docs: Rewrote README-emscripten.md

Fixes #6453.

docs/README-emscripten.md

@@ -1,8 +1,172 @@
 # Emscripten
 
-(This documentation is not very robust; we will update and expand this later.)
+## The state of things
 
-## A quick note about audio
+(As of September 2023, but things move quickly and we don't update this
+document often.)
+
+In modern times, all the browsers you probably care about (Chrome, Firefox,
+Edge, and Safari, on Windows, macOS, Linux, iOS and Android), support some
+reasonable base configurations:
+
+- WebAssembly (don't bother with asm.js any more)
+- WebGL (which will look like OpenGL ES 2 or 3 to your app).
+- Threads (see caveats, though!)
+- Game controllers
+- Autoupdating (so you can assume they have a recent version of the browser)
+
+All this to say we're at the point where you don't have to make a lot of
+concessions to get even a fairly complex SDL-based game up and running.
+
+
+## RTFM
+
+This document is a quick rundown of some high-level details. The
+documentation at [emscripten.org](https://emscripten.org/) is vast
+and extremely detailed for a wide variety of topics, and you should at
+least skim through it at some point.
+
+
+## Porting your app to Emscripten
+
+Many many things just need some simple adjustments and they'll compile
+like any other C/C++ code, as long as SDL was handling the platform-specific
+work for your program.
+
+First, you probably need this in at least one of your source files:
+
+```c
+#ifdef __EMSCRIPTEN__
+#include <emscripten.h>
+#endif
+```
+
+Second: assembly language code has to go. Replace it with C. You can even use
+[x86 SIMD intrinsic functions in Emscripten](https://emscripten.org/docs/porting/simd.html)!
+
+Third: Middleware has to go. If you have a third-party library you link
+against, you either need an Emscripten port of it, or the source code to it
+to compile yourself, or you need to remove it.
+
+Fourth: You still start in a function called main(), but you need to get out of
+it and into a function that gets called repeatedly, and returns quickly,
+called a mainloop.
+
+Somewhere in your program, you probably have something that looks like a more
+complicated version of this:
+
+```c
+void main(void)
+{
+    initialize_the_game();
+    while (game_is_still_running) {
+        check_for_new_input();
+        think_about_stuff();
+        draw_the_next_frame();
+    }
+    deinitialize_the_game();
+}
+```
+
+This will not work on Emscripten, because the main thread needs to be free
+to do stuff and can't sit in this loop forever. So Emscripten lets you set up
+a [mainloop](https://emscripten.org/docs/porting/emscripten-runtime-environment.html#browser-main-loop).
+
+```c
+static void mainloop(void)   /* this will run often, possibly at the monitor's refresh rate */
+{
+    if (!game_is_still_running) {
+        deinitialize_the_game();
+        #ifdef __EMSCRIPTEN__
+        emscripten_cancel_main_loop();  /* this should "kill" the app. */
+        #else
+        exit(0);
+        #endif
+    }
+
+    check_for_new_input();
+    think_about_stuff();
+    draw_the_next_frame();
+}
+
+void main(void)
+{
+    initialize_the_game();
+    #ifdef __EMSCRIPTEN__
+    emscripten_set_main_loop(mainloop, 0, 1);
+    #else
+    while (1) { mainloop(); }
+    #endif
+}
+```
+
+Basically, `emscripten_set_main_loop(mainloop, 0, 1);` says "run
+`mainloop` over and over until I end the program." The function will
+run, and return, freeing the main thread for other tasks, and then
+run again when it's time. The `1` parameter does some magic to make
+your main() function end immediately; this is useful because you
+don't want any shutdown code that might be sitting below this code
+to actually run if main() were to continue on, since we're just
+getting started.
+
+There's a lot of little details that are beyond the scope of this
+document, but that's the biggest intial set of hurdles to porting
+your app to the web.
+
+
+## Do you need threads?
+
+If you plan to use threads, they work on all major browsers now. HOWEVER,
+they bring with them a lot of careful considerations. Rendering _must_
+be done on the main thread. This is a general guideline for many
+platforms, but a hard requirement on the web.
+
+Many other things also must happen on the main thread; often times SDL
+and Emscripten make efforts to "proxy" work to the main thread that
+must be there, but you have to be careful (and read more detailed
+documentation than this for the finer points).
+
+Even when using threads, your main thread needs to set an Emscripten
+mainloop that runs quickly and returns, or things will fail to work
+correctly. More on this later.
+
+You should definitely read [Emscripten's pthreads docs](https://emscripten.org/docs/porting/pthreads.html)
+for all the finer points. Mostly SDL's thread API will work as expected,
+but is built on pthreads, so it shares the same little incompatibilities
+that are documented there, such as where you can use a mutex, and when
+a thread will start running, etc.
+
+
+IMPORTANT: You have to decide to either build something that uses
+threads or something that doesn't; you can't have one build
+that works everywhere. This is an Emscripten (or maybe WebAssembly?
+Or just web browsers in general?) limitation. If you aren't using
+threads, it's easier to not enable them at all, at build time.
+
+If you use threads, you _have to_ run from a web server that has
+[COOP/COEP headers set correctly](https://web.dev/why-coop-coep/)
+or your program will fail to start at all.
+
+If building with threads, `__EMSCRIPTEN_PTHREADS__` will be defined
+for checking with the C preprocessor, so you can build something
+different depending on what sort of build you're compiling.
+
+
+## Audio
+
+Audio works as expected at the API level, but not exactly like other
+platforms.
+
+You'll only see a single default audio device. Audio capture also works;
+if the browser pops up a prompt to ask for permission to access the
+microphone, the SDL_OpenAudioDevice call will succeed and start producing
+silence at a regular interval. Once the user approves the request, real
+audio data will flow. If the user denies it, the app is not informed and
+will just continue to receive silence.
+
+SDL3 allows you to open the audio device at any time; it'll just throw away
+audio data until the user interacts with the page, and then start feeding
+the browser seamlessly, but for SDL2, you need to manage this yourself.
 
