Fix: Process ETag header value to be RFC 9110 compliant.

Enclose ETag header value in double quotes.

The following definitions require the ETag header value to be enclosed in double quotes:

* RFC 9110 - https://www.rfc-editor.org/rfc/rfc9110#section-8.8.3
  ( and previous RFC 7232 - https://www.rfc-editor.org/rfc/rfc7232#section-2.3 )

* MDN docs - https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag#directives

Author: StefanOberhumer

examples/ChunkResponse/ChunkResponse.ino

@@ -98,7 +98,7 @@ void setup() {
   // curl -N -v -H "if-none-match: 4272" http://192.168.4.1/ --output -
   //
   server.on("/", HTTP_GET, [](AsyncWebServerRequest *request) {
-    String etag = String(htmlContentLength);
+    String etag = "\"" + String(htmlContentLength) + "\""; // RFC9110: ETag must be enclosed in double quotes
 
     if (request->header(asyncsrv::T_INM) == etag) {
       request->send(304);

src/AsyncWebServerRequest.cpp

@@ -35,7 +35,7 @@ void AsyncWebServerRequest::send(FS &fs, const String &path, const char *content
   // ETag validation
   if (this->hasHeader(asyncsrv::T_INM)) {
     // Generate server ETag from CRC in gzip trailer
-    char serverETag[9];
+    char serverETag[11];
     if (!_getEtag(gzFile, serverETag)) {
       // Compressed file not found or invalid
       send(404);
@@ -58,14 +58,15 @@ void AsyncWebServerRequest::send(FS &fs, const String &path, const char *content
 }
 
 /**
- * @brief Generates an ETag string from the CRC32 trailer of a GZIP file.
+ * @brief Generates an ETag string (enclosed into quotes) 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).
+ * and converts it into an 8-character hexadecimal ETag string (enclosed in double quotes and null-terminated).
+ * Double quotes for ETag value are required by RFC9110 section 8.8.3.
  *
  * @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).
+ *                Must be pre-allocated with at least 11 bytes (8 for hex digits + 2 for quotes + 1 for null terminator).
  *
  * @return true if the ETag was successfully generated, false otherwise (e.g., file too short or seek failed).
  */
@@ -79,15 +80,17 @@ bool AsyncWebServerRequest::_getEtag(File gzFile, char *etag) {
   uint32_t crc;
   gzFile.read(reinterpret_cast<uint8_t *>(&crc), sizeof(crc));
 
-  etag[0] = hexChars[(crc >> 4) & 0x0F];
-  etag[1] = hexChars[crc & 0x0F];
-  etag[2] = hexChars[(crc >> 12) & 0x0F];
-  etag[3] = hexChars[(crc >> 8) & 0x0F];
-  etag[4] = hexChars[(crc >> 20) & 0x0F];
-  etag[5] = hexChars[(crc >> 16) & 0x0F];
-  etag[6] = hexChars[(crc >> 28)];
-  etag[7] = hexChars[(crc >> 24) & 0x0F];
-  etag[8] = '\0';
+  etag[0] = '"';
+  etag[1] = hexChars[(crc >> 4) & 0x0F];
+  etag[2] = hexChars[crc & 0x0F];
+  etag[3] = hexChars[(crc >> 12) & 0x0F];
+  etag[4] = hexChars[(crc >> 8) & 0x0F];
+  etag[5] = hexChars[(crc >> 20) & 0x0F];
+  etag[6] = hexChars[(crc >> 16) & 0x0F];
+  etag[7] = hexChars[(crc >> 28)];
+  etag[8] = hexChars[(crc >> 24) & 0x0F];
+  etag[9] = '"';
+  etag[10] = '\0';
 
   return true;
 }

src/WebHandlers.cpp

@@ -210,7 +210,7 @@ void AsyncStaticWebHandler::handleRequest(AsyncWebServerRequest *request) {
   }
 
   // Get server ETag. If file is not GZ and we have a Template Processor, ETag is set to an empty string
-  char etag[9];
+  char etag[11];
   const char *tempFileName = request->_tempFile.name();
   const size_t lenFilename = strlen(tempFileName);
 
@@ -237,7 +237,8 @@ void AsyncStaticWebHandler::handleRequest(AsyncWebServerRequest *request) {
       size_t fileSize = request->_tempFile.size();
       etagValue = static_cast<uint32_t>(fileSize);
     }
-    snprintf(etag, sizeof(etag), "%08" PRIx32, etagValue);
+    // RFC9110 Section-8.8.3: Value of the ETag response must be enclosed in double quotes
+    snprintf(etag, sizeof(etag), "\"%08" PRIx32 "\"", etagValue);
   } else {
     etag[0] = '\0';
   }

src/WebResponses.cpp

@@ -732,7 +732,7 @@ AsyncFileResponse::AsyncFileResponse(FS &fs, const String &path, const char *con
     gzPath.concat(asyncsrv::T__gz);
     _content = fs.open(gzPath, fs::FileOpenMode::read);
 
-    char serverETag[9];
+    char serverETag[11];
     if (AsyncWebServerRequest::_getEtag(_content, serverETag)) {
       addHeader(T_Content_Encoding, T_gzip, false);
       _callback = nullptr;  // Unable to process zipped templates