Avoid relying on buggy import order in the legacy importer #245
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Historically, the legacy importer shim has strongly assumed that all calls to `canonicalize()` would doubled: once as a URL resolved relative to the current importer, and again as a URL passed to the importers list. However, this relies on buggy, non-spec-compliant behavior in Dart Sass: absolute URLs should never be passed to the current importer, only to the global list of importers. This change avoids relying on that behavior by instead disambiguating relative loads using a custom URL protocol prefix. All canonical URLs passed to Dart Sass now have the prefix `legacy-importer-` added to the beginning. This allows us to disambiguate relative loads, and ignore them for non-`file:` URLs, since relative loads and _only_ relative loads will have schemes beginning with `legacy-importer-`. To avoid regressing the developer experience, we then strip these prefixes from all URLs before they're passed to any developer-facing APIs or displayed to the user.
jathak
approved these changes
Sep 8, 2023
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Historically, the legacy importer shim has strongly assumed that all
calls to
canonicalize()would doubled: once as a URL resolvedrelative to the current importer, and again as a URL passed to the
importers list. However, this relies on buggy, non-spec-compliant
behavior in Dart Sass: absolute URLs should never be passed to the
current importer, only to the global list of importers.
This change avoids relying on that behavior by instead disambiguating
relative loads using a custom URL protocol prefix. All canonical URLs
passed to Dart Sass now have the prefix
legacy-importer-added tothe beginning. This allows us to disambiguate relative loads, and
ignore them for non-
file:URLs, since relative loads and onlyrelative loads will have schemes beginning with
legacy-importer-.To avoid regressing the developer experience, we then strip these
prefixes from all URLs before they're passed to any developer-facing
APIs or displayed to the user.
See sass/sass-spec#1935
See sass/dart-sass#2077