go: cmd/trace: requires HTML imports, which doesn't work on any major browser anymore

go version devel +d3595f7171 Wed Sep 18 07:29:33 2019 +0000 linux/amd64

This affects “View trace” on the web UI when running go tool trace. It doesn’t matter what the contents of the trace are; they can be from any test, for example via go test -trace=trace.out.

On Chromium 77.0.3865.75 it works, although with some warnings:

[Deprecation] HTML Imports is deprecated and will be removed in M80, around February 2020. Please use ES modules instead. See https://www.chromestatus.com/features/5144752345317376 and https://developers.google.com/web/updates/2019/07/web-components-time-to-upgrade for more details.
trace_viewer_html:3455 [Deprecation] document.registerElement is deprecated and will be removed in M80, around February 2020. Please use window.customElements.define instead. See https://www.chromestatus.com/features/4642138092470272 and https://developers.google.com/web/updates/2019/07/web-components-time-to-upgrade for more details.
(anonymous) @ trace_viewer_html:3455
(anonymous) @ trace_viewer_html:3455
trace_viewer_html:3635 [Deprecation] Element.createShadowRoot is deprecated and will be removed in M80, around February 2020. Please use Element.attachShadow instead. See https://www.chromestatus.com/features/4507242028072960 and https://developers.google.com/web/updates/2019/07/web-components-time-to-upgrade for more details.
_createLocalRoot @ trace_viewer_html:3635
_setupRoot @ trace_viewer_html:3607
_ready @ trace_viewer_html:3487
_tryReady @ trace_viewer_html:3487
_initFeatures @ trace_viewer_html:3930
__initialize @ trace_viewer_html:3438
createdCallback @ trace_viewer_html:3435
(anonymous) @ trace:94

On Firefox 69.0, it simply doesn’t work at all. http://localhost:$PORT/trace appears as a blank page. The log shows why:

ReferenceError: tr is not defined trace:59:5
    onResult http://[::]:8080/trace:59
    onreadystatechange http://[::]:8080/trace:43

The underying cause is that FireFox, as well as many other browsers, don’t support HTML imports. I’m not sure if they were ever widespread, but they’re clearly deprecated and non-portable at the moment. See https://caniuse.com/#feat=imports and https://developer.mozilla.org/en-US/docs/Web/Web_Components/HTML_Imports.

I realise that the docs say:

Note that while the various profiles available when launching ‘go tool trace’ work on every browser, the trace viewer itself (the ‘view trace’ page) comes from the Chrome/Chromium project and is only actively tested on that browser.

However, I don’t find that to be a good long-term plan. The tool shouldn’t depend on deprecated non-portable features, and it should work on all major browsers.

I’ve been recommending that Go developers make use of go tool trace alongside go tool pprof, but it feels much less useful if it needs a “only works on Chrome” footnote 😃

/cc @hyangah