cmd/go: treat VCS errors as hard errors in module search

If we're looking for a module for a/b/c/d/e,
we check for a module named a/b/c/d/e,
then a/b/c/d, then a/b/c, then a/b, then a.
If we know the source repo for a/b/c and that
fails, we should report that error instead of
continuing the loop: a/b and a are useless,
and the error from a/b/c contains important
information.

The errors are now a bit more verbose than
I'd like but they will suffice for Go 1.11.

$ go get github.com/bradfitz/private/sonos
go get github.com/bradfitz/private/sonos: git ls-remote -q origin in /Users/rsc/pkg/mod/cache/vcs/61e3c76780847e514802ec6af8f940f641c6017f711444f05c59cb17ac46d456: exit status 128:
	remote: Repository not found.
	fatal: repository 'https://github.com/bradfitz/private/' not found
$ go list launchpad.net/gocheck
can't load package: package launchpad.net/gocheck: unknown import path "launchpad.net/gocheck": bzr branch --use-existing-dir https://launchpad.net/~niemeyer/gocheck/trunk . in /Users/rsc/pkg/mod/cache/vcs/f46ce2ae80d31f9b0a29099baa203e3b6d269dace4e5357a2cf74bd109e13339: exec: "bzr": executable file not found in $PATH
$

Fixes #26885.
Fixes #26982.

Change-Id: I2f9cf1853d2d68af18adad668c80513b6ba220d6
Reviewed-on: https://go-review.googlesource.com/129683
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/modfetch/codehost/vcs.go

@@ -22,6 +22,17 @@ import (
 	"cmd/go/internal/str"
 )
 
+// A VCSError indicates an error using a version control system.
+// The implication of a VCSError is that we know definitively where
+// to get the code, but we can't access it due to the error.
+// The caller should report this error instead of continuing to probe
+// other possible module paths.
+type VCSError struct {
+	Err error
+}
+
+func (e *VCSError) Error() string { return e.Err.Error() }
+
 func NewRepo(vcs, remote string) (Repo, error) {
 	type key struct {
 		vcs    string
@@ -33,6 +44,9 @@ func NewRepo(vcs, remote string) (Repo, error) {
 	}
 	c := vcsRepoCache.Do(key{vcs, remote}, func() interface{} {
 		repo, err := newVCSRepo(vcs, remote)
+		if err != nil {
+			err = &VCSError{err}
+		}
 		return cached{repo, err}
 	}).(cached)
 

src/cmd/go/internal/modfetch/repo.go

@@ -237,6 +237,9 @@ func lookup(path string) (r Repo, err error) {
 func lookupCodeRepo(rr *get.RepoRoot) (codehost.Repo, error) {
 	code, err := codehost.NewRepo(rr.VCS, rr.Repo)
 	if err != nil {
+		if _, ok := err.(*codehost.VCSError); ok {
+			return nil, err
+		}
 		return nil, fmt.Errorf("lookup %s: %v", rr.Root, err)
 	}
 	return code, nil

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

@@ -14,6 +14,7 @@ import (
 	"strings"
 
 	"cmd/go/internal/cfg"
+	"cmd/go/internal/modfetch/codehost"
 	"cmd/go/internal/module"
 	"cmd/go/internal/par"
 	"cmd/go/internal/search"
@@ -133,6 +134,9 @@ func Import(path string) (m module.Version, dir string, err error) {
 
 	m, _, err = QueryPackage(path, "latest", Allowed)
 	if err != nil {
+		if _, ok := err.(*codehost.VCSError); ok {
+			return module.Version{}, "", err
+		}
 		return module.Version{}, "", &ImportMissingError{ImportPath: path}
 	}
 	return m, "", &ImportMissingError{ImportPath: path, Module: m}

src/cmd/go/internal/modload/query.go

@@ -6,6 +6,7 @@ package modload
 
 import (
 	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modfetch/codehost"
 	"cmd/go/internal/module"
 	"cmd/go/internal/semver"
 	"fmt"
@@ -223,6 +224,11 @@ func QueryPackage(path, query string, allowed func(module.Version) bool) (module
 	for p := path; p != "."; p = pathpkg.Dir(p) {
 		info, err := Query(p, query, allowed)
 		if err != nil {
+			if _, ok := err.(*codehost.VCSError); ok {
+				// A VCSError means we know where to find the code,
+				// we just can't. Abort search.
+				return module.Version{}, nil, err
+			}
 			if finalErr == errMissing {
 				finalErr = err
 			}

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

@@ -0,0 +1,11 @@
+[exec:bzr] skip 'tests NOT having bzr'
+[!net] skip
+
+env GO111MODULE=on
+env GOPROXY=
+
+! go list launchpad.net/gocheck
+stderr '"bzr": executable file not found'
+
+-- go.mod --
+module m