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<p>> <span style="font-size:12pt">As ctxt is *not* a top context as required by #jump, we need to put a (fake) return value (nil) on its stack. Otherwise, #jump will pop something different from the stack. Concretely, this caused the bug described in [1] (Scenario
1) because the latest stack top was the closure vector {chain}. This closure vector was accidently popped away so that in the final return statement, #pushRemoteTemp:inVectorAt: raised an error subscript bounds (because the next stack item was not variable).</span></p>
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<div class="x__rp_T4" id="x_Item.MessagePartBody">To refine this description even more: From the perspective of the #jump context, the latest stack top of its receiver was the block closure in #contextEnsure: (or #contextOn:do:) rather than the closure vector.
So after returning from #jump, the closure vector was on the stack top and thus got popped by the next bytecode statement. But that's rather a detail; it does not affect the problem and its solution. Just for protocol.</div>
<div class="x__rp_T4" id="x_Item.MessagePartBody">Btw: I found the new <a href="http://forum.world.st/Changeset-Concept-BytecodeDebugger-3-cs-td5110352.html" class="x_OWAAutoLink">
BytecodeDebugger</a> quite useful for investigating this bug :-)</div>
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<div class="x__rp_T4" id="x_Item.MessagePartBody">Best,</div>
<div class="x__rp_T4" id="x_Item.MessagePartBody">Christoph</div>
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<div id="x_divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>Von:</b> Squeak-dev <squeak-dev-bounces@lists.squeakfoundation.org> im Auftrag von commits@source.squeak.org <commits@source.squeak.org><br>
<b>Gesendet:</b> Montag, 27. Januar 2020 12:59:29<br>
<b>An:</b> squeak-dev@lists.squeakfoundation.org<br>
<b>Betreff:</b> [squeak-dev] The Inbox: Kernel-ct.1296.mcz</font>
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<div class="PlainText">A new version of Kernel was added to project The Inbox:<br>
<a href="http://source.squeak.org/inbox/Kernel-ct.1296.mcz">http://source.squeak.org/inbox/Kernel-ct.1296.mcz</a><br>
<br>
==================== Summary ====================<br>
<br>
Name: Kernel-ct.1296<br>
Author: ct<br>
Time: 27 January 2020, 12:59:15.057199 pm<br>
UUID: e60f4637-b4fc-9947-94d3-50e7aa58ab82<br>
Ancestors: Kernel-tonyg.1293<br>
<br>
Fixes context simulation bug in #contextEnsure: and #contextOn:do:<br>
<br>
As ctxt is *not* a top context as required by #jump, we need to put a (fake) return value (nil) on its stack. Otherwise, #jump will pop something different from the stack. Concretely, this caused the bug described in [1] (Scenario 1) because the latest stack
top was the closure vector {chain}. This closure vector was accidently popped away so that in the final return statement, #pushRemoteTemp:inVectorAt: raised an error subscript bounds (because the next stack item was not variable). Read the linked bug report
for more details.<br>
<br>
[1] <a href="http://forum.world.st/BUG-s-in-Context-control-jump-runUntilErrorOrReturnFrom-td5107263.html">
http://forum.world.st/BUG-s-in-Context-control-jump-runUntilErrorOrReturnFrom-td5107263.html</a><br>
<br>
=============== Diff against Kernel-tonyg.1293 ===============<br>
<br>
Item was changed:<br>
----- Method: Context class>>contextEnsure: (in category 'special context creation') -----<br>
contextEnsure: block<br>
"Create an #ensure: context that is ready to return from executing its receiver"<br>
<br>
| ctxt chain |<br>
ctxt := thisContext.<br>
+ [chain := thisContext sender cut: ctxt.<br>
+ ctxt push: nil.<br>
+ ctxt jump] ensure: block.<br>
- [chain := thisContext sender cut: ctxt. ctxt jump] ensure: block.<br>
"jump above will resume here without unwinding chain"<br>
^ chain!<br>
<br>
Item was changed:<br>
----- Method: Context class>>contextOn:do: (in category 'special context creation') -----<br>
contextOn: exceptionClass do: block<br>
"Create an #on:do: context that is ready to return from executing its receiver"<br>
<br>
| ctxt chain |<br>
ctxt := thisContext.<br>
+ [chain := thisContext sender cut: ctxt.<br>
+ ctxt push: nil.<br>
+ ctxt jump] on: exceptionClass do: block.<br>
- [chain := thisContext sender cut: ctxt. ctxt jump] on: exceptionClass do: block.<br>
"jump above will resume here without unwinding chain"<br>
^ chain!<br>
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