mod/module: allow versionless module path in CheckPath

In general, a canonical module path can omit the major
version (implying `@v0`), but this was not reflected
in the `module.CheckPath` function.

Changing this behavior fixes `cue mod edit` to allow
such module paths. It also changes some error messages,
as reflected in the tests.

Fixes #3262.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Ic1d64bf13b9afc81da5281d320e61fa704a1a947
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1197629
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cmd/cue/cmd/modinit.go

@@ -18,7 +18,6 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"strings"
 
 	"github.com/spf13/cobra"
 
@@ -56,13 +55,7 @@ func runModInit(cmd *Command, args []string) (err error) {
 		}
 		modulePath = args[0]
 		if err := module.CheckPath(modulePath); err != nil {
-			// It might just be lacking a major version.
-			if err1 := module.CheckPathWithoutVersion(modulePath); err1 != nil {
-				if strings.Contains(modulePath, "@") {
-					err1 = err
-				}
-				return fmt.Errorf("invalid module name %q: %v", modulePath, err1)
-			}
+			return err
 		}
 	}
 

cmd/cue/cmd/testdata/script/modedit_nomajorversion.txtar

@@ -4,15 +4,16 @@
 
 exec cue mod init test.example
 cmp cue.mod/module.cue want-module
-
-# TODO this should not fail.
-! exec cue mod edit --module other.example
-cmp stderr want-stderr
+exec cue mod edit --module other.example
+cmp cue.mod/module.cue want-module-2
 
 -- want-module --
 module: "test.example"
 language: {
 	version: "v0.10.0"
 }
--- want-stderr --
-invalid argument "other.example" for "--module" flag: malformed module path "other.example": no major version found in module path
+-- want-module-2 --
+module: "other.example"
+language: {
+	version: "v0.10.0"
+}

cmd/cue/cmd/testdata/script/modinit_badmodule.txtar

@@ -1,4 +1,4 @@
 ! exec cue mod init bad
 cmp stderr want-stderr
 -- want-stderr --
-invalid module name "bad": missing dot in first path element
+malformed module path "bad": missing dot in first path element

cmd/cue/cmd/testdata/script/modinit_badmoduleversion.txtar

@@ -1,6 +1,4 @@
-# TODO reduce error message redundancy
-
 ! exec cue mod init bad.com@v2.2
 cmp stderr want-stderr
 -- want-stderr --
-invalid module name "bad.com@v2.2": malformed module path "bad.com@v2.2": path can contain major version only
+malformed module path "bad.com@v2.2": path can contain major version only

mod/module/path.go

@@ -148,9 +148,11 @@ func CheckPathWithoutVersion(basePath string) (err error) {
 // ASCII digits, dots (U+002E), and dashes (U+002D);
 // it must contain at least one dot and cannot start with a dash.
 //
-// Second, there must be a final major version of the form
+// Second, there may be a final major version of the form
 // @vN where N looks numeric
 // (ASCII digits) and must not begin with a leading zero.
+// Without such a major version, the major version is assumed
+// to be v0.
 //
 // Third, no path element may begin with a dot.
 func CheckPath(mpath string) (err error) {
@@ -164,18 +166,19 @@ func CheckPath(mpath string) (err error) {
 	}()
 
 	basePath, vers, ok := SplitPathVersion(mpath)
-	if !ok {
-		return fmt.Errorf("no major version found in module path")
-	}
-	if semver.Major(vers) != vers {
-		return fmt.Errorf("path can contain major version only")
+	if ok {
+		if semver.Major(vers) != vers {
+			return fmt.Errorf("path can contain major version only")
+		}
+		if !tagPat.MatchString(vers) {
+			return fmt.Errorf("non-conforming version %q", vers)
+		}
+	} else {
+		basePath = mpath
 	}
 	if err := CheckPathWithoutVersion(basePath); err != nil {
 		return err
 	}
-	if !tagPat.MatchString(vers) {
-		return fmt.Errorf("non-conforming version %q", vers)
-	}
 	return nil
 }