cmd/go/internal/modload: move fetch to import.go

From a comment in CL 262341. It makes more sense in import.go than in
mvs.go.

Change-Id: If4dfa1091077e110c5041bc849d99bc0be2bd8e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/262780
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 39 insertions, 0 deletions

diff --git a/src/cmd/go/internal/modload/import.go b/src/cmd/go/internal/modload/import.go
index 76fe6745d9..8641cfec08 100644
--- a/src/cmd/go/internal/modload/import.go
+++ b/src/cmd/go/internal/modload/import.go
@@ -451,3 +451,42 @@ func dirInModule(path, mpath, mdir string, isLocal bool) (dir string, haveGoFile
 
 	return dir, res.haveGoFiles, res.err
 }
+
+// fetch downloads the given module (or its replacement)
+// and returns its location.
+//
+// The isLocal return value reports whether the replacement,
+// if any, is local to the filesystem.
+func fetch(ctx context.Context, mod module.Version) (dir string, isLocal bool, err error) {
+	if mod == Target {
+		return ModRoot(), true, nil
+	}
+	if r := Replacement(mod); r.Path != "" {
+		if r.Version == "" {
+			dir = r.Path
+			if !filepath.IsAbs(dir) {
+				dir = filepath.Join(ModRoot(), dir)
+			}
+			// Ensure that the replacement directory actually exists:
+			// dirInModule does not report errors for missing modules,
+			// so if we don't report the error now, later failures will be
+			// very mysterious.
+			if _, err := os.Stat(dir); err != nil {
+				if os.IsNotExist(err) {
+					// Semantically the module version itself “exists” — we just don't
+					// have its source code. Remove the equivalence to os.ErrNotExist,
+					// and make the message more concise while we're at it.
+					err = fmt.Errorf("replacement directory %s does not exist", r.Path)
+				} else {
+					err = fmt.Errorf("replacement directory %s: %w", r.Path, err)
+				}
+				return dir, true, module.VersionError(mod, err)
+			}
+			return dir, true, nil
+		}
+		mod = r
+	}
+
+	dir, err = modfetch.Download(ctx, mod)
+	return dir, false, err
+}