Add ETag handling and cache for pre-compressed .gz files

Add ETag handling and cache for pre-compressed .gz files in AsyncFileResponse
This patch improves AsyncFileResponse by adding ETag and cache support for pre-compressed .gz files.
Pre-compressed files (e.g., index.html.gz) were already being served when available. This update introduces automatic generation of an ETag for such files, based on the CRC32 checksum located in the gzip trailer (last 4 bytes).
When a .gz file from FS is served:
- An `ETag` header is included, derived from the gzip trailer CRC32
- A `Cache-Control: no-cache` header is added to ensure proper validation
This enhancement enables better caching and validation mechanisms for clients, improving efficiency while ensuring updated content is properly detected.

Author: JosePineiro

src/ESPAsyncWebServer.h

@@ -368,12 +368,52 @@ class AsyncWebServerRequest {
   }
 
   void send(FS &fs, const String &path, const char *contentType = asyncsrv::empty, bool download = false, AwsTemplateProcessor callback = nullptr) {
+    // If-None-Match header
+    if (this->hasHeader(asyncsrv::T_INM) && !download && fs.exists(path + asyncsrv::T__gz)) {
+      // CRC32-based ETag of the trailer, bytes 4-7 from the end
+      File file = fs.open(path + asyncsrv::T__gz, fs::FileOpenMode::read);
+      if (file && file.size() >= 8) {
+        file.seek(file.size() - 8);
+
+        uint8_t trailer[4];
+        if (file.read(trailer, sizeof(trailer)) == sizeof(trailer)) {
+          char serverETag[11];
+          _getEtag(trailer, serverETag);
+          
+          // Compare with client's If-None-Match header
+          const char *clientETag = this->getHeader(asyncsrv::T_INM)->value().c_str();
+          if (strcmp(clientETag, serverETag) == 0) {
+            file.close();
+            this->send(304); // Not Modified
+            return;
+          }
+        }
+        file.close();
+      }
+    }
+    
+    // If we get here, create and send the normal response
     if (fs.exists(path) || (!download && fs.exists(path + asyncsrv::T__gz))) {
       send(beginResponse(fs, path, contentType, download, callback));
     } else {
       send(404);
     }
   }
+
+  void _getEtag(uint8_t trailer[4], char* serverETag) {
+    serverETag[0] = '"';
+    for (int i = 0; i < 4; ++i) {
+      uint8_t byte = trailer[i];
+      uint8_t highNibble = (byte >> 4) & 0x0F;
+      uint8_t lowNibble  = byte & 0x0F;
+
+      serverETag[1 + i * 2] = (highNibble < 10) ? ('0' + highNibble) : ('A' + highNibble - 10);
+      serverETag[2 + i * 2] = (lowNibble  < 10) ? ('0' + lowNibble)  : ('A' + lowNibble - 10);
+    }
+    serverETag[9] = '"';
+    serverETag[10] = '\0';
+  }
+
   void send(FS &fs, const String &path, const String &contentType, bool download = false, AwsTemplateProcessor callback = nullptr) {
     send(fs, path, contentType.c_str(), download, callback);
   }

src/WebResponses.cpp

@@ -657,8 +657,25 @@ AsyncFileResponse::AsyncFileResponse(FS &fs, const String &path, const char *con
     _callback = nullptr;  // Unable to process zipped templates
     _sendContentLength = true;
     _chunked = false;
+    _content = fs.open(_path, fs::FileOpenMode::read);
+    _contentLength = _content.size();
+
+    // CRC32-based ETag of the trailer, bytes 4-7 from the end
+    if (_content && _contentLength >= 8) {
+      _content.seek(_contentLength - 8);
+      uint8_t trailer[4];
+      if (_content.read(trailer, sizeof(trailer)) == sizeof(trailer)) {
+        char serverETag[11];
+        _getEtag(trailer, serverETag);
+        addHeader(T_ETag, serverETag, false);
+        addHeader(T_Cache_Control, T_no_cache, false);
+      }
+  
+    // Return to the beginning of the file
+    _content.seek(0);
+    }
   }
-
+  
   _content = fs.open(_path, fs::FileOpenMode::read);
   _contentLength = _content.size();
 
@@ -682,6 +699,20 @@ AsyncFileResponse::AsyncFileResponse(FS &fs, const String &path, const char *con
   addHeader(T_Content_Disposition, buf, false);
 }
 
+void AsyncFileResponse::_getEtag(uint8_t trailer[4], char* serverETag) {
+  serverETag[0] = '"';
+  for (int i = 0; i < 4; ++i) {
+    uint8_t byte = trailer[i];
+    uint8_t highNibble = (byte >> 4) & 0x0F;
+    uint8_t lowNibble  = byte & 0x0F;
+
+    serverETag[1 + i * 2] = (highNibble < 10) ? ('0' + highNibble) : ('A' + highNibble - 10);
+    serverETag[2 + i * 2] = (lowNibble  < 10) ? ('0' + lowNibble)  : ('A' + lowNibble - 10);
+  }
+  serverETag[9] = '"';
+  serverETag[10] = '\0';
+}
+
 AsyncFileResponse::AsyncFileResponse(File content, const String &path, const char *contentType, bool download, AwsTemplateProcessor callback)
   : AsyncAbstractResponse(callback) {
   _code = 200;