chore: Add context to errors thrown on cbor decode (#874)

We have the issue in Internet Identity that people complain about hard to
diagnose issues. In particular, we regularly encounter users faced with the
"failed to parse" message thrown out of `borc`. It would be very helpful
to know what the input was that caused that error.

docs/CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Changed
 
 - feat: make `IdbStorage` `get/set` methods generic
+- chore: add context to errors thrown when failing to decode CBOR values.
 
 ## [1.2.0] - 2024-03-25
 

packages/agent/src/cbor.ts

@@ -4,7 +4,7 @@ import borc from 'borc';
 import * as cbor from 'simple-cbor';
 import { CborEncoder, SelfDescribeCborSerializer } from 'simple-cbor';
 import { Principal } from '@dfinity/principal';
-import { concat, fromHex } from './utils/buffer';
+import { concat, fromHex, toHex } from './utils/buffer';
 
 // We are using hansl/simple-cbor for CBOR serialization, to avoid issues with
 // encoding the uint64 values that the HTTP handler of the client expects for
@@ -125,5 +125,9 @@ export function decode<T>(input: ArrayBuffer): T {
     },
   });
 
-  return decoder.decodeFirst(buffer);
+  try {
+    return decoder.decodeFirst(buffer);
+  } catch(e: unknown) {
+    throw new Error(`Failed to decode CBOR: ${e}, input: ${toHex(buffer)}`);
+  }
 }