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

src/AsyncWebServerRequest.cpp

@@ -0,0 +1,65 @@
+ #include <ESPAsyncWebServer.h>
+
+ void AsyncWebServerRequest::send(FS &fs, const String &path, const char *contentType, bool download, AwsTemplateProcessor callback) {
+    const String gzPath = path + asyncsrv::T__gz;
+    const bool useCompressedVersion  = !download && fs.exists(gzPath);
+
+    // If-None-Match header
+    if (useCompressedVersion && this->hasHeader(asyncsrv::T_INM)) {
+      // CRC32-based ETag of the trailer, bytes 4-7 from the end
+      File file = fs.open(gzPath, fs::FileOpenMode::read);
+      if (file && file.size() >= 18) {  // 18 is the minimum size of valid gzip file
+        file.seek(file.size() - 8);
+
+        uint8_t crcFromGzipTrailer[4];
+        if (file.read(crcFromGzipTrailer, sizeof(crcFromGzipTrailer)) == sizeof(crcFromGzipTrailer)) {
+          char serverETag[11]; // " + 8 hex chars + " + '\0'
+          _getEtag(crcFromGzipTrailer, serverETag);
+
+          // Compare with client's If-None-Match header
+          const AsyncWebHeader* inmHeader = this->getHeader(asyncsrv::T_INM);
+          if (inmHeader && inmHeader->value().equals(serverETag)) {
+            file.close();
+            this->send(304); // Not Modified
+            return;
+          }
+        }
+        file.close();
+      }
+    }
+
+    // If we get here, create and send the normal response
+    if (fs.exists(path) || useCompressedVersion) {
+      send(beginResponse(fs, path, contentType, download, callback));
+    } else {
+      send(404);
+    }
+  }
+
+/**
+ * @brief Generates an ETag string from a 4-byte trailer using basic loop implementation
+ * 
+ * This function converts a 4-byte array into a hexadecimal ETag string enclosed in quotes.
+ *  
+ * @param trailer[4] Input array of 4 bytes to convert to hexadecimal
+ * @param serverETag Output buffer that must be at least 11 bytes long to store the ETag
+ *                   Must be pre-allocated with minimum 11 bytes (8 hex + 2 quotes + 1 null terminator)
+ */
+void AsyncWebServerRequest::_getEtag(uint8_t trailer[4], char* serverETag) {
+  static constexpr char hexChars[] = "0123456789ABCDEF";
+
+  uint32_t data;
+  memcpy(&data, trailer, 4);
+
+  serverETag[0] = '"';
+  serverETag[1] = hexChars[(data >> 4) & 0x0F];
+  serverETag[2] = hexChars[data & 0x0F];
+  serverETag[3] = hexChars[(data >> 12) & 0x0F];
+  serverETag[4] = hexChars[(data >> 8) & 0x0F];
+  serverETag[5] = hexChars[(data >> 20) & 0x0F];
+  serverETag[6] = hexChars[(data >> 16) & 0x0F];
+  serverETag[7] = hexChars[(data >> 28)];
+  serverETag[8] = hexChars[(data >> 24) & 0x0F];
+  serverETag[9] = '"';
+  serverETag[10] = '\0';
+}

src/ESPAsyncWebServer.h

@@ -334,6 +334,8 @@ class AsyncWebServerRequest {
   void addInterestingHeader(__unused const String &name) {
   }
 
+  static void _getEtag(uint8_t trailer[4], char* serverETag);
+
   /**
      * @brief issue HTTP redirect response with Location header
      *
@@ -367,13 +369,7 @@ class AsyncWebServerRequest {
     send(beginResponse(code, contentType, content, len, callback));
   }
 
-  void send(FS &fs, const String &path, const char *contentType = asyncsrv::empty, bool download = false, AwsTemplateProcessor callback = nullptr) {
-    if (fs.exists(path) || (!download && fs.exists(path + asyncsrv::T__gz))) {
-      send(beginResponse(fs, path, contentType, download, callback));
-    } else {
-      send(404);
-    }
-  }
+  void send(FS &fs, const String &path, const char *contentType = asyncsrv::empty, bool download = false, AwsTemplateProcessor callback = nullptr);
   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

@@ -649,22 +649,41 @@ void AsyncFileResponse::_setContentTypeFromPath(const String &path) {
 AsyncFileResponse::AsyncFileResponse(FS &fs, const String &path, const char *contentType, bool download, AwsTemplateProcessor callback)
   : AsyncAbstractResponse(callback) {
   _code = 200;
-  _path = path;
+  const String gzPath = path + asyncsrv::T__gz;
 
-  if (!download && !fs.exists(_path) && fs.exists(_path + T__gz)) {
-    _path = _path + T__gz;
+  if (!download && !fs.exists(path) && fs.exists(gzPath)) {
+    _path = gzPath;
+    _content = fs.open(gzPath, fs::FileOpenMode::read);
+    _contentLength = _content.size();
     addHeader(T_Content_Encoding, T_gzip, false);
-    _callback = nullptr;  // Unable to process zipped templates
+    _callback = nullptr; // Unable to process zipped templates
     _sendContentLength = true;
     _chunked = false;
+
+    // CRC32-based ETag of the trailer, bytes 4-7 from the end
+    _content.seek(_contentLength - 8);
+    uint8_t crcInTrailer[4];
+    if (_content.read(crcInTrailer, sizeof(crcInTrailer)) == sizeof(crcInTrailer)) {
+      char serverETag[11];
+      AsyncWebServerRequest::_getEtag(crcInTrailer, 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();
+  if (!_content) {
+    _path = path;
+    _content = fs.open(path, fs::FileOpenMode::read);
+    _contentLength = _content.size();
+  }
 
-  if (strlen(contentType) == 0) {
+  if (*contentType != '\0') {
     _setContentTypeFromPath(path);
-  } else {
+  }
+  else {
     _contentType = contentType;
   }
 
@@ -675,7 +694,8 @@ AsyncFileResponse::AsyncFileResponse(FS &fs, const String &path, const char *con
   if (download) {
     // set filename and force download
     snprintf_P(buf, sizeof(buf), PSTR("attachment; filename=\"%s\""), filename);
-  } else {
+  }
+  else {
     // set filename and force rendering
     snprintf_P(buf, sizeof(buf), PSTR("inline"));
   }