Supporting ETag http headers on static files

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libraries/WebServer/examples/WebServer/README.md

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+# Arduino-ESP32 WebServer Example for WebServer Library
+
+This example shows different techniques on how to use and extend the WebServer for specific purposes
+
+It is a small project in it's own and has some files to use on the web server to show how to use simple REST based services.
+
+This example requires some space for a filesystem and runs fine boards with 4 MByte flash using the following options:
+
+* Board: ESP32 Dev Module
+* Partition Scheme: Default 4MB with spiffs (1.2MB APP/1.5MB SPIFFS)
+    but LittleFS will be used in the partition (not SPIFFS)
+
+It features
+
+* Setup a web server
+* redirect when accessing the url with servername only
+* get real time by using builtin NTP functionality
+* send HTML responses from Sketch (see builtinfiles.h)
+* use a LittleFS file system on the data partition for static files
+* use http ETag Header for client side caching of static files
+* use custom ETag calculation for static files
+* extended FileServerHandler for uploading and deleting static files
+* uploading files using drag & drop
+* serve APIs using REST services (/api/list, /api/sysinfo)
+* define HTML response when no file/api/handler was found
+
+## Supported Targets
+
+Currently, this example supports the following targets.
+
+| Supported Targets | ESP32 | ESP32-S2 | ESP32-C3 |
+| ----------------- | ----- | -------- | -------- |
+|                   | yes   | yes      | yes      |
+
+## Use the Example
+
+How to install the Arduino IDE: [Install Arduino IDE](https://github.com/espressif/arduino-esp32/tree/master/docs/arduino-ide).
+
+* In the file `secrets.h` you can add the home WiFi network name ans password.
+* Compile and upload to the device.
+* Have a look into the monitoring output.
+* Open <http://webserver> or <http://(ip-address)> using a browser.
+* You will be redirected to <http://webserver/$upload.htm> as there are no files yet in the file system.
+* Drag the files from the data folder onto the drop area shown in the browser.
+* See below for more details
+
+## Implementing a web server
+
+The WebServer library offers a simple path to implement a web server on a ESP32 based board.
+
+The advantage on using the WebServer instead of the plain simple WiFiServer is that the WebServer
+takes much care about the http protocol conventions and features and allows easily access to parameters.
+It offers plug-in capabilities by registering specific functionalities that will be outlined below.
+
+### Initialization
+
+In the setup() function in the webserver.ino sketch file the following steps are implemented to make the webserver available on the local network.
+
+* Create a webserver listening to port 80 for http requests.
+* Initialize the access to the filesystem in the free flash memory.
+* Connect to the local WiFi network. Here is only a straight-forward implementation hard-coding network name and passphrase. You may consider to use something like the WiFiManager library in real applications.
+* Register the device in DNS using a known hostname.
+* Registering several plug-ins (see below).
+* Starting the web server.
+
+### Running
+
+In the loop() function the web server will be given time to receive and send network packages by calling
+`server.handleClient();`.
+
+## Registering simple functions to implement RESTful services
+
+Registering function is the simplest integration mechanism available to add functionality. The server offers the `on(path, function)` methods that take the URL and the function as parameters.
+
+There are 2 functions implemented that get registered to handle incoming GET requests for given URLs.
+
+The JSON data format is used often for such services as it is the "natural" data format of the browser using javascript.
+
+When the **handleSysInfo()** function is registered and a browser requests for <http://webserver/api/sysinfo> the function will be called and can collect the requested information.
+
+> ```CPP
+> server.on("/api/sysinfo", handleSysInfo);
+> ```
+
+The result in this case is a JSON object that is assembled in the result String variable and the returned as a response to the client also giving the information about the data format.
+
+You can try this request in a browser by opening <http://webserver/api/sysinfo> in the address bar.
+
+> ```CPP
+> server.on("/api/sysinfo", handleList);
+> ```
+
+The function **handleList()** is registered the same way to return the list of files in the file system also returning a JSON object including name, size and the last modification timestamp.
+
+You can try this request in a browser by opening <http://webserver/api/list> in the address bar.
+
+## Registering a function to send out some static content from a String
+
+This is an example of registering a inline function in the web server.
+The 2. parameter of the on() method is a so called CPP lamda function (without a name)
+that actually has only one line of functionality by sending a string as result to the client.
+
+> ``` cpp
+> server.on("/$upload.htm", []() {
+>   server.send(200, "text/html", FPSTR(uploadContent));
+> });
+> ```
+
+Here the text from a static String with html code is returned instead of a file from the filesystem.
+The content of this string can be found in the file `builtinfiles.h`. It contains a small html+javascript implementation
+that allows uploading new files into the empty filesystem.
+
+Just open <http://webserver/$upload.htm> and drag some files from the data folder on the drop area.
+
+## Registering a function to handle requests to the server without a path
+
+Often servers are addressed by using the base URL like <http://webserver/> where no further path details is given.
+Of course we like the user to be redirected to something usable. Therefore the `handleRoot()` function is registered:
+
+> ``` cpp
+> server.on("/$upload.htm", handleRoot);
+> ```
+
+The `handleRoot()` function checks the filesystem for the file named **/index.htm** and creates a redirect to this file when the file exists.
+Otherwise the redirection goes to the built-in **/$upload.htm** web page.
+
+## Using the serveStatic plug-in
+
+The **serveStatic** plug in is part of the library and handles delivering files from the filesystem to the client. It can be customized in some ways.
+
+> ``` cpp
+> server.enableCORS(true);
+> server.enableETag(true);
+> server.serveStatic("/", LittleFS, "/");
+> ```
+
+### Cross-Origin Ressource Sharing (CORS)
+
+The `enableCORS(true)` function adds a `Access-Control-Allow-Origin: *` http-header to all responses to the client
+to inform that it is allowed to call URLs and services on this server from other web sites.
