[gjs: 2/7] debugger: Handle special return values from Environment.getVariable()

[Philip Chimento <pchimento src gnome org>]

commit e5b1e32ae2f2afe3e3ce388aafcc1245ae41545e
Author: Philip Chimento <philip chimento gmail com>
Date:   Fri Dec 18 18:29:49 2020 -0800

    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

 installed-tests/debugger/backtrace.debugger        |  4 ++++
 installed-tests/debugger/backtrace.debugger.js     | 10 ++++++++--
 installed-tests/debugger/backtrace.debugger.output | 15 +++++++++++++++
 modules/script/_bootstrap/debugger.js              | 12 ++++++++++++
 4 files changed, 39 insertions(+), 2 deletions(-)
---
diff --git a/installed-tests/debugger/backtrace.debugger b/installed-tests/debugger/backtrace.debugger
index 261d388a..6f9d9516 100644
--- a/installed-tests/debugger/backtrace.debugger
+++ b/installed-tests/debugger/backtrace.debugger
@@ -7,4 +7,8 @@ c
 backtrace full
 bt full
 where
+c
+# test printing locals when exception is thrown before initialization of a value
+c
+bt full
 q
diff --git a/installed-tests/debugger/backtrace.debugger.js b/installed-tests/debugger/backtrace.debugger.js
index 9b84935b..7816d4d9 100644
--- a/installed-tests/debugger/backtrace.debugger.js
+++ b/installed-tests/debugger/backtrace.debugger.js
@@ -1,10 +1,16 @@
 // SPDX-License-Identifier: MIT OR LGPL-2.0-or-later
 // SPDX-FileCopyrightText: 2018 Philip Chimento <philip chimento gmail com>
 debugger;
-[[1, 2, 3, 4, 5], [6, 7, 8, 9, 10]].forEach(array => {
+[[1, 2, 3, 4, 5], [6, 7, 8, 9, 10]].every(array => {
     debugger;
-    array.forEach(num => {
+    array.every(num => {
         debugger;
         print(num);
+        return false;
     });
+    return false;
 });
+function mistake(array) {
+    let {uninitialized_} = array.shift();
+}
+mistake([]);
diff --git a/installed-tests/debugger/backtrace.debugger.output 
b/installed-tests/debugger/backtrace.debugger.output
index f5353e29..a74e0b06 100644
--- a/installed-tests/debugger/backtrace.debugger.output
+++ b/installed-tests/debugger/backtrace.debugger.output
@@ -22,5 +22,20 @@ array = [object Array]
 db> where
 #0    <anonymous>([object Array], 0, [object Array]) at backtrace.debugger.js:5:4
 #1    toplevel at backtrace.debugger.js:4:36
+db> c
+Debugger statement, <anonymous>(1, 0, [object Array]) at backtrace.debugger.js:7:8
+db> # test printing locals when exception is thrown before initialization of a value
+db> c
+1
+Unwinding due to exception. (Type 'c' to continue unwinding.)
+#0    mistake([object Array]) at backtrace.debugger.js:14:33
+   14      let {uninitialized_} = array.shift();
+Exception value is:
+$1 = [object Error]
+TypeError: array.shift() is undefined
+db> bt full
+#0    mistake([object Array]) at backtrace.debugger.js:14:33
+uninitialized_ = <uninitialized>
+#1    toplevel at backtrace.debugger.js:16:7
 db> q
 Program exited with code 0
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];