Refactor ETag to use GZIP CRC32 trailer

Refactor ETag generation to read CRC32 checksum from GZIP file trailer
- Changed _getEtag() signature to accept File handle instead of byte array
- Function now seeks to GZIP trailer and extracts CRC32 checksum
- Added error handling for file operations (seek/read failures)
- Returns boolean to indicate success/failure of ETag generation
- Maintains same hex conversion logic for consistent ETag format
- Simplifies code following DRY principle by eliminating duplicate read file

This refactoring maintains full backward compatibility

Author: JosePineiro

src/AsyncWebServerRequest.cpp

@@ -29,20 +29,16 @@ void AsyncWebServerRequest::send(FS &fs, const String &path, const char *content
   const String gzPath = path + asyncsrv::T__gz;
   File gzFile = fs.open(gzPath, fs::FileOpenMode::read);
 
-  // Compressed file not found or invalid
-  if (!gzFile.seek(gzFile.size() - 8)) {
-    send(404);
-    gzFile.close();
-    return;
-  }
-
   // ETag validation
   if (this->hasHeader(asyncsrv::T_INM)) {
     // Generate server ETag from CRC in gzip trailer
-    uint8_t crcInTrailer[4];
-    gzFile.read(crcInTrailer, 4);
     char serverETag[9];
-    _getEtag(crcInTrailer, serverETag);
+    if (!_getEtag(gzFile, serverETag)) {
+      // Compressed file not found or invalid
+      send(404);
+      gzFile.close();
+      return;
+    }
 
     // Compare with client's ETag
     const AsyncWebHeader *inmHeader = this->getHeader(asyncsrv::T_INM);
@@ -59,27 +55,39 @@ void AsyncWebServerRequest::send(FS &fs, const String &path, const char *content
 }
 
 /**
- * @brief Generates an ETag string from a 4-byte trailer
+ * @brief Generates an ETag string from the CRC32 trailer of a GZIP file.
+ *
+ * This function reads the CRC32 checksum (4 bytes) located at the end of a GZIP-compressed file
+ * and converts it into an 8-character hexadecimal ETag string (null-terminated).
  *
- * This function converts a 4-byte array into a hexadecimal ETag string enclosed in quotes.
+ * @param gzFile  Opened file handle pointing to the GZIP file.
+ * @param eTag    Output buffer to store the generated ETag.
+ *                Must be pre-allocated with at least 9 bytes (8 for hex digits + 1 for null terminator).
  *
- * @param trailer[4] Input array of 4 bytes to convert to hexadecimal
- * @param serverETag Output buffer to store the ETag
- *                   Must be pre-allocated with minimum 9 bytes (8 hex + 1 null terminator)
+ * @return true if the ETag was successfully generated, false otherwise (e.g., file too short or seek failed).
  */
-void AsyncWebServerRequest::_getEtag(uint8_t trailer[4], char *serverETag) {
+bool AsyncWebServerRequest::_getEtag(File gzFile, char *etag) {
   static constexpr char hexChars[] = "0123456789ABCDEF";
 
+  // Compressed file not found or invalid
+  if (!gzFile.seek(gzFile.size() - 8))
+    return false;
+
+  uint8_t crcInTrailer[4];
+  gzFile.read(crcInTrailer, 4);
+
   uint32_t data;
-  memcpy(&data, trailer, 4);
+  memcpy(&data, crcInTrailer, 4);
+
+  etag[0] = hexChars[(data >> 4) & 0x0F];
+  etag[1] = hexChars[data & 0x0F];
+  etag[2] = hexChars[(data >> 12) & 0x0F];
+  etag[3] = hexChars[(data >> 8) & 0x0F];
+  etag[4] = hexChars[(data >> 20) & 0x0F];
+  etag[5] = hexChars[(data >> 16) & 0x0F];
+  etag[6] = hexChars[(data >> 28)];
+  etag[7] = hexChars[(data >> 24) & 0x0F];
+  etag[8] = '\0';
 
-  serverETag[0] = hexChars[(data >> 4) & 0x0F];
-  serverETag[1] = hexChars[data & 0x0F];
-  serverETag[2] = hexChars[(data >> 12) & 0x0F];
-  serverETag[3] = hexChars[(data >> 8) & 0x0F];
-  serverETag[4] = hexChars[(data >> 20) & 0x0F];
-  serverETag[5] = hexChars[(data >> 16) & 0x0F];
-  serverETag[6] = hexChars[(data >> 28)];
-  serverETag[7] = hexChars[(data >> 24) & 0x0F];
-  serverETag[8] = '\0';
+  return true;
 }

src/ESPAsyncWebServer.h

@@ -275,7 +275,7 @@ class AsyncWebServerRequest {
   void _send();
   void _runMiddlewareChain();
 
-  static void _getEtag(uint8_t trailer[4], char *serverETag);
+  static bool _getEtag(File gzFile, char *eTag);
 
 public:
   File _tempFile;

src/WebResponses.cpp

@@ -711,24 +711,19 @@ AsyncFileResponse::AsyncFileResponse(FS &fs, const String &path, const char *con
   _content = fs.open(path, fs::FileOpenMode::read);
   if (_content.available()) {
     _path = path;
-    _contentLength = _content.size();
   } else {
     // Try to open the compressed version (.gz)
     _path = path + asyncsrv::T__gz;
     _content = fs.open(_path, fs::FileOpenMode::read);
-    _contentLength = _content.size();
 
-    if (_content.seek(_contentLength - 8)) {
+    char serverETag[9];
+    if (AsyncWebServerRequest::_getEtag(_content, serverETag)) {
       addHeader(T_Content_Encoding, T_gzip, false);
       _callback = nullptr;  // Unable to process zipped templates
       _sendContentLength = true;
       _chunked = false;
 
       // Add ETag and cache headers
-      uint8_t crcInTrailer[4];
-      _content.read(crcInTrailer, sizeof(crcInTrailer));
-      char serverETag[9];
-      AsyncWebServerRequest::_getEtag(crcInTrailer, serverETag);
       addHeader(T_ETag, serverETag, true);
       addHeader(T_Cache_Control, T_no_cache, true);
 
@@ -739,6 +734,9 @@ AsyncFileResponse::AsyncFileResponse(FS &fs, const String &path, const char *con
       return;
     }
   }
+  
+  _contentLength = _content.size();
+
 
   if (*contentType == '\0') {
     _setContentTypeFromPath(path);