 Modern web browsers will not permit web pages to produce sound before the
 user has interacted with them; this is for several reasons, not the least
@@ -35,42 +199,141 @@ all to make it happen.
 Please see the discussion at https://github.com/libsdl-org/SDL/issues/6385
 for some Javascript code to steal for this approach.
 
+SDL3 allows you to open the audio device at any time; it'll just throw away
+audio data until the user interacts with the page, and then start feeding
+the browser seamlessly, but for SDL2, you need to manage this yourself.
+
+
+## Rendering
+
+If you use SDL's 2D render API, it will use GLES2 internally, which
+Emscripten will turn into WebGL calls. You can also use OpenGL ES 2
+directly by creating a GL context and drawing into it.
+
+Calling SDL_RenderPresent (or SDL_GL_SwapWindow) will not actually
+present anything on the screen until your return from your mainloop
+function.
+
 
 ## Building SDL/emscripten
 
+First: do you _really_ need to build SDL from source?
+
+If you aren't developing SDL itself, have a desire to mess with its source
+code, or need something on the bleeding edge, don't build SDL. Just use
+Emscripten's packaged version!
+
+Compile and link your app with `-s USE_SDL=2` and it'll use a build of
+SDL packaged with Emscripten. This comes from the same source code and
+fixes the Emscripten project makes to SDL are generally merged into SDL's
+revision control, so often this is much easier for app developers.
+
+`-s USE_SDL=1` will select Emscripten's JavaScript reimplementation of SDL
+1.2 instead; if you need SDL 1.2, this might be fine, but we generally
+recommend you don't use SDL 1.2 in modern times.
+
+
+If you want to build SDL, though...
+
 SDL currently requires at least Emscripten 3.1.35 to build. Newer versions
 are likely to work, as well.
 
