cmd/go: do not turn list ./nonexist into a network lookup

If you're in a directory corresponding to x/y
and you run go list ./z, we do at some point
want to turn that into x/y/z. But if ./z does
not exist that will make the go command
check the network to see if it can find x/y/z.
That's clearly wrong: ./z means that directory,
nothing else. And it turns a typo into a long delay,
which is even worse.

Fixes #26874.

Change-Id: Iec15fa7b359af11b6a4fc6cb082e593658fb6e41
Reviewed-on: https://go-review.googlesource.com/129061
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/go/internal/modload/load.go

@@ -116,10 +116,24 @@ func ImportPaths(patterns []string) []*search.Match {
 					} else if path := pathInModuleCache(dir); path != "" {
 						pkg = path
 					} else {
+						pkg = ""
 						if !iterating {
 							base.Errorf("go: directory %s outside available modules", base.ShortPath(dir))
 						}
+					}
+					info, err := os.Stat(dir)
+					if err != nil || !info.IsDir() {
+						// If the directory does not exist,
+						// don't turn it into an import path
+						// that will trigger a lookup.
 						pkg = ""
+						if !iterating {
+							if err != nil {
+								base.Errorf("go: no such directory %v", m.Pattern)
+							} else {
+								base.Errorf("go: %s is not a directory", m.Pattern)
+							}
+						}
 					}
 					m.Pkgs[i] = pkg
 				}

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

@@ -28,6 +28,18 @@ stdout ^m/vendor$
 stdout ^m/y$
 ! stdout ^m/y/z
 
+# non-existent directory should not prompt lookups
+! go build -mod=readonly example.com/nonexist
+stderr 'import lookup disabled'
+
+! go build -mod=readonly ./nonexist
+! stderr 'import lookup disabled'
+stderr '^go: no such directory ./nonexist'
+
+! go build -mod=readonly ./go.mod
+! stderr 'import lookup disabled'
+stderr '^go: ./go.mod is not a directory'
+
 -- x/go.mod --
 module m
 

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

@@ -9,11 +9,14 @@ go list -f '{{.ImportPath}}' $GOROOT/src/math
 stdout ^math$
 go list -f '{{.ImportPath}}' .
 stdout ^x$
+! go list -f '{{.ImportPath}}' $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
+stderr '^go: no such directory.*quote@v1.5.2'
+go mod download rsc.io/quote@v1.5.2
 go list -f '{{.ImportPath}}' $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
 stdout '^rsc.io/quote$'
 go list -f '{{.ImportPath}}' $GOPATH/pkg/mod/rsc.io/sampler@v1.3.0
 stdout '^rsc.io/sampler$'
-go get rsc.io/sampler@v1.3.1
+go get -d rsc.io/sampler@v1.3.1
 go list -f '{{.ImportPath}}' $GOPATH/pkg/mod/rsc.io/sampler@v1.3.1
 stdout '^rsc.io/sampler$'
 ! go list -f '{{.ImportPath}}' $GOPATH/pkg/mod/rsc.io/sampler@v1.3.0