XSnap REPL

[kriskowal]

Describe the bug

The xsrepl tool for debugging xsnap has been broken for some time.

To Reproduce

Steps to reproduce the behavior:

1. Go to 'packages/xsnap'
2. Run yarn xsrepl

Expected behavior

This should open up a CLI REPL you can use to send commands to an xsnap process. It’s useful for debugging xsnap.

Platform Environment

• agoric-sdk 5cd99eae25a3194dd04e77bc5bae23cd9ead981d

Additional context

The tool doesn’t have validation in CI so is doomed to rot. We should either delete it or CI it.

[mhofman]

I used this fairly recently, maybe a few weeks ago, what's broken about it?

[mhofman]

I just verified that it's working for me on master as of 7a08c6af641c62d8b0ef4a0c35090b6216a5be34

[kriskowal]

I’ve lost track of the circumstances this was originally reported on, but content that it’s working on master until we get a more specific isolation.

[erights]

Steps to reproduce the behavior:

1. Go to 'packages/xsnap'
2. Run yarn xsrepl

Doing that runs the xsrepl at agoric-sdk/node_modules/.bin/xsrepl. In that repl, a thrown error seems to cause that repl to hang forever, or until I type a control-C.

$ yarn xsrepl
yarn run v1.22.22
$ /Users/markmiller/src/ongithub/agoric/agoric-sdk/node_modules/.bin/xsrepl
xs> 2n/0n;
(Error#1)
Error#1: Uncaught exception in <unnamed xsnap worker>: RangeError: (anonymous-543): zero divider
  at runToIdle (file:///Users/markmiller/src/ongithub/agoric/agoric-sdk/packages/xsnap/src/xsnap.js:334:15)
  at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

Indeed, in xsnap package.json scripts at

agoric-sdk/packages/xsnap/package.json

Lines 13 to 14 in 3c66f4b

	"scripts": {
	"repl": "node src/xsrepl.js",

there is an entry for "repl": but no entry for "xsrepl":. So if I do a yarn repl, it does run src/xsrepl.js, but this also hangs forever on a thrown error:

$ yarn repl
yarn run v1.22.22
$ node src/xsrepl.js
xs> 2n/0n
(Error#1)
Error#1: Uncaught exception in <unnamed xsnap worker>: RangeError: (anonymous-543): zero divider
  at runToIdle (file:///Users/markmiller/src/ongithub/agoric/agoric-sdk/packages/xsnap/src/xsnap.js:334:15)
  at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

Reopening.

[erights]

Attn @kriskowal @mhofman @phoddie

[phoddie]

Is there something Team Moddable should look at here?

[mhofman]

I don't think there is anything Moddable needs to do here. We just need to better handle errors thrown in evaluated commands from our repl.

Arguably this should be filed as a separate issue.

[mhofman]

Potential fix:

diff --git a/packages/xsnap/src/xsrepl.js b/packages/xsnap/src/xsrepl.js
index 3eede346bb..bfeece538b 100755
--- a/packages/xsnap/src/xsrepl.js
+++ b/packages/xsnap/src/xsrepl.js
@@ -31,7 +31,12 @@ async function main() {
    * @returns {Promise<Uint8Array>}
    */
   async function handleCommand(message) {
-    console.log(decoder.decode(message));
+    const { success, result, error } = JSON.parse(decoder.decode(message));
+    if (success) {
+      console.log(result);
+    } else {
+      console.error(error);
+    }
     return new Uint8Array();
   }
 
@@ -50,11 +55,22 @@ async function main() {
     });
     function handleCommand(request) {
       const command = new TextDecoder().decode(request);
-      let result = compartment.evaluate(command);
-      if (result === undefined) {
-        result = null;
+      let result;
+      let error;
+      let success;
+      try {
+        result = compartment.evaluate(command);
+        success = true;
+      } catch (e) {
+        success = false;
+        try {
+          const {name, message, stack} = e;
+          error = {name, message, stack};
+        } catch (_e) {
+          error = String(e);
+        }
       }
-      issueCommand(new TextEncoder().encode(JSON.stringify(result, null, 4)).buffer);
+      issueCommand(new TextEncoder().encode(JSON.stringify({success, result, error}, null, 4)).buffer);
     }
   `);

[erights]

Time for a PR?