cmd/go/internal/modload: refactor version filtering for exclude

Query and other functions now accept an "allowed" function that
returns an error (previously, the function returned a bool). If the
error is equivalent to ErrDisallowed, it indicates the version is
excluded (or, in a future CL, retracted). This provides predicates a
chance to explain why a version is not allowed.

When a query refers to a specific revision (by version, branch, tag,
or commit name), most callers will not use the Allowed predicate. This
allows commands like 'go list -m' and 'go mod download' to handle
disallowed versions when explicitly requested. 'go get' will reject
excluded versions though.

When a query does not refer to a specific revision (for example,
"latest"), disallowed versions will not be considered.

When an "allowed" predicate returns an error not equivalent to
ErrDisallowed, it may be ignored or returned, depending on the
case. This never happens for excluded versions, but it may happen for
retractions (in a future CL). This indicates a list of retractions
could not be loaded. This frequently happens when offline, and it
shouldn't cause a fatal or warning in most cases.

For #24031

Change-Id: I4df6fb6bd60e3e0259e5b3b4bf71a307b4b32298
Reviewed-on: https://go-review.googlesource.com/c/go/+/228379
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
9 files changed
tree: 96b289a0bf8c4bfe583a8ca155309748b517c021
  1. .gitattributes
  2. .github/
  3. .gitignore
  4. AUTHORS
  5. CONTRIBUTING.md
  6. CONTRIBUTORS
  7. LICENSE
  8. PATENTS
  9. README.md
  10. SECURITY.md
  11. api/
  12. doc/
  13. favicon.ico
  14. lib/
  15. misc/
  16. robots.txt
  17. src/
  18. test/
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