cmdget: accept package paths

DO NOT MERGE

The test fails because go-internal's proxy
doesn't support .../@v/list requests.

Fixes rogpeppe#62

cmdget.go

@@ -5,6 +5,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"sort"
 	"strings"
 
 	"gopkg.in/errgo.v2/fmt/errors"
@@ -14,11 +15,13 @@ import (
 )
 
 var getCommand = &Command{
-	UsageLine: "get [-vcs] [-u] [-f] [module...]",
+	UsageLine: "get [-vcs] [-u] [-f] [module/package...]",
 	Short:     "start hacking a module",
 	Long: `
 The get command checks out Go module dependencies
 into a directory where they can be edited.
+If a package is named, its containing module
+is checked out.
 
 It uses $GOHACK/<module> as the destination directory,
 or $HOME/gohack/<module> if $GOHACK is empty.
@@ -61,12 +64,39 @@ func runGet1(args []string) error {
 		// Perhaps we should be more resilient in that case?
 		return errors.Notef(err, nil, "cannot get module info")
 	}
-	for _, mpath := range args {
-		m := mods[mpath]
-		if m == nil {
-			errorf("module %q does not appear to be in use", mpath)
+	// Args could be package paths or modules.
+	// Resolve them all to modules, deduplicate, and sort.
+	resolved := make(map[string]bool)
+	for _, arg := range args {
+		if _, ok := mods[arg]; ok {
+			// module in use
+			resolved[arg] = true
 			continue
 		}
+		// Either a package path, or an unused module.
+		// Try to resolve as a package path.
+		out, err := runCmd(cwd, "go", "list", "-f", "{{with .Module}}{{.Path}}{{end}}", arg)
+		if err == nil {
+			// Resolved to a module. Is it in use?
+			out = strings.TrimSpace(out)
+			if _, ok := mods[out]; ok {
+				resolved[out] = true
+				continue
+			}
+		}
+		// Either an unused module, or an invalid package path,
+		// or a valid package path that resolves to an unused module,
+		// or something else.
+		errorf("module/package %q does not appear to be in use", arg)
+	}
+	mpaths := make([]string, 0, len(resolved))
+	for mpath := range resolved {
+		mpaths = append(mpaths, mpath)
+	}
+	sort.Strings(mpaths)
+
+	for _, mpath := range mpaths {
+		m := mods[mpath] // must be present, by construction (above)
 		// Early check that we can replace the module, so we don't
 		// do all the work to check it out only to find we can't
 		// add the replace directive.

testdata/get-pkg.txt

@@ -0,0 +1,34 @@
+cd repo
+go get golang.org/x/text
+env GOHACK=$WORK/gohack
+gohack get golang.org/x/text/language
+# Check that golang.org/x/text/language resolved
+# to golang.org/x/text by examining stdout.
+# Trust the other get tests to ensure that everything else went ok.
+stdout '^golang.org/x/text  => .*/gohack/golang.org/x/text$'
+! stderr .+
+
+gohack undo
+! stderr .+
+
+# Check that deduplication works across modules and packages.
+gohack get golang.org/x/text golang.org/x/text/language golang.org/x/text/unused
+stdout '^golang.org/x/text => .*/gohack/golang.org/x/text$'
+! stdout language
+! stdout unused
+! stderr .+
+
+-- repo/main.go --
+package main
+import (
+	"golang.org/x/text/language"
+	_ "golang.org/x/text/unused"
+)
+
+var _ = language.Make
+
+func main() {
+}
+
+-- repo/go.mod --
+module example.com/repo

testdata/help.txt

@@ -1,8 +1,8 @@
 # --help flag produces output to stderr and fails
 ! gohack get --help
-stderr '^usage: get \[-vcs] \[-u] \[-f] \[module...]\nRun ''gohack help get'' for details.\n'
+stderr '^usage: get \[-vcs] \[-u] \[-f] \[module/package...]\nRun ''gohack help get'' for details.\n'
 ! stdout .+
 
 gohack help get
-stdout '^usage: get \[-vcs] \[-u] \[-f] \[module...]$'
+stdout '^usage: get \[-vcs] \[-u] \[-f] \[module/package...]$'
 ! stderr .+