Crash when trying to write to the same text buffer from python after undo #53530

Closed
opened 2017-12-09 19:53:16 +01:00 by Ilya Portnov · 8 comments

System Information
Reproduces both on Ubuntu and Debian, different nvidia cards. Also reproduces on MacOS x64 and Win 10 x64.

Blender Version
Broken: both official 2.79 build and blender-2.79-e1eb1fbfca-linux-glibc219-x86_64 from builder.blender.org.

Short description of error

There is a scenario, when python script writes to the text buffer, then there is undo, and after that if script tries to write to the same buffer - blender crashes.

Initially this was found with Sverchok addon (see https://github.com/nortikin/sverchok/issues/1918 - an issue in sverchok repo, there also people saying they can reproduce this). Then I was able to reproduce the same with much simpler addon.

Exact steps for others to reproduce the error

  • Install sample script as an addon. Source is: https://gist.github.com/portnov/d9f5cab6188fa1b8d32b64f742c9087d, will also attach as zip for simplicity.
  • This addon registers a panel under T panel / Misc in 3D view. Go to that panel and press "Init text buffer".
  • You can observe that there is new empty text buffer in the scene.
  • Press "Write some text".
  • You can observe that a line of text appeared in the text buffer.
  • Press Ctrl-Z
  • You can observe that the line of text appeared previously disappeared. IMHO this looks like a bug by itself, but different one.
  • Press "Write some text" again.

Actual Result: on my Debian computer, blender crashes (will attach txt file). On Ubuntu computer, blender writes "Attempt to free NULL pointer" to console and hangs while eating 100% CPU.
Expected result: the same line of text appears again in the text buffer.

My humble guesses of what is going on: Python script stores a reference to Text object (it is initialized when you press "Init text buffer"). When user presses Ctrl-Z, that Text object is for some reason invalidated on C side. And when Python tries to write to the same object again, we have a crash.

**System Information** Reproduces both on Ubuntu and Debian, different nvidia cards. Also reproduces on MacOS x64 and Win 10 x64. **Blender Version** Broken: both official 2.79 build and blender-2.79-e1eb1fbfca-linux-glibc219-x86_64 from builder.blender.org. **Short description of error** There is a scenario, when python script writes to the text buffer, then there is undo, and after that if script tries to write to the same buffer - blender crashes. Initially this was found with Sverchok addon (see https://github.com/nortikin/sverchok/issues/1918 - an issue in sverchok repo, there also people saying they can reproduce this). Then I was able to reproduce the same with much simpler addon. **Exact steps for others to reproduce the error** * Install sample script as an addon. Source is: https://gist.github.com/portnov/d9f5cab6188fa1b8d32b64f742c9087d, will also attach as zip for simplicity. * This addon registers a panel under T panel / Misc in 3D view. Go to that panel and press "Init text buffer". * You can observe that there is new empty text buffer in the scene. * Press "Write some text". * You can observe that a line of text appeared in the text buffer. * Press Ctrl-Z * You can observe that the line of text appeared previously disappeared. IMHO this looks like a bug by itself, but different one. * Press "Write some text" again. Actual Result: on my Debian computer, blender crashes (will attach txt file). On Ubuntu computer, blender writes "Attempt to free NULL pointer" to console and hangs while eating 100% CPU. Expected result: the same line of text appears again in the text buffer. My humble guesses of what is going on: Python script stores a reference to Text object (it is initialized when you press "Init text buffer"). When user presses Ctrl-Z, that Text object is for some reason invalidated on C side. And when Python tries to write to the same object again, we have a crash.
Author

Added subscriber: @portnov

Added subscriber: @portnov
Author

blender.crash.txt - crash.txt from blender.

[blender.crash.txt](https://archive.blender.org/developer/F1359404/blender.crash.txt) - crash.txt from blender.
Author

test.zip - sample script zipped.

[test.zip](https://archive.blender.org/developer/F1359413/test.zip) - sample script zipped.

Added subscribers: @mont29, @ideasman42, @VukGardasevic

Added subscribers: @mont29, @ideasman42, @VukGardasevic

The issue you are describing is noted in the Python documentation:
https://docs.blender.org/api/master/info_gotcha.html#undo-redo

Undo invalidates all the bpy.types.

The issue about text disappearing is listed as an To Do https://wiki.blender.org/index.php/Dev:Source/Development/Todo/UserInterface#Undo.2C_Redo_and_Operator_Context

Something like this seems to work:

bl_info = {
    "name": "Test Addon Crash Text",
    "author": ("portnov@bk.ru"),
    "version": (0, 0, 2),
    "blender": (2, 7, 8),
    "location": "?",
    "description": "Just a test",
    "warning": "",
    "wiki_url": "github.com",
    "category": "Test"}


import bpy
from bpy.props import StringProperty
from bpy.types import WindowManager


class TextWrite(bpy.types.Operator):
    bl_label = "Write some text"
    bl_idname = "node.write_text"
    bl_options = {'REGISTER', 'UNDO'}

    def execute(self, context):
        name = "test.log"

        textblock = None
        if name in bpy.data.texts:
            textblock = bpy.data.texts[name]

        text1 = context.window_manager.str1
        text2 = context.window_manager.str2

        if textblock:
            print("textblock hash is :", hash(textblock))
            print("textblock is {}".format(textblock))
            textblock.write("\n\nHello there!")
            textblock.write("\n%s \n%s" % (text1, text2))
        else:
            self.report({'WARNING'}, "No text block found!")

