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The Inbox: Kernel-ct.1296.mcz

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The Inbox: Kernel-ct.1296.mcz

A new version of Kernel was added to project The Inbox:
http://source.squeak.org/inbox/Kernel-ct.1296.mcz

==================== Summary ====================

Name: Kernel-ct.1296
Author: ct
Time: 27 January 2020, 12:59:15.057199 pm
UUID: e60f4637-b4fc-9947-94d3-50e7aa58ab82
Ancestors: Kernel-tonyg.1293

Fixes context simulation bug in #contextEnsure: and #contextOn:do:

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.

[1] http://forum.world.st/BUG-s-in-Context-control-jump-runUntilErrorOrReturnFrom-td5107263.html

=============== Diff against Kernel-tonyg.1293 ===============

Item was changed:
----- Method: Context class>>contextEnsure: (in category 'special context creation') -----
contextEnsure: block
"Create an #ensure: context that is ready to return from executing its receiver"

| ctxt chain |
ctxt := thisContext.
+ [chain := thisContext sender cut: ctxt.
+ ctxt push: nil.
+ ctxt jump] ensure: block.
- [chain := thisContext sender cut: ctxt. ctxt jump] ensure: block.
"jump above will resume here without unwinding chain"
^ chain!

Item was changed:
----- Method: Context class>>contextOn:do: (in category 'special context creation') -----
contextOn: exceptionClass do: block
"Create an #on:do: context that is ready to return from executing its receiver"

| ctxt chain |
ctxt := thisContext.
+ [chain := thisContext sender cut: ctxt.
+ ctxt push: nil.
+ ctxt jump] on: exceptionClass do: block.
- [chain := thisContext sender cut: ctxt. ctxt jump] on: exceptionClass do: block.
"jump above will resume here without unwinding chain"
^ chain!

Christoph Thiede

Re: The Inbox: Kernel-ct.1296.mcz

> 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).

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.

Btw: I found the new BytecodeDebugger quite useful for investigating this bug :-)

Best,

Christoph

Von: Squeak-dev <[hidden email]> im Auftrag von [hidden email] <[hidden email]>
Gesendet: Montag, 27. Januar 2020 12:59:29
An: [hidden email]
Betreff: [squeak-dev] The Inbox: Kernel-ct.1296.mcz

A new version of Kernel was added to project The Inbox:
http://source.squeak.org/inbox/Kernel-ct.1296.mcz

==================== Summary ====================

Name: Kernel-ct.1296
Author: ct
Time: 27 January 2020, 12:59:15.057199 pm
UUID: e60f4637-b4fc-9947-94d3-50e7aa58ab82
Ancestors: Kernel-tonyg.1293

Fixes context simulation bug in #contextEnsure: and #contextOn:do:

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.

[1] http://forum.world.st/BUG-s-in-Context-control-jump-runUntilErrorOrReturnFrom-td5107263.html

=============== Diff against Kernel-tonyg.1293 ===============

Item was changed:
----- Method: Context class>>contextEnsure: (in category 'special context creation') -----
contextEnsure: block
         "Create an #ensure: context that is ready to return from executing its receiver"

         | ctxt chain |
         ctxt := thisContext.
+        [chain := thisContext sender cut: ctxt.
+        ctxt push: nil.
+        ctxt jump] ensure: block.
-        [chain := thisContext sender cut: ctxt. ctxt jump] ensure: block.
         "jump above will resume here without unwinding chain"
         ^ chain!

Item was changed:
----- Method: Context class>>contextOn:do: (in category 'special context creation') -----
contextOn: exceptionClass do: block
         "Create an #on:do: context that is ready to return from executing its receiver"

         | ctxt chain |
         ctxt := thisContext.
+        [chain := thisContext sender cut: ctxt.
+        ctxt push: nil.
+        ctxt jump] on: exceptionClass do: block.
-        [chain := thisContext sender cut: ctxt. ctxt jump] on: exceptionClass do: block.
         "jump above will resume here without unwinding chain"
         ^ chain!

Carpe Squeak!