debugger: Handle special return values from Environment.getVariable()

Debugger.Environment.getVariable() may return one of the three sentinel
values {missingArguments: true}, {optimizedOut: true}, or {uninitialized:
true} if a variable does not have a value for some reason. Handle these
special values in debuggeeValueToString().

"missingArguments" means that a function argument was optimized out,
"optimizedOut" means that a regular variable was optimized out, and
"uninitialized" means that a variable is present in the environment but
hasn't been initialized yet, for example when breaking on an exception
thrown while initializing it.

Also adds a test for the latter case (printing a backtrace with locals,
when breaking on an exception that leaves a local variable uninitialized.)

https://searchfox.org/mozilla-central/rev/6a6a366031680829746b5d2362610b868fd9571a/js/src/debugger/Debugger.cpp#1412-1435

1 files changed, 12 insertions, 0 deletions

diff --git a/modules/script/_bootstrap/debugger.js b/modules/script/_bootstrap/debugger.js
index d1b22ed9..0cffe703 100644
--- a/modules/script/_bootstrap/debugger.js
+++ b/modules/script/_bootstrap/debugger.js
@@ -48,6 +48,18 @@ function summarizeObject(dv) {
 }
 
 function debuggeeValueToString(dv, style = {pretty: options.pretty}) {
+    // Special sentinel values returned by Debugger.Environment.getVariable()
+    if (typeof dv === 'object' && dv !== null) {
+        if (dv.missingArguments)
+            return ['<missing>', undefined];
+        if (dv.optimizedOut)
+            return ['<optimized out>', undefined];
+        if (dv.uninitialized)
+            return ['<uninitialized>', undefined];
+        if (!(dv instanceof Debugger.Object))
+            return ['<unexpected object>', JSON.stringify(dv, null, 4)];
+    }
+
     const dvrepr = dvToString(dv);
     if (!style.pretty || dv === null || typeof dv !== 'object')
         return [dvrepr, undefined];