cmd/go: make go list -m -u all not complain about missing checksums

This is a band-aid of a fix for Go 1.21, to create space to work on
a real fix for Go 1.22, if in fact the real fix is different. It simply
disables the go.sum update check during go list -m -u.
I don't have a self-contained test for the breakage. See #61605.
All existing tests continue to pass.

For #61605.
After merging into the Go 1.21 branch we can move #61605 to the Go 1.22 milestone.

Change-Id: Ib155710092003f08d2a6ce0aefa8e0270cad5a5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/513778
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/modload/list.go

@@ -110,7 +110,13 @@ func ListModules(ctx context.Context, args []string, mode ListMode, reuseFile st
 
 	if err == nil {
 		requirements = rs
-		if !ExplicitWriteGoMod {
+		// TODO(#61605): The extra ListU clause fixes a problem with Go 1.21rc3
+		// where "go mod tidy" and "go list -m -u all" fight over whether the go.sum
+		// should be considered up-to-date. The fix for now is to always treat the
+		// go.sum as up-to-date during list -m -u. Probably the right fix is more targeted,
+		// but in general list -u is looking up other checksums in the checksum database
+		// that won't be necessary later, so it makes sense not to write the go.sum back out.
+		if !ExplicitWriteGoMod && mode&ListU == 0 {
 			err = commitRequirements(ctx, WriteOpts{})
 		}
 	}