cmd/go: emit go list error for local non-existant packages

In CL 129061, a check was added for patterns that reference
nonexistent local directories. While this prevented unnecessary
network lookups (fixing #26874), it caused "go list -e" to exit with
an error instead of listing packages with error messages.

This change avoids the network lookup and does not exit for these
kinds of packages. Errors are still reported by
internal/load.LoadImport for packages that don't exist.

Fixes #28023

Change-Id: I0a648269e437aed3a95bfb05461a397264f3793f
Reviewed-on: https://go-review.googlesource.com/c/151800
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

@@ -90,7 +90,7 @@ func ImportPaths(patterns []string) []*search.Match {
 				// the exact version of a particular module increases during
 				// the loader iterations.
 				m.Pkgs = str.StringList(fsDirs[i])
-				for i, pkg := range m.Pkgs {
+				for j, pkg := range m.Pkgs {
 					dir := pkg
 					if !filepath.IsAbs(dir) {
 						dir = filepath.Join(cwd, pkg)
@@ -124,19 +124,15 @@ func ImportPaths(patterns []string) []*search.Match {
 					}
 					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.
+						// If the directory is local but does not exist, don't return it
+						// while loader is iterating, since this would trigger a fetch.
+						// After loader is done iterating, we still need to return the
+						// path, so that "go list -e" produces valid output.
+						if iterating {
 							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
+					m.Pkgs[j] = pkg
 				}
 
 			case strings.Contains(m.Pattern, "..."):

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

@@ -34,11 +34,11 @@ stderr 'import lookup disabled'
 
 ! go build -mod=readonly ./nonexist
 ! stderr 'import lookup disabled'
-stderr '^go: no such directory ./nonexist'
+stderr 'unknown import path "m/nonexist": cannot find package'
 
 ! go build -mod=readonly ./go.mod
 ! stderr 'import lookup disabled'
-stderr '^go: ./go.mod is not a directory'
+stderr 'unknown import path "m/go.mod": cannot find package'
 
 -- x/go.mod --
 module m

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

@@ -10,7 +10,9 @@ 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'
+stderr 'unknown import path "rsc.io/quote": cannot find package'
+go list -e -f '{{with .Error}}{{.}}{{end}}' $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
+stdout 'unknown import path "rsc.io/quote": cannot find package'
 go mod download rsc.io/quote@v1.5.2
 go list -f '{{.ImportPath}}' $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
 stdout '^rsc.io/quote$'