cmd/go: with -mod=vendor, don't panic if there are duplicate requirements

In loadModFile with -mod=vendor, load the vendor list and use it to
initialize the module graph before calling updateRoots.

In updateLazyRoots with any mode other than "mod", return the original
*Requirements if no roots needed to be upgraded, even if there are
inconsistencies. This means 'go list -m -mod=readonly' and -mod=vendor
may succeed if there are duplicate requirements or requirements on
versions of the main module.

Fixes #47565

Change-Id: I4640dffc4a7359367cc910da0d29e3538bfd1ca4
Reviewed-on: https://go-review.googlesource.com/c/go/+/340252
Trust: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 6 insertions, 11 deletions

diff --git a/src/cmd/go/testdata/script/mod_tidy_lazy_self.txt b/src/cmd/go/testdata/script/mod_tidy_lazy_self.txt
index ffcea18603..9abbabd2eb 100644
--- a/src/cmd/go/testdata/script/mod_tidy_lazy_self.txt
+++ b/src/cmd/go/testdata/script/mod_tidy_lazy_self.txt
@@ -2,18 +2,13 @@
 # 'go mod tidy' should not panic if the main module initially
 # requires an older version of itself.
 
+# A module may require an older version of itself without error. This is
+# inconsistent (the required version is never selected), but we still get
+# a reproducible build list.
+go list -m all
+stdout '^golang.org/issue/46078$'
 
-# A module that explicitly requires an older version of itself should be
-# rejected as inconsistent: we enforce that every explicit requirement is the
-# selected version of its module path, but the selected version of the main
-# module is always itself — not some explicit version.
-
-! go list -m all
-stderr '^go: updates to go\.mod needed; to update it:\n\tgo mod tidy$'
-
-
-# The suggested 'go mod tidy' command should succeed (not crash).
-
+# 'go mod tidy' should fix this (and not crash).
 go mod tidy