cmd/go: fix modload infinite directory loop

It is possible to enter the parent-walking directory loop in a way that
it will loop forever - if mdir is empty, and d reaches ".". To avoid
this, make sure that the 'd = filepath.Dir(d)' step only happens if the
parent directory is actually different than the current directory.

This fixes some of the tests like TestImport/golang.org_x_net_context,
which were never finishing before.

While at it, also fix TestImport/golang.org_x_net, which seems to have
the wrong expected error. The root of the x/net repo doesn't have a
go.mod file, nor is part of a module itself, so it seems like the
expected error should reflect that.

After these two changes, 'go test cmd/go/internal/modload' passes on my
linux/amd64 machine.

Fixes #27080.

Change-Id: Ie8bab0f9fbc9f447844cbbc64117420d9087db1b
Reviewed-on: https://go-review.googlesource.com/129778
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/modload/import.go

@@ -181,7 +181,7 @@ func dirInModule(path, mpath, mdir string, isLocal bool) (dir string, haveGoFile
 	// So we only check local module trees
 	// (the main module, and any directory trees pointed at by replace directives).
 	if isLocal {
-		for d := dir; d != mdir && len(d) > len(mdir); d = filepath.Dir(d) {
+		for d := dir; d != mdir && len(d) > len(mdir); {
 			haveGoMod := haveGoModCache.Do(d, func() interface{} {
 				_, err := os.Stat(filepath.Join(d, "go.mod"))
 				return err == nil
@@ -190,6 +190,13 @@ func dirInModule(path, mpath, mdir string, isLocal bool) (dir string, haveGoFile
 			if haveGoMod {
 				return "", false
 			}
+			parent := filepath.Dir(d)
+			if parent == d {
+				// Break the loop, as otherwise we'd loop
+				// forever if d=="." and mdir=="".
+				break
+			}
+			d = parent
 		}
 	}
 

src/cmd/go/internal/modload/import_test.go

@@ -21,7 +21,7 @@ var importTests = []struct {
 	},
 	{
 		path: "golang.org/x/net",
-		err:  "missing module for import: golang.org/x/net@.* provides golang.org/x/net",
+		err:  "cannot find module providing package golang.org/x/net",
 	},
 	{
 		path: "golang.org/x/text",