GIF interlace option not working

[peterdn]

Prerequisites

• I have written a descriptive issue
• I have verified that I am using the latest version of ImageMagick
• I have searched open and closed issues to ensure it has not already been reported

Description

The -interlace CLI option does not appear to work with GIFs, either with -interlace line or -interlace gif.

Steps to Reproduce

Begin with a non-interlaced GIF:

>  identify -verbose test.gif | grep Interlace
  Interlace: None
  Interlace: None
  Interlace: None
  Interlace: None

Use mogrify to add interlacing:

> mogrify -verbose -interlace gif test.gif

The command terminates successfully, but the GIF is not interlaced:

> identify -verbose test.gif | grep Interlace
  Interlace: None
  Interlace: None
  Interlace: None
  Interlace: None

The same issue is seen with convert.

System Configuration

• ImageMagick version: ImageMagick 7.0.10-30 Q16 x86_64 2020-09-13
• Environment (Operating system, version and so on): WSL - Ubuntu 20.04.1 LTS

[peterdn]

After having a quick look in coders/gif.c I found the following that appears to clear the output interlace option if we're adjoining (which we should be, for a multi-frame GIF), and the first frame isn't NULL (which it shouldn't be if the input GIF is valid):

  if ((write_info->adjoin != MagickFalse) &&
      (GetNextImageInList(image) != (Image *) NULL))
    write_info->interlace=NoInterlace;

(link)

Commenting this out fixes the issue, and I therefore get:

> utilities/magick identify -verbose test.gif | grep Interlace
  Interlace: None
  Interlace: None
  Interlace: None
  Interlace: None

> utilities/magick mogrify -verbose -interlace gif test.gif

> utilities/magick identify -verbose test.gif | grep Interlace
  Interlace: GIF
  Interlace: GIF
  Interlace: GIF
  Interlace: GIF

I'm happy to file PR for this however I don't understand the full implications of doing this. Is there a particular reason why interlacing would need to be disabled under the above conditions?