 
 Build:
 
-    $ mkdir build
-    $ cd build
-    $ emconfigure ../configure --host=asmjs-unknown-emscripten --disable-assembly --disable-threads --disable-cpuinfo CFLAGS="-O2"
-    $ emmake make
+This works on Linux/Unix and macOS. Please send comments about Windows.
+
+Make sure you've [installed emsdk](https://emscripten.org/docs/getting_started/downloads.html)
+first, and run `source emsdk_env.sh` at the command line so it finds the
+tools.
+
+(These configure options might be overkill, but this has worked for me.)
+
+```bash
+cd SDL
+mkdir build
+cd build
+emconfigure ../configure --host=wasm32-unknown-emscripten --disable-pthreads --disable-assembly --disable-cpuinfo CFLAGS="-s USE_SDL=0 -O3"
+emmake make -j4
+```
+
+If you want to build with thread support, something like this works:
+
+```bash
+emconfigure ../configure --host=wasm32-unknown-emscripten --enable-pthreads --disable-assembly --disable-cpuinfo CFLAGS="-s USE_SDL=0 -O3 -pthread" LDFLAGS="-pthread"
+```
 
 Or with cmake:
 
-    $ mkdir build
-    $ cd build
-    $ emcmake cmake ..
-    $ emmake make
+```bash
+mkdir build
+cd build
+emcmake cmake ..
+emmake make -j4
+```
 
 To build one of the tests:
 
-    $ cd test/
-    $ emcc -O2 --js-opts 0 -g4 testdraw2.c -I../include ../build/.libs/libSDL2.a ../build/libSDL2_test.a -o a.html
+```bash
+cd test/
+emcc -O2 --js-opts 0 -g4 testdraw2.c -I../include ../build/.libs/libSDL2.a ../build/libSDL2_test.a -o a.html
+```
+
+## Building your app
+
+You need to compile with `emcc` instead of `gcc` or `clang` or whatever, but
+mostly it uses the same command line arguments as Clang.
+
+Link against the SDL/build/.libs/libSDL2.a file you generated by building SDL,
+link with `-s USE_SDL=2` to use Emscripten's prepackaged SDL2 build.
+
+Usually you would produce a binary like this:
+
+```bash
+gcc -o mygame mygame.c  # or whatever
+```
+
+But for Emscripten, you want to output something else:
+
+```bash
+gcc -o index.html mygame.c  # or whatever
+```
+
+This will produce several files...support Javascript and WebAssembly (.wasm)
+files. The `-o index.html` will produce a simple HTML page that loads and
+runs your app. You will (probably) eventually want to replace or customize
+that file and do `-o index.js` instead to just build the code pieces.
+
+If you're working on a program of any serious size, you'll likely need to
+link with `-s ALLOW_MEMORY_GROWTH=1 -s MAXIMUM_MEMORY=1gb` to get access
+to more memory. If using pthreads, you'll need the `-s MAXIMUM_MEMORY=1gb`
+or the app will fail to start on iOS browsers, but this might be a bug that
+goes away in the future.
+
+
+
+## Debugging
+
+Debugging web apps is a mixed bag. You should compile and link with
+`-gsource-map`, which embeds a ton of source-level debugging information into
+the build, and make sure _the app source code is available on the web server_,
+which is often a scary proposition for various reasons.
+
+When you debug from the browser's tools and hit a breakpoint, you can step
+through the actual C/C++ source code, though, which can be nice.
+
+If you try debugging in Firefox and it doesn't work well for no apparent
+reason, try Chrome, and vice-versa. Sometimes it's just like that.
+
+SDL_Log() will write to the Javascript console, and honestly I find
+printf-style debugging to be easier than setting up a build for proper
+debugging, so use whatever tools work best for you.
 
-Uses GLES2 renderer or software
 
-Some other SDL2 libraries can be easily built (assuming SDL2 is installed somewhere):
+## Questions?
 
-SDL_mixer (http://www.libsdl.org/projects/SDL_mixer/):
+Please give us feedback on this document at [the SDL bug tracker](https://github.com/libsdl-org/SDL/issues).
+If something is wrong or unclear, we want to know!
 
-    $ EMCONFIGURE_JS=1 emconfigure ../configure
-    build as usual...
 
-SDL_gfx (http://cms.ferzkopp.net/index.php/software/13-sdl-gfx):
 
-    $ EMCONFIGURE_JS=1 emconfigure ../configure --disable-mmx
-    build as usual...