Support for pre-compressed and ETag in download (#222)

* Support for pre-compressed and ETag in download 

When downloading file request:
1. **Gzipped file serving**:
   - Automatically detects and serves pre-compressed `.gz` files when uncompressed originals are missing
   - Properly sets `Content-Encoding: gzip` headers
   - Implements `If-None-Match` header comparison for 304 (Not Modified) responses (RFC 7232)
   - Implements `ETag` header using CRC-32 from gzip trailer (bytes 4-7 from end)
   - Optimize for speed
Changes affect:
void AsyncWebServerRequest::send(FS &fs, const String &path, const char *contentType, bool download, AwsTemplateProcessor callback) 
AsyncWebServerResponse *
  AsyncWebServerRequest::beginResponse(FS &fs, const String &path, const char *contentType, bool download, AwsTemplateProcessor callback) 
AsyncFileResponse::AsyncFileResponse(FS &fs, const String &path, const char *contentType, bool download, AwsTemplateProcessor callback)
  : AsyncAbstractResponse(callback)

* ci(pre-commit): Apply automatic fixes

* Update src/AsyncWebServerRequest.cpp

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Add files via upload

---------

Co-authored-by: pre-commit-ci-lite[bot] <117423508+pre-commit-ci-lite[bot]@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people

src/AsyncWebServerRequest.cpp

@@ -4,54 +4,58 @@
  * @brief Sends a file from the filesystem to the client, with optional gzip compression and ETag-based caching.
  *
  * This method serves files over HTTP from the provided filesystem. If a compressed version of the file
- * (with a `.gz` extension) exists and the `download` flag is not set, it serves the compressed file.
+ * (with a `.gz` extension) exists and uncompressed version does not exist, it serves the compressed file.
  * It also handles ETag caching using the CRC32 value from the gzip trailer, responding with `304 Not Modified`
  * if the client's `If-None-Match` header matches the generated ETag.
  *
  * @param fs Reference to the filesystem (SPIFFS, LittleFS, etc.).
  * @param path Path to the file to be served.
  * @param contentType Optional MIME type of the file to be sent.
  *                    If contentType is "" it will be obtained from the file extension
- * @param download If true, forces the file to be sent as a download (disables gzip compression).
+ * @param download If true, forces the file to be sent as a download.
  * @param callback Optional template processor for dynamic content generation.
  *                 Templates will not be processed in compressed files.
  *
  * @note If neither the file nor its compressed version exists, responds with `404 Not Found`.
  */
 void AsyncWebServerRequest::send(FS &fs, const String &path, const char *contentType, bool download, AwsTemplateProcessor callback) {
+  // Check uncompressed file first
+  if (fs.exists(path)) {
+    send(beginResponse(fs, path, contentType, download, callback));
+    return;
+  }
+
+  // Handle compressed version
   const String gzPath = path + asyncsrv::T__gz;
-  const bool useCompressedVersion = !download && fs.exists(gzPath);
+  File gzFile = fs.open(gzPath, "r");
 
-  // 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);
+  // Compressed file not found or invalid
+  if (!gzFile.seek(gzFile.size() - 8)) {
+    send(404);
+    gzFile.close();
+    return;
+  }
 
-      uint8_t crcFromGzipTrailer[4];
-      if (file.read(crcFromGzipTrailer, sizeof(crcFromGzipTrailer)) == sizeof(crcFromGzipTrailer)) {
-        char serverETag[9];
-        _getEtag(crcFromGzipTrailer, serverETag);
+  // 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);
 
-        // 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();
+    // Compare with client's ETag
+    const AsyncWebHeader *inmHeader = this->getHeader(asyncsrv::T_INM);
+    if (inmHeader && inmHeader->value() == serverETag) {
+      gzFile.close();
+      this->send(304);  // Not Modified
+      return;
     }
   }
 
-  // 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);
-  }
+  // Send compressed file response
+  gzFile.close();
+  send(beginResponse(fs, path, contentType, download, callback));
 }
 
 /**

src/WebResponses.cpp

@@ -670,38 +670,52 @@ void AsyncFileResponse::_setContentTypeFromPath(const String &path) {
 #endif
 }
 
+/**
+ * @brief Constructor for AsyncFileResponse that handles file serving with compression support
+ *
+ * This constructor creates an AsyncFileResponse object that can serve files from a filesystem,
+ * with automatic fallback to gzip-compressed versions if the original file is not found.
+ * It also handles ETag generation for caching and supports both inline and download modes.
+ *
+ * @param fs Reference to the filesystem object used to open files
+ * @param path Path to the file to be served (without compression extension)
+ * @param contentType MIME type of the file content (empty string for auto-detection)
+ * @param download If true, file will be served as download attachment; if false, as inline content
+ * @param callback Template processor callback for dynamic content processing
+ */
 AsyncFileResponse::AsyncFileResponse(FS &fs, const String &path, const char *contentType, bool download, AwsTemplateProcessor callback)
   : AsyncAbstractResponse(callback) {
-  _code = 200;
-  const String gzPath = path + asyncsrv::T__gz;
-
-  if (!download && !fs.exists(path) && fs.exists(gzPath)) {
-    _path = gzPath;
-    _content = fs.open(gzPath, fs::FileOpenMode::read);
+  // Try to open the uncompressed version first
+  _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();
-    addHeader(T_Content_Encoding, T_gzip, false);
-    _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)) {
+    if (_content.seek(_contentLength - 8)) {
+      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, false);
-      addHeader(T_Cache_Control, T_no_cache, false);
-    }
+      addHeader(T_ETag, serverETag, true);
+      addHeader(T_Cache_Control, T_no_cache, true);
 
-    // Return to the beginning of the file
-    _content.seek(0);
-  }
-
-  if (!_content) {
-    _path = path;
-    _content = fs.open(path, fs::FileOpenMode::read);
-    _contentLength = _content.size();
+      _content.seek(0);
+    } else {
+      // File is corrupted or invalid
+      _code = 404;
+      return;
+    }
   }
 
   if (*contentType != '\0') {
@@ -710,18 +724,19 @@ AsyncFileResponse::AsyncFileResponse(FS &fs, const String &path, const char *con
     _contentType = contentType;
   }
 
-  int filenameStart = path.lastIndexOf('/') + 1;
-  char buf[26 + path.length() - filenameStart];
-  char *filename = (char *)path.c_str() + filenameStart;
-
   if (download) {
-    // set filename and force download
+    // Extract filename from path and set as download attachment
+    int filenameStart = path.lastIndexOf('/') + 1;
+    char buf[26 + path.length() - filenameStart];
+    char *filename = (char *)path.c_str() + filenameStart;
     snprintf_P(buf, sizeof(buf), PSTR("attachment; filename=\"%s\""), filename);
+    addHeader(T_Content_Disposition, buf, false);
   } else {
-    // set filename and force rendering
-    snprintf_P(buf, sizeof(buf), PSTR("inline"));
+    // Serve file inline (display in browser)
+    addHeader(T_Content_Disposition, PSTR("inline"), false);
   }
-  addHeader(T_Content_Disposition, buf, false);
+
+  _code = 200;
 }
 
 AsyncFileResponse::AsyncFileResponse(File content, const String &path, const char *contentType, bool download, AwsTemplateProcessor callback)