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

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

@@ -657,6 +657,23 @@ 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);
@@ -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;