Table is stuck in an infinite loop sometimes

[MihaMarkic]

Steps to Reproduce

1. Create a Table with two rows and 6 columns, all containing a Text widget.
2. When it is supposed to be displayed as child of a DefaultTabController->Scaffold->TabBarView it might get stuck in an infinite loop.
3. I'll describe where and how it is stuck below

It gets stuck in RenderTable._computeColumnWidths method, specifically in this while loop:

int availableColumns = columns;
      while (deficit > 0.0 && totalFlex > 0.0) {
        double newTotalFlex = 0.0;
        for (int x = 0; x < columns; x += 1) {
          if (flexes[x] != null) {
            final double newWidth = widths[x] - deficit * flexes[x] / totalFlex;
            assert(newWidth.isFinite);
            if (newWidth <= minWidths[x]) {
              // shrank to minimum
              deficit -= widths[x] - minWidths[x];
              widths[x] = minWidths[x];
              flexes[x] = null;
              availableColumns -= 1;
            } else {
              deficit -= widths[x] - newWidth;
              widths[x] = newWidth;
              newTotalFlex += flexes[x];
            }
            assert(widths[x] >= 0.0);
          }
        }
        totalFlex = newTotalFlex;
      }

The problem is that deficit is a really small value 2.842170943040401e-14 and it won't get to or below 0.
Other values are:

widths: [65.9047619047619, 65.9047619047619, 65.90476190476191, 65.90476190476191, 65.90476190476191, 65.90476190476191]
minWidths: [0, 0, 0, 0, 0, 0]
flexes: [1, 1, 1, 1, 1, 1]
totalFlex: 6

Basically newWidth <= minWidths[x] is always false and deficit doesn't decrease because widths[x] - newWidth is always 0.

This problem happens only on some screens and it is consistent when it starts (i.e. works on emulator and not on device - Nexus 6, or the other way round).

Logs

No useful logs (when it gets stuck nothing is reported).

No issues found! (ran in 3.4s)

[√] Flutter (Channel stable, v1.0.0, on Microsoft Windows [Version 10.0.17763.253], locale sl-SI)  
    • Flutter version 1.0.0 at D:\SDK\flutter                                                      
    • Framework revision 5391447fae (8 weeks ago), 2018-11-29 19:41:26 -0800                       
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    • Platform android-28, build-tools 28.0.3                                                      
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    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1248-b01)                  
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[√] Connected device (2 available)                                                                 
    • Nexus 6                   • ZX1G4256JM    • android-arm • Android 7.1.1 (API 25)             
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• No issues found!                                                                                 

[MihaMarkic]

Looks similar to #23723
Hopefully additional data (variables values) I provided will help

[zoechi]

Do you also get an exception?

[MihaMarkic]

@zoechi No, it just loops infinitely because the condition is never satisfied (rounding with doubles problem, I guess).

[MihaMarkic]

And of course, app becomes unresponsive due to this loop.

[MihaMarkic]

Also, I'd push fix for this one sooner, not just "Goals" in coming years. Not because of me, well, also, but because this bug is insidious and happens only on certain screen size combinations. Imagine having an app in production and it becomes unresponsive for random customers/devices without any feedback.

[zoechi]

@jason-simmons ?

[jason-simmons]

@Hixie

[jonahwilliams]

We've had similar rounding issues in slivers, we should use a small delta for our while-loop

[jonahwilliams]

I was able to reproduce this almost immediately, will have PR up shortly

[MihaMarkic]

Instead of using a small delta (how small should be the challenge - are you sure it will cover all values?), perhaps you might consider checking whether deficit changed in a last loop, or something slightly more reliable.

[jonahwilliams]

Feel free to comment on PR

[zoechi]

Looks similar to #23723

Seems different because #27112 doesn't fix it.

[MihaMarkic]

@zoechi You're right, that's different.