tgtPath = target.nativeFilePath();
+    if (::link(srcPath, tgtPath) == 0) {
+        if (::unlink(srcPath) == 0)
+            return true;
+
+        // if we managed to link but can't unlink the source, it's likely
+        // it's in a directory we don't have write access to; fail the
+        // renaming instead
+        int savedErrno = errno;
+
+        // this could fail too, but there's nothing we can do about it now
+        ::unlink(tgtPath);
+
+        error = QSystemError(savedErrno, QSystemError::StandardLibraryError);
+        return false;
+    }
+
+    switch (errno) {
+    case EACCES:
+    case EEXIST:
+    case ENAMETOOLONG:
+    case ENOENT:
+    case ENOTDIR:
+    case EROFS:
+    case EXDEV:
+        // accept the error from link(2) (especially EEXIST) and don't retry
+        break;
+
+    default:
+        // fall back to rename()
+        // ### Race condition. If a file is moved in after this, it /will/ be
+        // overwritten.
+        if (::rename(srcPath, tgtPath) == 0)
+            return true;
+    }
+
+    error = QSystemError(errno, QSystemError::StandardLibraryError);
+    return false;
+}
+
+//static
+bool QFileSystemEngine::renameOverwriteFile(const QFileSystemEntry &source, const QFileSystemEntry &target, QSystemError &error)
+{
     if (::rename(source.nativeFilePath().constData(), target.nativeFilePath().constData()) == 0)
         return true;
     error = QSystemError(errno, QSystemError::StandardLibraryError);