        return {'FINISHED'}


class TextInit(bpy.types.Operator):
    bl_label = "Init text textblock"
    bl_idname = "node.init_textblock"
    bl_options = {'REGISTER', 'UNDO'}

    def execute(self, context):
        name = "test.log"

        if name not in bpy.data.texts:
            bpy.data.texts.new(name)

        return {'FINISHED'}


class PreviewsExamplePanel(bpy.types.Panel):
    bl_label = "Example Panel"
    bl_idname = "OBJECT_PT_previews"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'TOOLS'

    def draw(self, context):
        layout = self.layout
        wm = context.window_manager

        layout.prop(wm, "str1")

        row = layout.row()
        row.prop(wm, "str2")

        layout.operator("node.init_textblock")
        layout.operator("node.write_text")


def register():

    WindowManager.str1 = StringProperty(name="Str1")
    WindowManager.str2 = StringProperty(name="Str2")

    bpy.utils.register_class(TextInit)
    bpy.utils.register_class(TextWrite)
    bpy.utils.register_class(PreviewsExamplePanel)


def unregister():

    del WindowManager.str1
    del WindowManager.str2

    bpy.utils.unregister_class(PreviewsExamplePanel)
    bpy.utils.unregister_class(TextWrite)
    bpy.utils.unregister_class(TextInit)


if __name__ == "__main__":
    register()

You can notice that the object hash changes on Undo.
However, there could be some other issues related to valid calls using (getting the bpy,data.texts without storing it in a global in some corner cases).
@mont29, @ideasman42 should this be regarded as a bug? Thanks.

The issue you are describing is noted in the Python documentation: https://docs.blender.org/api/master/info_gotcha.html#undo-redo Undo invalidates all the bpy.types. The issue about text disappearing is listed as an To Do https://wiki.blender.org/index.php/Dev:Source/Development/Todo/UserInterface#Undo.2C_Redo_and_Operator_Context Something like this seems to work: ``` bl_info = { "name": "Test Addon Crash Text", "author": ("portnov@bk.ru"), "version": (0, 0, 2), "blender": (2, 7, 8), "location": "?", "description": "Just a test", "warning": "", "wiki_url": "github.com", "category": "Test"} import bpy from bpy.props import StringProperty from bpy.types import WindowManager class TextWrite(bpy.types.Operator): bl_label = "Write some text" bl_idname = "node.write_text" bl_options = {'REGISTER', 'UNDO'} def execute(self, context): name = "test.log" textblock = None if name in bpy.data.texts: textblock = bpy.data.texts[name] text1 = context.window_manager.str1 text2 = context.window_manager.str2 if textblock: print("textblock hash is :", hash(textblock)) print("textblock is {}".format(textblock)) textblock.write("\n\nHello there!") textblock.write("\n%s \n%s" % (text1, text2)) else: self.report({'WARNING'}, "No text block found!") return {'FINISHED'} class TextInit(bpy.types.Operator): bl_label = "Init text textblock" bl_idname = "node.init_textblock" bl_options = {'REGISTER', 'UNDO'} def execute(self, context): name = "test.log" if name not in bpy.data.texts: bpy.data.texts.new(name) return {'FINISHED'} class PreviewsExamplePanel(bpy.types.Panel): bl_label = "Example Panel" bl_idname = "OBJECT_PT_previews" bl_space_type = 'VIEW_3D' bl_region_type = 'TOOLS' def draw(self, context): layout = self.layout wm = context.window_manager layout.prop(wm, "str1") row = layout.row() row.prop(wm, "str2") layout.operator("node.init_textblock") layout.operator("node.write_text") def register(): WindowManager.str1 = StringProperty(name="Str1") WindowManager.str2 = StringProperty(name="Str2") bpy.utils.register_class(TextInit) bpy.utils.register_class(TextWrite) bpy.utils.register_class(PreviewsExamplePanel) def unregister(): del WindowManager.str1 del WindowManager.str2 bpy.utils.unregister_class(PreviewsExamplePanel) bpy.utils.unregister_class(TextWrite) bpy.utils.unregister_class(TextInit) if __name__ == "__main__": register() ``` You can notice that the object hash changes on Undo. However, there could be some other issues related to valid calls using (getting the bpy,data.texts without storing it in a global in some corner cases). @mont29, @ideasman42 should this be regarded as a bug? Thanks.
Author

Thanks for the information.

Okay, if there are some (i beleive very good) reasons to invalidate bpy objecs on undo, let it be so. But, I do not think that crash is a good behaivour in such a case. IMHO blender should either throw python exception, or maybe just stop processing this script and print error to console, or show some pop-up error message, but not crash.

Thanks for the information. Okay, if there are some (i beleive very good) reasons to invalidate bpy objecs on undo, let it be so. But, I do not think that crash is a good behaivour in such a case. IMHO blender should either throw python exception, or maybe just stop processing this script and print error to console, or show some pop-up error message, but not crash.
Member

Added subscriber: @tetha.z

Added subscriber: @tetha.z

Changed status from 'Open' to: 'Resolved'

Changed status from 'Open' to: 'Resolved'
Campbell Barton self-assigned this 2017-12-11 12:30:48 +01:00
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Reference: blender/blender#53530
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