cmd/go: allow configuring module cache directory with GOMODCACHE

Adds a GOMODCACHE environment variable that's used by cmd/go to determine the
location of the module cache. The default value of GOMODCACHE will be
GOPATH[0]/pkg/mod, the default location of the module cache before this change.

Replace the cmd/go/internal/modfetch.PkgMod variable which previously held the
location of the module cache with the new cmd/go/internal/cfg.GOMODCACHE
variable, for consistency with many of the other environment variables that
affect the behavior of cmd/go.  (Most of the changes in this CL are due to
moving/renaming the variable.)

The value of cfg.GOMODCACHE is now set using a variable initializer. It was
previously set in cmd/go/internal/modload.Init.

The location of GOPATH/pkg/sumdb is unchanged by this CL. While it was
previously determined using the value of PkgMod, it now is determined
independently dirctly from the value of GOPATH[0].

Fixes #34527

Change-Id: Id4d31d217b3507d6057c8ef7c52af1a0606603e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/219538
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/src/cmd/go/internal/cfg/cfg.go b/src/cmd/go/internal/cfg/cfg.go
index 61dc6bdda6..7f8f8e92be 100644
--- a/src/cmd/go/internal/cfg/cfg.go
+++ b/src/cmd/go/internal/cfg/cfg.go
@@ -236,6 +236,7 @@ var (
 	GOROOTpkg    = filepath.Join(GOROOT, "pkg")
 	GOROOTsrc    = filepath.Join(GOROOT, "src")
 	GOROOT_FINAL = findGOROOT_FINAL()
+	GOMODCACHE   = envOr("GOMODCACHE", gopathDir("pkg/mod"))
 
 	// Used in envcmd.MkEnv and build ID computations.
 	GOARM    = envOr("GOARM", fmt.Sprint(objabi.GOARM))
@@ -253,6 +254,8 @@ var (
 	GOINSECURE = Getenv("GOINSECURE")
 )
 
+var SumdbDir = gopathDir("pkg/sumdb")
+
 // GetArchEnv returns the name and setting of the
 // GOARCH-specific architecture environment variable.
 // If the current architecture has no GOARCH-specific variable,
@@ -364,3 +367,11 @@ func isGOROOT(path string) bool {
 	}
 	return stat.IsDir()
 }
+
+func gopathDir(rel string) string {
+	list := filepath.SplitList(BuildContext.GOPATH)
+	if len(list) == 0 || list[0] == "" {
+		return ""
+	}
+	return filepath.Join(list[0], rel)
+}