Fixing ImgToPicture Double Processing Bug

by Alex Johnson 42 views

This article delves into a detailed bug within the ImgToPicture functionality of the go-html-asset-manager, which causes double-processing of images when the configuration ID matches both a tag and a class. We'll explore the context, impact, and root cause of this issue, along with a recommended fix and the reasons it remained undetected for so long.

Understanding the ImgToPicture Bug

In the realm of web development, optimizing images for various devices and screen sizes is crucial for delivering a seamless user experience. The ImgToPicture manipulation in the go-html-asset-manager plays a vital role in this process by converting <img> elements into responsive <picture> elements. These <picture> elements, through the use of <source> tags, allow developers to specify multiple image formats and sizes, enabling browsers to select the most appropriate image based on the user's device and network conditions.

However, a subtle but significant bug has been identified within this functionality. This bug manifests when the configuration ID used to identify target elements matches both a tag name and elements possessing that same value as a class. In such scenarios, the same element undergoes processing twice, leading to the creation of nested <picture> elements. This not only results in invalid HTML structure but also disrupts the intended responsive image behavior.

The core problem lies in how the code identifies elements for processing. The system uses both FindNodesByTag and FindNodesByClassname to locate elements matching the provided ID. When an element's tag and class match the ID, it's added to the processing list twice, causing the double-wrapping issue.

The Impact of Double Processing

The consequences of this double-processing bug are far-reaching, impacting both the structural integrity of the HTML and the user experience. Let's break down the key issues:

  • Invalid HTML Structure: The most immediate consequence is the generation of invalid HTML. Nesting <picture> elements within each other violates HTML semantics, as <picture> elements are designed to be top-level elements containing <source> and <img> tags. This structural issue can lead to unpredictable browser behavior and accessibility problems.
  • Broken Responsive Image Functionality: The primary purpose of using <picture> elements is to enable responsive images. When nested, this functionality is severely compromised. Browsers may struggle to interpret the nested structure, potentially ignoring the <source> elements and failing to select the optimal image for the user's device.
  • Performance Degradation: While not as direct as the structural and functional issues, double-processing can also lead to performance degradation. The extra processing steps consume server resources and can slightly increase page load times, especially on pages with numerous images.

Root Cause Analysis: Diving into the Code

To understand the bug fully, let's examine the problematic code snippet:

func manipulateWithConfig(s3Client *s3.Client, debug bool, conf *config.Config, imgtopic *config.ImgToPicConfig, doc *html.Node) error {
 rawElements := htmlparsing.FindNodesByTag(imgtopic.ID, doc)
 rawElements = append(rawElements, htmlparsing.FindNodesByClassname(imgtopic.ID, doc)...) // <-- BUG 🔴 appends instead of conditional assignment

 if debug {
 fmt.Printf("Found %v raw elements for %q\n", len(rawElements), imgtopic.ID)
 }

 var imgs []*html.Node
 for _, e := range rawElements {
 imgs = append(imgs, htmlparsing.FindNodesByTag("img", e)...)
 }

 if debug {
 fmt.Printf("Found %v img elements for %q\n", len(imgs), imgtopic.ID)
 }

 for _, ie := range imgs {
 err := manipulateImg(s3Client, debug, conf, imgtopic, ie)
 if err != nil {
 return err
 }
 }
 return nil
}

The critical line is:

rawElements = append(rawElements, htmlparsing.FindNodesByClassname(imgtopic.ID, doc)...)

This line unconditionally appends the results of FindNodesByClassname to the rawElements slice, regardless of whether elements were already found by FindNodesByTag. This is the root cause of the double-processing issue. If an element matches both the tag name and the class name specified in imgtopic.ID, it will be added to rawElements twice.

Step-by-Step Execution Example

Consider this configuration:

{
 "img-to-picture": [
 {
 "id": "img",
 "source-sizes": ["100vw"]
 }
 ]
}

And this HTML:

<img class="img" src="/example.png"/>

Here's how the bug unfolds:

  1. `FindNodesByTag(