+
+The feature is disabled by default (in the current version) and when you like to disable this then you should call `enableCORS(false)` during setup.
+
+* Web sites providing high sensitive information like online banking this is disabled most of the times.
+* Web sites providing advertising information or reusable scripts / images this is enabled.
+
+### enabling ETag support
+
+To enable this in the embedded web server the `enableETag()` can be used.
+(next to enableCORS)
+
+In the simplest version just call `enableETag(true)` to enable the internal ETag generation that calcs the hint using a md5 checksum in base64 encoded form. This is an simple approach that adds some time for calculation on every request but avoids network traffic.
+
+The headers will look like:
+
+``` txt
+If-None-Match: "GhZka3HevoaEBbtQOgOqlA=="
+ETag: "GhZka3HevoaEBbtQOgOqlA=="
+```
+
+
+### ETag support customization
+
+The enableETag() function has an optional second optional parameter to provide a function for ETag calculation of files.
+
+The function enables eTags for all files by using calculating a value from the last write timestamp:
+
+``` cpp
+server.enableETag(true, [](FS &fs, const String &path) -> String {
+  File f = fs.open(path, "r");
+  String eTag = String(f.getLastWrite(), 16);  // use file modification timestamp to create ETag
+  f.close();
+  return (eTag);
+});
+```
+
+The headers will look like:
+
+``` txt
+ETag: "63bbaeb5"
+If-None-Match: "63bbaeb5"
+```
+
+
+## Registering a full-featured handler as plug-in
+
+The example also implements the class `FileServerHandler` derived from the class `RequestHandler` to plug in functionality
+that can handle more complex requests without giving a fixed URL.
+It implements uploading and deleting files in the file system that is not implemented by the standard server.serveStatic functionality.
+
+This class has to implements several functions and works in a more detailed way:
+
+* The `canHandle()` method can inspect the given http method and url to decide weather the RequestFileHandler can handle the incoming request or not.
+
+  In this case the RequestFileHandler will return true when the request method is an POST for upload or a DELETE for deleting files.
+
+  The regular GET requests will be ignored and therefore handled by the also registered server.serveStatic handler.
+
+* The function `handle()` then implements the real deletion of the file.
+
+* The `canUpload()`and `upload()` methods work similar while the `upload()` method is called multiple times to create, append data and close the new file.
+
+## File upload
+
+By opening <http://webserver/$upload.htm> you can easily upload files by dragging them over the drop area.
+
+Just take the files from the data folder to create some files that can explore the server functionality.
+
+Files will be uploaded to the root folder of the file system. and you will see it next time using  <http://webserver/files.htm>.
+
+The filesize that is uploaded is not known when the upload mechanism in function
+FileServerHandler::upload gets started.
+
+Uploading a file that fits into the available filesystem space
+can be found in the Serial output:
+
+``` txt
+starting upload file /file.txt...
+finished.
+1652 bytes uploaded.
+```
+
+Uploading a file that doesn't fit can be detected while uploading when writing to the filesystem fails.
+However upload cannot be aborted by the current handler implementation.
+
+The solution implemented here is to delete the partially uploaded file and wait for the upload ending.
+The following can be found in the Serial output:
+
+``` txt
+starting upload file /huge.jpg...
+./components/esp_littlefs/src/littlefs/lfs.c:584:error: No more free space 531
+  write error!
+finished.
+```
+
+You can see on the Serial output that one filesystem write error is reported.
+
+Please be patient and wait for the upload ending even when writing to the filesystem is disabled
+it maybe take more than a minute.
+
+## Registering a special handler for "file not found"
+
+Any other incoming request that was not handled by the registered plug-ins above can be detected by registering
+
+> ``` cpp
+> // handle cases when file is not found
+> server.onNotFound([]() {
+>   // standard not found in browser.
+>   server.send(404, "text/html", FPSTR(notFoundContent));
+> });
+> ```
+
+This allows sending back an "friendly" result for the browser. Here a simple html page is created from a static string.
+You can easily change the html code in the file `builtinfiles.h`.
+
+## customizations
+
+You may like to change the hostname and the timezone in the lines:
+
+> ``` cpp
+> #define HOSTNAME "webserver"
+> #define TIMEZONE "CET-1CEST,M3.5.0,M10.5.0/3"
+> ```
+
+## Troubleshooting
+
+Have a look in the Serial output for some additional runtime information.
+
+## Contribute
+
+To know how to contribute to this project, see [How to contribute.](https://github.com/espressif/arduino-esp32/blob/master/CONTRIBUTING.rst)
+
+If you have any **feedback** or **issue** to report on this example/library, please open an issue or fix it by creating a new PR. Contributions are more than welcome!
+
+Before creating a new issue, be sure to try Troubleshooting and check if the same issue was already created by someone else.
+
+## Resources
+
+* Official ESP32 Forum: [Link](https://esp32.com)
+* Arduino-ESP32 Official Repository: [espressif/arduino-esp32](https://github.com/espressif/arduino-esp32)
+* ESP32 Datasheet: [Link to datasheet](https://www.espressif.com/sites/default/files/documentation/esp32_datasheet_en.pdf)
+* ESP32-S2 Datasheet: [Link to datasheet](https://www.espressif.com/sites/default/files/documentation/esp32-s2_datasheet_en.pdf)
+* ESP32-C3 Datasheet: [Link to datasheet](https://www.espressif.com/sites/default/files/documentation/esp32-c3_datasheet_en.pdf)
+* Official ESP-IDF documentation: [ESP-IDF](https://idf.espressif.com)