1 files changed, 67 insertions, 0 deletions
diff --git a/zen/file_update_handle.h b/zen/file_update_handle.h
new file mode 100644
index 00000000..aa9edebd
--- /dev/null
+++ b/zen/file_update_handle.h
@@ -0,0 +1,67 @@
+#ifndef FILE_UPDATE_HANDLE_H_INCLUDED
+#define FILE_UPDATE_HANDLE_H_INCLUDED
+
+#include "win.h" //includes "windows.h"
+#include "long_path_prefix.h"
+
+namespace
+{
+//manage file handle to update existing files (temporarily resetting read-only if necessary)
+//CreateFileCmd: lambda directly returning non-owned file handle from ::CreateFile()
+class FileUpdateHandle
+{
+public:
+    template <class CreateFileCmd>
+    FileUpdateHandle(const Zstring& filename, CreateFileCmd cmd) :
+        filenameFmt(zen::applyLongPathPrefix(filename)),
+        hFile(INVALID_HANDLE_VALUE),
+        attr(INVALID_FILE_ATTRIBUTES)
+    {
+        hFile = cmd();
+        if (hFile != INVALID_HANDLE_VALUE)
+            return;
+
+        const DWORD lastError = ::GetLastError();
+        if (lastError == ERROR_ACCESS_DENIED) //function fails if file is read-only
+        {
+            zen::ScopeGuard guardErrorCode = zen::makeGuard([&]() { ::SetLastError(lastError); }); //transactional behavior: ensure cleanup (e.g. network drop) -> cref [!]
+
+            //read-only file attribute may cause trouble: temporarily reset it
+            const DWORD tmpAttr = ::GetFileAttributes(filenameFmt.c_str());
+            if (tmpAttr != INVALID_FILE_ATTRIBUTES && (tmpAttr & FILE_ATTRIBUTE_READONLY))
+            {
+                if (::SetFileAttributes(filenameFmt.c_str(), FILE_ATTRIBUTE_NORMAL))
+                {
+                    guardErrorCode.dismiss();
+                    attr = tmpAttr; //"create" guard on read-only attribute
+
+                    //now try again
+                    hFile = cmd();
+                }
+            }
+        }
+    }
+
+    ~FileUpdateHandle()
+    {
+        if (hFile != INVALID_HANDLE_VALUE)
+            ::CloseHandle(hFile);
+
+        if (attr != INVALID_FILE_ATTRIBUTES)
+            ::SetFileAttributes(filenameFmt.c_str(), attr);
+    }
+
+    //may return INVALID_FILE_ATTRIBUTES, in which case ::GetLastError() may be called directly after FileUpdateHandle()
+    HANDLE get() const { return hFile; }
+
+private:
+    FileUpdateHandle(const FileUpdateHandle&);
+    FileUpdateHandle& operator=(const FileUpdateHandle&);
+
+    Zstring filenameFmt;
+    HANDLE hFile;
+    DWORD attr;
+};
+}
+
+#endif // FILE_UPDATE_HANDLE_H_INCLUDED