cmd/go: only add a 'go' directive to the main module when the go.mod file will be written

Then, write the 'go.mod' file with that version before further
processing. That way, if the command errors out due to a change in
behavior, the reason for the change in behavior will be visible in the
file diffs.

If the 'go.mod' file cannot be written (due to -mod=readonly or
-mod=vendor), assume Go 1.11 instead of the current Go release.
(cmd/go has added 'go' directives automatically, including in 'go mod
init', since Go 1.12.)

For #44976

Change-Id: If9d4af557366f134f40ce4c5638688ba3bab8380
Reviewed-on: https://go-review.googlesource.com/c/go/+/302051
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/src/cmd/go/internal/modload/modfile.go b/src/cmd/go/internal/modload/modfile.go
index c6667d0bf7..2afa831583 100644
--- a/src/cmd/go/internal/modload/modfile.go
+++ b/src/cmd/go/internal/modload/modfile.go
@@ -257,10 +257,13 @@ func indexModFile(data []byte, modFile *modfile.File, needsFix bool) *modFileInd
 	}
 
 	i.goVersionV = ""
-	if modFile.Go != nil {
+	if modFile.Go == nil {
+		rawGoVersion.Store(Target, "")
+	} else {
 		// We're going to use the semver package to compare Go versions, so go ahead
 		// and add the "v" prefix it expects once instead of every time.
 		i.goVersionV = "v" + modFile.Go.Version
+		rawGoVersion.Store(Target, modFile.Go.Version)
 	}
 
 	i.require = make(map[module.Version]requireMeta, len(modFile.Require))