cmd/go/internal/list: ensure that cfg.BuildMod is initialized before reading it in 'go list -m'

The default value of cfg.BuildMod depends on the 'go' version in the
go.mod file. The go.mod file is read and parsed, and its settings are
applied, in modload.InitMod.

As it turns out, modload.Enabled does not invoke InitMod, so
cfg.BuildMod is not necessarily set even if modload.Enabled returns
true.

Updates #33848

Change-Id: I13a4dd80730528e6f1a5acc492fcfe07cb59d94e
Reviewed-on: https://go-review.googlesource.com/c/go/+/202917
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

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

@@ -381,12 +381,20 @@ func runList(cmd *base.Command, args []string) {
 			base.Fatalf("go list -test cannot be used with -m")
 		}
 
+		buildModIsDefault := (cfg.BuildMod == "")
 		if modload.Init(); !modload.Enabled() {
 			base.Fatalf("go list -m: not using modules")
 		}
+
+		modload.InitMod() // Parses go.mod and sets cfg.BuildMod.
 		if cfg.BuildMod == "vendor" {
-			base.Fatalf("go list -m: can't list modules with -mod=vendor\n\tuse -mod=mod or -mod=readonly to ignore the vendor directory")
+			if buildModIsDefault {
+				base.Fatalf("go list -m: can't list modules using the vendor directory\n\tUse -mod=mod or -mod=readonly to ignore it.")
+			} else {
+				base.Fatalf("go list -m: can't list modules with -mod=vendor\n\tUse -mod=mod or -mod=readonly to ignore the vendor directory.")
 			}
+		}
+
 		modload.LoadBuildList()
 
 		mods := modload.ListModules(args, *listU, *listVersions)

src/cmd/go/internal/modload/init.go

@@ -290,6 +290,9 @@ func die() {
 
 // InitMod sets Target and, if there is a main module, parses the initial build
 // list from its go.mod file, creating and populating that file if needed.
+//
+// As a side-effect, InitMod sets a default for cfg.BuildMod if it does not
+// already have an explicit value.
 func InitMod() {
 	if len(buildList) > 0 {
 		return

src/cmd/go/testdata/script/mod_convert_vendor_json.txt

@@ -10,7 +10,7 @@ stderr '\s*cd \.\. && go mod init'
 # The command we suggested should succeed.
 cd ..
 go mod init
-go list -m all
+go list -mod=mod -m all
 stdout '^m$'
 
 -- $WORK/test/vendor/vendor.json --

src/cmd/go/testdata/script/mod_convert_vendor_manifest.txt

@@ -10,7 +10,7 @@ stderr '\s*cd \.\. && go mod init'
 # The command we suggested should succeed.
 cd ..
 go mod init
-go list -m all
+go list -mod=mod -m all
 stdout '^m$'
 
 -- $WORK/test/vendor/manifest --

src/cmd/go/testdata/script/mod_getmode_vendor.txt

@@ -11,7 +11,7 @@ stdout '^rsc.io/quote v1.5.1 .*vendor[\\/]rsc.io[\\/]quote$'
 stdout '^golang.org/x/text v0.0.0.* .*vendor[\\/]golang.org[\\/]x[\\/]text[\\/]language$'
 
 ! go list -mod=vendor -m rsc.io/quote@latest
-stderr 'go list -m: can''t list modules with -mod=vendor\n\tuse -mod=mod or -mod=readonly to ignore the vendor directory'
+stderr 'go list -m: can''t list modules with -mod=vendor\n\tUse -mod=mod or -mod=readonly to ignore the vendor directory.'
 ! go get -mod=vendor -u
 stderr 'flag provided but not defined: -mod'
 

src/cmd/go/testdata/script/mod_vendor.txt

@@ -35,7 +35,7 @@ stdout 'src[\\/]vendor[\\/]x'
 # 'go list -mod=vendor -m' does not have enough information to list modules
 # accurately, and should fail.
 ! go list -mod=vendor -f {{.Dir}} -m x
-stderr 'can''t list modules with -mod=vendor\n\tuse -mod=mod or -mod=readonly to ignore the vendor directory'
+stderr 'can''t list modules with -mod=vendor\n\tUse -mod=mod or -mod=readonly to ignore the vendor directory.'
 
 # 'go list -mod=mod' should report packages outside the import graph,
 # but 'go list -mod=vendor' should error out for them.

src/cmd/go/testdata/script/mod_vendor_auto.txt

@@ -17,10 +17,10 @@ stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]printversion$'
 stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]version$'
 
 ! go list -m all
-stderr 'can''t list modules with -mod=vendor'
+stderr 'can''t list modules with -mod=vendor\n\tUse -mod=mod or -mod=readonly to ignore the vendor directory.'
 
 ! go list -m -f '{{.Dir}}' all
-stderr 'can''t list modules with -mod=vendor'
+stderr 'can''t list modules with -mod=vendor\n\tUse -mod=mod or -mod=readonly to ignore the vendor directory.'
 
 # An explicit -mod=mod should force the vendor directory to be ignored.
 env GOFLAGS=-mod=mod
@@ -103,6 +103,15 @@ stdout '^'$WORK'[/\\]auto$'
 stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]printversion$'
 stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]version$'
 
+# ...but 'go list -m' should continue to fail, this time without
+# referring to a -mod default that the user didn't set.
+! go list -m all
+stderr 'can''t list modules using the vendor directory\n\tUse -mod=mod or -mod=readonly to ignore it.'
+
+! go list -m -f '{{.Dir}}' all
+stderr 'can''t list modules using the vendor directory\n\tUse -mod=mod or -mod=readonly to ignore it.'
+
+
 # 'go mod init' should work if there is already a GOPATH-mode vendor directory
 # present. If there are no module dependencies, -mod=vendor should be used by
 # default and should not fail the consistency check even though no module