sqlite/ext/wasm/README.md

This directory houses the Web Assembly (WASM) parts of the sqlite3 build.

It requires emscripten and that the build environment be set up for emscripten. A mini-HOWTO for setting that up follows...

First, install the Emscripten SDK, as documented here and summarized below for Linux environments:

# Clone the emscripten repository:
$ sudo apt install git
$ git clone https://github.com/emscripten-core/emsdk.git
$ cd emsdk

# Download and install the latest SDK tools:
$ ./emsdk install latest

# Make the "latest" SDK "active" for the current user:
$ ./emsdk activate latest

Those parts only need to be run once, but the SDK can be updated using:

$ git pull
$ ./emsdk activate latest

The following needs to be run for each shell instance which needs the emcc compiler:

# Activate PATH and other environment variables in the current terminal:
$ source ./emsdk_env.sh

$ which emcc
/path/to/emsdk/upstream/emscripten/emcc

Optionally, add that to your login shell's resource file (~/.bashrc or equivalent).

That env script needs to be sourced for building this application from the top of the sqlite3 build tree:

$ make fiddle

Or:

$ cd ext/wasm
$ make

That will generate the fiddle application under ext/fiddle, as fiddle.html. That application cannot, due to XMLHttpRequest security limitations, run if the HTML file is opened directly in the browser (i.e. if it is opened using a file:// URL), so it needs to be served via an HTTP server. For example, using althttpd:

$ cd ext/wasm/fiddle
$ althttpd -page fiddle.html

That will open the system's browser and run the fiddle app's page.

Note that when serving this app via althttpd, it must be a version from 2022-05-17 or newer so that it recognizes the .wasm file extension and responds with the mimetype application/wasm, as the WASM loader is pedantic about that detail.

Testing on a remote machine that is accessed via SSH

NB: The following are developer notes, last validated on 2022-08-18

• Remote: Install git, emsdk, and althttpd
  • Use a version of althttpd that adds HTTP reply header lines to enable SharedArrayBuffers. These header lines are required:

            Cross-Origin-Opener-Policy: same-origin
            Cross-Origin-Embedder-Policy: require-corp

• Remote: Install the SQLite source tree. CD to ext/wasm
• Remote: "make" to build WASM
• Remote: althttpd --port 8080 --popup
• Local: ssh -L 8180:localhost:8080 remote
• Local: Point your web-browser at http://localhost:8180/testing1.html

In order to enable SharedArrayBuffers, the web-browser requires that the two extra Cross-Origin lines be present in HTTP reply headers and that the request must come from "localhost". Since the web-server is on a different machine from the web-broser, the localhost requirement means that the connection must be tunneled using SSH.

Known Quirks and Limitations

Some "impedence mismatch" between C and WASM/JavaScript is to be expected.

No I/O

sqlite3 shell commands which require file I/O or pipes are disabled in the WASM build.

exit() Triggered from C

When C code calls exit(), as happens (for example) when running an "unsafe" command when safe mode is active, WASM's connection to the sqlite3 shell environment has no sensible choice but to shut down because exit() leaves it in a state we can no longer recover from. The JavaScript-side application attempts to recognize this and warn the user that restarting the application is necessary. Currently the only way to restart it is to reload the page. Restructuring the shell code such that it could be "rebooted" without restarting the JS app would require some invasive changes which are not currently on any TODO list but have not been entirely ruled out long-term.