🏗✨ Add a test for sourcemaps generation during minified compilation

[rsimha]

Background:

During minified compilation, we generate a sourcemap file called v0.js.map in addition to the runtime file v0.js. Sourcemap generation frequently breaks due to untested compilation toolchain changes. See #27681.

PR highlights:

• Adds a task called gulp check-sourcemaps
• Compiles the runtime with sourcemaps (gulp dist --core_runtime_only --full_sourcemaps)
• Verifies the the sourceRoot field exists, and is of the correct format
• Verifies that the sources array exists, and all paths in it point to valid files
• Verifies that the mappings object exists, decodes it, and verifies that a mapping exists for a known sentinel piece of code

Usage:

# Build core runtime and check generated sourcemaps
gulp check-sourcemaps

# Check sourcemaps from previous invocation of gulp dist --full_sourcemaps
gulp check-sourcemaps --nobuild

Passing example:

Failing example (incorrect sourceRoot: wrong sourcemap domain):

Failing example (incorrect sources: revert fix in #27665):

Failing example (incorrect mappings: modify sentinel piece of code):

Fixes #27681

[jridgewell]

Looking at the source code, source-map-unpacker doesn't actually regenerate the inputs from the sourcemap. It's just printing the included sourcesContent to a file. We need actual tests of the sourcema's mappings.

[rsimha]

Looking at the source code , source-map-unpacker doesn't actually regenerate the inputs from the sourcemap. It's just printing the included sourcesContent to a file.

Pretty sure this is not true. I made a known change to the source code in v0.js.map and it showed up as a diff. From this line, I'm pretty certain js.sourcesContent[idx] is coming from the map file, not from the original source.

Edit: Unless I misunderstood the original intent of a sourcemap file? Is the sourcesContent not what should be checked?

[amp-owners-bot]

Hey @estherkim, these files were changed:

build-system/pr-check/checks.js

[rsimha]

@jridgewell I've added a step that compares ASTs of src/amp.js and dist/extracted/src/amp.js. There are diffs which you will see in these logs.

[rsimha]

As discussed offline with @jridgewell, we're still in search of an effective way to extract the mappings from v0.js.map and ensure their correctness (via say, a sentinel value). Until then, there's still some value in making sure the sourcemap file is generated, and the correct sourcemap URL is present.

I've modified the scope of this PR to add the gulp check-sourcemaps task and verify the sourcemaps URL. The rest will be done in a follow up PR when time permits.

[rsimha]

@choumx @jridgewell All changes made. This is now ready for review and merge.

[rsimha]

Added a new check for all the paths in the sources array.

[rsimha]

Added yet another check that implements #27681 (comment) and inspects the mappings field for a known sentinel value.