Change causing a crash while image decoding

Steps to reproduce

The following cl seems to be causing a frequent null check crash in my app:

8023527

UnknownError: Null check operator used on a null value
at MultiFrameImageStreamCompleter._decodeNextFrameAndSchedule (image_stream.dart:1133)

Sometimes the code line is 1111 depending on the version of flutter I have. That points to the _codec vairable being null checked before being disposed here:

flutter/packages/flutter/lib/src/painting/image_stream.dart

Line 1111 in b2a4a05

_codec!.dispose();

The crash happens very frequently. Maybe we can revert this commit?

Expected results

Don't crash due to a null check.

Actual results

It crashes.

Code sample

n/a

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The codec_ variable is null checked a few lines above that. Furthermore that line is inside a block that acceess a property on codec. Seems like that would crash first?

flutter/packages/flutter/lib/src/painting/image_stream.dart

Lines 1091 to 1111 in b2a4a05

    
           if (_codec == null) { 
        
             // codec was disposed during getNextFrame 
        
             return; 
        
           } 
        
           if (_codec!.frameCount == 1) { 
        
             // ImageStreamCompleter listeners removed while waiting for next frame to 
        
             // be decoded. 
        
             // There's no reason to emit the frame without active listeners. 
        
             if (!hasListeners) { 
        
               return; 
        
             } 
        
             // This is not an animated image, just return it and don't schedule more 
        
             // frames. 
        
             _emitFrame( 
        
               ImageInfo(image: _nextFrame!.image.clone(), scale: _scale, debugLabel: debugLabel), 
        
             ); 
        
             _nextFrame!.image.dispose(); 
        
             _nextFrame = null; 
        
             _codec!.dispose();

I believe the _emitFrame call here must have a side effect that occasionally causes the _codec to be disposed - maybe the receiver of the frame disposes the codec for some reason in certain circumstances? I could conceive of that calling some callback in client code which in-turn calls dispose on the image stream or something.

flutter/packages/flutter/lib/src/painting/image_stream.dart

Line 1105 in b2a4a05

_emitFrame(

It's not if it can happen. It definitely can happening - it's happening now.

The code should protect itself by using a ? instead of a ! or check for null first before attempting to dispose.

Note also I see it doing this null check on _codec in two places so we probably want to protect that second location too.

Yeah that makes sense. THe listeners run immediately so one could decide to dispose the image. So just changing the final !.dispose to a ?dispose should be safe.

#164537 has a way to reproduce this in Wonderous.

Captured a stack trace from Wonderous:

#0      MultiFrameImageStreamCompleter._maybeDispose (package:flutter/src/painting/image_stream.dart:1160:95)
#1      ImageStreamCompleter.removeListener (package:flutter/src/painting/image_stream.dart:659:7)
#2      MultiFrameImageStreamCompleter.removeListener (package:flutter/src/painting/image_stream.dart:1144:11)
#3      RetryImage.loadImage.<anonymous closure> (package:wonders/logic/common/retry_image.dart:82:23)
#4      ImageStreamCompleter.removeListener (package:flutter/src/painting/image_stream.dart:656:17)
#5      _PendingImage.removeListener (package:flutter/src/painting/image_cache.dart:652:15)
#6      ImageCache.putIfAbsent.listener (package:flutter/src/painting/image_cache.dart:424:22)
#7      ImageStreamCompleter.setImage (package:flutter/src/painting/image_stream.dart:745:25)
#8      _DelegatingImageStreamCompleter.addImage (package:wonders/logic/common/retry_image.dart:106:5)
#9      RetryImage.loadImage.<anonymous closure> (package:wonders/logic/common/retry_image.dart:62:19)
#10     ImageStreamCompleter.setImage (package:flutter/src/painting/image_stream.dart:745:25)
#11     MultiFrameImageStreamCompleter._emitFrame (package:flutter/src/painting/image_stream.dart:1130:5)
#12     MultiFrameImageStreamCompleter._decodeNextFrameAndSchedule (package:flutter/src/painting/image_stream.dart:1107:7)

_emitFrame calls setImage, which invokes listener onImage callbacks. The callback may remove the last listener, resulting in a call to _maybeDispose which clears the _codec.

So adding the _codec null check after the _emitFrame call would solve this. I'll create a PR.

Check for a null codec in MultiFrameImageStreamCompleter after callin…

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Change causing a crash while image decoding #164944

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	if (_codec == null) {
	// codec was disposed during getNextFrame
	return;
	}

	if (_codec!.frameCount == 1) {
	// ImageStreamCompleter listeners removed while waiting for next frame to
	// be decoded.
	// There's no reason to emit the frame without active listeners.
	if (!hasListeners) {
	return;
	}
	// This is not an animated image, just return it and don't schedule more
	// frames.
	_emitFrame(
	ImageInfo(image: _nextFrame!.image.clone(), scale: _scale, debugLabel: debugLabel),
	);
	_nextFrame!.image.dispose();
	_nextFrame = null;

	_codec!.dispose();

Change causing a crash while image decoding #164944

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jonahwilliams commented on Mar 10, 2025

apwilson commented on Mar 10, 2025

jonahwilliams commented on Mar 10, 2025

jason-simmons commented on Mar 11, 2025

github-actions commented on Mar 26, 2025

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