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In wasm builds, ifdef out shell commands which require file I/O, pipes, or which trigger an exit() (.quit and .exit). Documented some of the quirks and limitations of the C/WASM crossover. Keep the JS code from calling into the C code after an exit() has been triggered.

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FossilOrigin-Name: bee436e62a956e49b0df4a92abff2c89f2b44e21d8f593716df0331f8fc49814
This commit is contained in:
stephan
2022-05-18 22:58:34 +00:00
parent b0dae2b3c3
commit 02520cc8f6
5 changed files with 132 additions and 25 deletions
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66
ext/fiddle/fiddle.in.html
View File

@ -53,13 +53,22 @@
.button-bar button {
margin: 0.25em 1em;
}
label {
label[for] {
cursor: pointer;
}
fieldset {
border-radius: 0.5em;
}
.error {
color: red;
background-color: yellow;
}
.hidden {
position: absolute !important;
opacity: 0 !important;
pointer-events: none !important;
display: none !important;
}
</style>
</head>
<body>
@ -97,6 +106,34 @@ select * from t;</textarea>
<label for='opt-cb-sbs'>Side-by-side</label>
</div>
</fieldset>
<hr>
<header>
Notes and Caveats
(<input id='cb-notes-caveats' type='checkbox'>
<label for='cb-notes-caveats'>hide</label>)
</header>
<div id='notes-caveats'>
<p>
This JavaScript application runs a C application which has been
compiled into WASM (Web Assembly). As such, it has certain
limitations. Those include, but are not limited to:
</p>
<ul>
<li>It <strong>cannot recover after a call to
<code>exit()</code></strong>. If the native code triggers
an exit, reloading the page is the only way to restart
the application. (Making the app restartable without reloading
the page would require significant surgery in the C code.)
</li>
<li>It <strong>cannot perform any file I/O</strong>, and running
any command which attempts to do so might trigger an
<code>exit()</code>.
</li>
<li>A number of dot-commands available in the CLI shell are
explicitly removed from this version of the shell.
</li>
</ul>
</div><!-- #notes-caveats -->
<script type='text/javascript'>
(function(){
/**
@ -126,7 +163,14 @@ select * from t;</textarea>
btnClearOut.addEventListener('click',function(){
taOutput.value = '';
},false);
const doExec = Module.cwrap('fiddle_exec', null, ['string']);
const doExec = function f(sql){
if(!f._) f._ = Module.cwrap('fiddle_exec', null, ['string']);
if(Module._isDead){
Module.printErr("shell module has exit()ed. Cannot run SQL.");
}else{
f._(sql);
}
};
const btnRun = document.querySelector('#btn-run');
btnRun.addEventListener('click',function(){
const sql = taInput.value.trim();
@ -136,11 +180,18 @@ select * from t;</textarea>
},false);
doExec(null)/*sets up the db and outputs the header*/;
let e = document.querySelector('#opt-cb-sbs');
const mainWrapper = document.querySelector('#main-wrapper');
e.addEventListener('change', function(){
mainWrapper.classList[this.checked ? 'add' : 'remove']('side-by-side');
}, false);
document.querySelector('#opt-cb-sbs')
.addEventListener('change', function(){
document.querySelector('#main-wrapper').classList[
this.checked ? 'add' : 'remove'
]('side-by-side');
}, false);
document.querySelector('#cb-notes-caveats')
.addEventListener('change', function(){
document.querySelector('#notes-caveats').classList[
this.checked ? 'add' : 'remove'
]('hidden');
}, false);
/* For all buttons with data-cmd=X, map a click handler which
calls doExec(X). */
@ -217,6 +268,7 @@ select * from t;</textarea>
window.onerror = function(/*message, source, lineno, colno, error*/) {
const err = arguments[4];
if(err && 'ExitStatus'==err.name){
Module._isDead = true;
Module.printErr("FATAL ERROR:", err.message);
Module.printErr("Restarting the app requires reloading the page.");
const taOutput = document.querySelector('#output');

25
ext/fiddle/index.md
View File

@ -64,7 +64,30 @@ 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.
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.
[emscripten]: https://emscripten.org