cue/load: add Config.AcceptLegacyModules

In some cases, we want to be able to accept legacy module files (module
files lacking a major version) because there's no easy way to fix them
by running `cue mod fix`. When running the cue command, `cue mod fix`
should always be a possibility - the problem exists principally when
using `cue/load` directly.

We don't want to make this the default, because interpreting module
files in this way is problematic, as it will discard fields that are
legitimately part of the non-legacy module schema.

So we add a boolean field that explicitly enables this behavior.

Fixes #3219.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I24d66eb680be3267adbe0a972705cc3414936362
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1196156
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1196230
Reviewed-by: Paul Jolly <paul@myitcv.io>

cue/load/config.go

@@ -138,6 +138,10 @@ type Config struct {
 	// the module field of an existing cue.mod file.
 	Module string
 
+	// AcceptLegacyModules causes the module resolution code
+	// to accept module files that lack a language.version field.
+	AcceptLegacyModules bool
+
 	// modFile holds the contents of the module file, or nil
 	// if no module file was present. If non-nil, then
 	// after calling Config.complete, modFile.Module will be
@@ -414,6 +418,13 @@ func (c *Config) loadModule() error {
 	parseModFile := modfile.ParseNonStrict
 	if c.Registry == nil {
 		parseModFile = modfile.ParseLegacy
+	} else if c.AcceptLegacyModules {
+		// Note: technically this does not support all legacy module
+		// files because some old module files might contain non-concrete
+		// data, but that seems like an OK restriction for now at least,
+		// given that no actual instances of non-concrete data in
+		// module files have been discovered in the wild.
+		parseModFile = modfile.FixLegacy
 	}
 	mf, err := parseModFile(data, mod)
 	if err != nil {

cue/load/loader_test.go

@@ -358,7 +358,75 @@ root:   $CWD/testdata/testmod
 dir:    $CWD/testdata/testmod/cycle
 display:./cycle
 files:
-    $CWD/testdata/testmod/cycle/cycle.cue`}}
+    $CWD/testdata/testmod/cycle/cycle.cue`}, {
+		name: "AcceptLegacyModuleWithLegacyModule",
+		cfg: &Config{
+			Dir:                 testdata("testmod_legacy"),
+			AcceptLegacyModules: true,
+		},
+		want: `path:   test.example/foo@v0
+module: test.example/foo@v0
+root:   $CWD/testdata/testmod_legacy
+dir:    $CWD/testdata/testmod_legacy
+display:.
+files:
+    $CWD/testdata/testmod_legacy/foo.cue`}, {
+		name: "AcceptLegacyModuleWithNonLegacyModule",
+		cfg: &Config{
+			Dir:                 testdataDir,
+			Tools:               true,
+			AcceptLegacyModules: true,
+		},
+		args: []string{"./imports"},
+		want: `path:   mod.test/test/imports@v0
+module: mod.test/test@v0
+root:   $CWD/testdata/testmod
+dir:    $CWD/testdata/testmod/imports
+display:./imports
+files:
+    $CWD/testdata/testmod/imports/imports.cue
+imports:
+    mod.test/catch: $CWD/testdata/testmod/cue.mod/pkg/mod.test/catch/catch.cue
+    mod.test/helper:helper1: $CWD/testdata/testmod/cue.mod/pkg/mod.test/helper/helper1.cue`}, {
+		name: "MismatchedModulePathInConfig",
+		cfg: &Config{
+			Dir:    testdataDir,
+			Tools:  true,
+			Module: "wrong.test@v0",
+		},
+		args: []string{"./imports"},
+		want: `err:    inconsistent modules: got "mod.test/test@v0", want "wrong.test@v0"
+path:   ""
+module: wrong.test@v0
+root:   ""
+dir:    ""
+display:""`}, {
+		name: "ModulePathInConfigWithoutMajorVersion",
+		cfg: &Config{
+			Dir:    testdataDir,
+			Tools:  true,
+			Module: "mod.test/test",
+		},
+		args: []string{"./imports"},
+		want: `err:    inconsistent modules: got "mod.test/test@v0", want "mod.test/test"
+path:   ""
+module: mod.test/test
+root:   ""
+dir:    ""
+display:""`}, {
+		name: "ModulePathInConfigWithoutMajorVersionAndMismatchedPath",
+		cfg: &Config{
+			Dir:    testdataDir,
+			Tools:  true,
+			Module: "mod.test/wrong",
+		},
+		args: []string{"./imports"},
+		want: `err:    inconsistent modules: got "mod.test/test@v0", want "mod.test/wrong"
+path:   ""
+module: mod.test/wrong
+root:   ""
+dir:    ""
+display:""`}}
 	tdtest.Run(t, testCases, func(t *tdtest.T, tc *loadTest) {
 		pkgs := Instances(tc.args, tc.cfg)
 

cue/load/testdata/testmod_legacy/cue.mod/module.cue

@@ -0,0 +1,7 @@
+// Note: no major version suffix.
+module: "test.example/foo"
+
+// Note: no language.version field present.
+
+// Note: extra field not allowed by later strict module versions.
+extraField: 124

cue/load/testdata/testmod_legacy/foo.cue

@@ -0,0 +1,2 @@
+package foo
+