Segfault when referencing a removed action via Python #39600

New Issue

Jason van Gumster · 2014-04-04T19:29:13+02:00

Jason van Gumster commented

2014-04-04 19:29:13 +02:00

System Information
Arch Linux, Nvidia Quadro K5000M

Blender Version
Broken: master - 2dedf2a

Short description of error
Blender crashes if you try to reference an action datablock after it's been removed via Python.

Exact steps for others to reproduce the error
Use the attached .blend file

Open the file and run the embedded python script.
Blender will crash if you click anywhere in the interface

Also attached is a gdb backtrace

action_ref-segfault.blend

action_ref-segfault_backtrace.txt

**System Information** Arch Linux, Nvidia Quadro K5000M **Blender Version** Broken: master - 2dedf2a **Short description of error** Blender crashes if you try to reference an action datablock after it's been removed via Python. **Exact steps for others to reproduce the error** Use the attached .blend file 1. Open the file and run the embedded python script. 2. Blender will crash if you click anywhere in the interface Also attached is a gdb backtrace [action_ref-segfault.blend](https://archive.blender.org/developer/F83952/action_ref-segfault.blend) [action_ref-segfault_backtrace.txt](https://archive.blender.org/developer/F83953/action_ref-segfault_backtrace.txt)

Jason van Gumster commented

2014-04-04 19:29:13 +02:00

Changed status to: 'Open'

Jason van Gumster commented

2014-04-04 19:29:13 +02:00

Added subscriber: @JasonvanGumster

Lukas Tönne commented

2014-04-05 12:21:12 +02:00

Added subscribers: @ideasman42, @LukasTonne

Lukas Tönne commented

2014-04-05 12:21:12 +02:00

Unfortunately there is no good way to support explicit deletion of ID data blocks from python ...

ID.user_clear simply sets the user count to 0 - without any kind of check. I'm not sure if there is a legit use case for this function, but it has to be used with extreme care and should be avoided whenever possible.
bpy.data.***.remove is even more dangerous, it directly deletes ID blocks from the library, but also doesn't clear any references to these blocks in the general case, leaving you with dangling pointers and invalid python objects (such as the action variable in your case)

According to @ideasman42 there is no proper way to support these things efficiently. There is a build option WITH_PYTHON_SAFETY which would at least keep python variables valid, but still doesn't solve issues with remove() function calls.

Some data blocks have "unlink" functionality build into their freeing functions (Object, Group, Text, ...) which should help keep the C pointer sane, but this is not universally implemented (in particular Action doesn't do it yet).

TL;DR: Never ever use bpy.data.xxx.remove() unless you're absolutely sure what you are doing (basically: be sure there are no references to that data block left). Sorry, it's a difficult technical issue :(

Unfortunately there is no good way to support explicit deletion of ID data blocks from python ... * `ID.user_clear` simply sets the user count to 0 - without any kind of check. I'm not sure if there is a legit use case for this function, but it has to be used with extreme care and should be avoided whenever possible. * `bpy.data.***.remove` is even more dangerous, it directly deletes ID blocks from the library, but also doesn't clear any references to these blocks in the general case, leaving you with dangling pointers and invalid python objects (such as the `action` variable in your case) According to @ideasman42 there is no proper way to support these things efficiently. There is a build option WITH_PYTHON_SAFETY which would at least keep python variables valid, but still doesn't solve issues with `remove()` function calls. *Some* data blocks have "unlink" functionality build into their freeing functions (Object, Group, Text, ...) which should help keep the C pointer sane, but this is not universally implemented (in particular Action doesn't do it yet). TL;DR: **Never ever** use bpy.data.xxx.remove() unless you're absolutely sure what you are doing (basically: be sure there are no references to that data block left). Sorry, it's a difficult technical issue :(

Lukas Tönne commented

2014-04-05 12:23:05 +02:00

Changed status from 'Open' to: 'Archived'

Lukas Tönne closed this issue

2014-04-05 12:23:05 +02:00

Lukas Tönne self-assigned this 2014-04-05 12:23:05 +02:00

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Segfault when referencing a removed action via Python #39600