cmd: disable *.go domains lookup in go get command

Using 'go get x.go' instead of 'go build x.go' or some other
go command is a common mistake. By that mistake, a user gets
a misleading error message about unsuccessful `x.go` domain lookup.
This improvement handles such cases, by validating, whether the
argument hasn't specified version, has .go suffix, and either has
no slashes or such file locally exists. Handled both GOPATH
and GOMOD modes.

Fixes #38478

Change-Id: I583a4ef7f7ca8901deb07ebc811e2b3c0e828fa6
Reviewed-on: https://go-review.googlesource.com/c/go/+/229938
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/get/get.go

@@ -193,8 +193,24 @@ func downloadPaths(patterns []string) []string {
 	for _, arg := range patterns {
 		if strings.Contains(arg, "@") {
 			base.Fatalf("go: cannot use path@version syntax in GOPATH mode")
+			continue
+		}
+
+		// Guard against 'go get x.go', a common mistake.
+		// Note that package and module paths may end with '.go', so only print an error
+		// if the argument has no slash or refers to an existing file.
+		if strings.HasSuffix(arg, ".go") {
+			if !strings.Contains(arg, "/") {
+				base.Errorf("go get %s: arguments must be package or module paths", arg)
+				continue
+			}
+			if fi, err := os.Stat(arg); err == nil && !fi.IsDir() {
+				base.Errorf("go get: %s exists as a file, but 'go get' requires package arguments", arg)
+			}
 		}
 	}
+	base.ExitIfErrors()
+
 	var pkgs []string
 	for _, m := range search.ImportPathsQuiet(patterns) {
 		if len(m.Pkgs) == 0 && strings.Contains(m.Pattern(), "...") {

src/cmd/go/internal/modget/get.go

@@ -310,6 +310,20 @@ func runGet(cmd *base.Command, args []string) {
 			continue
 		}
 
+		// Guard against 'go get x.go', a common mistake.
+		// Note that package and module paths may end with '.go', so only print an error
+		// if the argument has no version and either has no slash or refers to an existing file.
+		if strings.HasSuffix(arg, ".go") && vers == "" {
+			if !strings.Contains(arg, "/") {
+				base.Errorf("go get %s: arguments must be package or module paths", arg)
+				continue
+			}
+			if fi, err := os.Stat(arg); err == nil && !fi.IsDir() {
+				base.Errorf("go get: %s exists as a file, but 'go get' requires package arguments", arg)
+				continue
+			}
+		}
+
 		// If no version suffix is specified, assume @upgrade.
 		// If -u=patch was specified, assume @patch instead.
 		if vers == "" {

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

@@ -0,0 +1,60 @@
+# Tests Issue #38478
+# Tests that go get in GOPATH mode returns a specific error if the argument
+# ends with '.go', and either has no slash or refers to an existing file.
+
+env GO111MODULE=off
+
+# argument doesn't have .go suffix
+go get test
+
+# argument has .go suffix, is a file and exists
+! go get test.go
+stderr 'go get test.go: arguments must be package or module paths'
+
+# argument has .go suffix, doesn't exist and has no slashes
+! go get test_missing.go
+stderr 'go get test_missing.go: arguments must be package or module paths'
+
+# argument has .go suffix, is a file and exists in sub-directory
+! go get test/test.go
+stderr 'go get: test/test.go exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, doesn't exist and has slashes
+! go get test/test_missing.go
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, is a symlink and exists
+[symlink] symlink test_sym.go -> test.go
+[symlink] ! go get test_sym.go
+[symlink] stderr 'go get test_sym.go: arguments must be package or module paths'
+[symlink] rm test_sym.go
+
+# argument has .go suffix, is a symlink and exists in sub-directory
+[symlink] symlink test/test_sym.go -> test.go
+[symlink] ! go get test/test_sym.go
+[symlink] stderr 'go get: test/test_sym.go exists as a file, but ''go get'' requires package arguments'
+[symlink] rm test_sym.go
+
+# argument has .go suffix, is a directory and exists
+mkdir test_dir.go
+! go get test_dir.go
+stderr 'go get test_dir.go: arguments must be package or module paths'
+rm test_dir.go
+
+# argument has .go suffix, is a directory and exists in sub-directory
+mkdir test/test_dir.go
+! go get test/test_dir.go
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+rm test/test_dir.go
+
+
+-- test.go --
+package main
+func main() {println("test")}
+
+
+-- test/test.go --
+package main
+func main() {println("test")}

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

@@ -0,0 +1,68 @@
+# Tests Issue #38478
+# Tests that go get in GOMOD mode returns a specific error if the argument
+# ends with '.go', has no version, and either has no slash or refers to an
+# existing file.
+
+env GO111MODULE=on
+
+# argument doesn't have .go suffix and has no version
+! go get test
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix and has version
+! go get test.go@v1.0.0
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, is a file and exists
+! go get test.go
+stderr 'go get test.go: arguments must be package or module paths'
+
+# argument has .go suffix, doesn't exist and has no slashes
+! go get test_missing.go
+stderr 'arguments must be package or module paths'
+
+# argument has .go suffix, is a file and exists in sub-directory
+! go get test/test.go
+stderr 'go get: test/test.go exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, doesn't exist and has slashes
+! go get test/test_missing.go
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, is a symlink and exists
+[symlink] symlink test_sym.go -> test.go
+[symlink] ! go get test_sym.go
+[symlink] stderr 'go get test_sym.go: arguments must be package or module paths'
+[symlink] rm test_sym.go
+
+# argument has .go suffix, is a symlink and exists in sub-directory
+[symlink] symlink test/test_sym.go -> test.go
+[symlink] ! go get test/test_sym.go
+[symlink] stderr 'go get: test/test_sym.go exists as a file, but ''go get'' requires package arguments'
+[symlink] rm test_sym.go
+
+# argument has .go suffix, is a directory and exists
+mkdir test_dir.go
+! go get test_dir.go
+stderr 'go get test_dir.go: arguments must be package or module paths'
+rm test_dir.go
+
+# argument has .go suffix, is a directory and exists in sub-directory
+mkdir test/test_dir.go
+! go get test/test_dir.go
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+rm test/test_dir.go
+
+
+-- test.go --
+package main
+func main() {println("test")}
+
+
+-- test/test.go --
+package main
+func main() {println("test")}