sqlite/ext/fiddle/index.md

2.9 KiB

This directory houses a "fiddle"-style application which embeds a Web Assembly (WASM) build of the sqlite3 shell app into an HTML page, effectively running the shell in a client-side browser.

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:
$ 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. 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

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

$ make fiddle

Or:

$ cd ext/fiddle
$ 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/fiddle
$ althttpd -debug 1 -jail 0 -port 9090 -root .

Then browse to http://localhost:9090/fiddle.html.

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.

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.