add header synchronisation for shrinked files

Author: weese

cpp/MMapEncryptedFile.h

@@ -10,7 +10,7 @@
 struct EncryptedFileHeader {
     uint16_t magic;
     uint16_t version;
-    uint64_t size;
+    volatile uint64_t size;
     uint8_t  encryptedDataKey[16];
     uint8_t  iv[16];
     uint16_t keyHash;
@@ -147,6 +147,7 @@ class MMapEncryptedFile
         if (newSize < size()) {
             // the order is important here to avoid inconsistencies
             reinterpret_cast<EncryptedFileHeader*>(file_.data())->size = FixEndianness((uint64_t)newSize);
+            file_.sync(0, sizeof(EncryptedFileHeader));
             file_.resize(newSize + sizeof(EncryptedFileHeader), strictResize);
         } else {
             // the order is important here to avoid inconsistencies

cpp/MMapFile.h

@@ -284,6 +284,25 @@ class MMapFile {
     inline bool readOnly() const { return readOnly_; }
     inline bool isOpen() const { return fd_ >= 0; }
 
+    inline void sync(size_t offset, size_t length) {
+        if (length == 0) [[unlikely]] {
+            return; // Nothing to sync
+        }
+        assertFileIsOpen();
+
+        // compute page-aligned offset and length
+        size_t pageSize = sysconf(_SC_PAGE_SIZE);
+        size_t alignedOffset = offset - (offset % pageSize);
+        length += (offset - alignedOffset);
+        if (alignedOffset + length > size_) [[unlikely]] {
+            length = size_ - alignedOffset; // Adjust length to not exceed the size of the file
+        }
+
+        if (msync(data_ + alignedOffset, length, MS_SYNC) == -1) [[unlikely]] {
+            throw std::runtime_error("Failed to sync memory-mapped file: " + std::string(strerror(errno)));
+        }
+    }
+
     inline void assertFileIsOpen() const {
         if (fd_ < 0) [[unlikely]] {
             throw std::runtime_error("File is not open");