cue/load: add package import cycle error

When two or more packages import each other resulting in a cycle,
cue/load used to cause an infinite loop.
Error instead, since we explicitly don't support import cycles,
per https://cuelang.org/docs/references/spec/#import-declarations:

    It is illegal for a package to import itself, directly or indirectly

Fixes #849

Closes #2469 as merged as of commit ce73397.

Signed-off-by: Artem V. Navrotskiy <[email protected]>
Change-Id: I1928717b3df3c5e6c48eb4fa30966f26079e19c6
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1167597
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

cue/load/import.go

@@ -51,6 +51,19 @@ import (
 //	_       anonymous files (which may be marked with _)
 //	*       all packages
 func (l *loader) importPkg(pos token.Pos, p *build.Instance) []*build.Instance {
+	retErr := func(errs errors.Error) []*build.Instance {
+		// XXX: move this loop to ReportError
+		for _, err := range errors.Errors(errs) {
+			p.ReportError(err)
+		}
+		return []*build.Instance{p}
+	}
+
+	for _, item := range l.stk {
+		if item == p.ImportPath {
+			return retErr(&PackageError{Message: errors.NewMessagef("package import cycle not allowed")})
+		}
+	}
 	l.stk.Push(p.ImportPath)
 	defer l.stk.Pop()
 
@@ -61,14 +74,6 @@ func (l *loader) importPkg(pos token.Pos, p *build.Instance) []*build.Instance {
 		return []*build.Instance{p}
 	}
 
-	retErr := func(errs errors.Error) []*build.Instance {
-		// XXX: move this loop to ReportError
-		for _, err := range errors.Errors(errs) {
-			p.ReportError(err)
-		}
-		return []*build.Instance{p}
-	}
-
 	fp := newFileProcessor(cfg, p, l.tagger)
 
 	if p.PkgName == "" {

cue/load/loader_test.go

@@ -330,6 +330,23 @@ module: mod.test/test
 root:   $CWD/testdata/testmod
 dir:    $CWD/testdata/testmod/tagsbad
 display:./tagsbad`,
+	}, {
+		cfg: &Config{
+			Dir: testdataDir,
+		},
+		args: args("./cycle"),
+		want: `
+err:    import failed: import failed: import failed: package import cycle not allowed:
+    $CWD/testdata/testmod/cycle/cycle.cue:3:8
+    $CWD/testdata/testmod/cue.mod/pkg/mod.test/cycle/bar/bar.cue:3:8
+    $CWD/testdata/testmod/cue.mod/pkg/mod.test/cycle/foo/foo.cue:3:8
+path:   mod.test/test/cycle
+module: mod.test/test
+root:   $CWD/testdata/testmod
+dir:    $CWD/testdata/testmod/cycle
+display:./cycle
+files:
+    $CWD/testdata/testmod/cycle/cycle.cue`,
 	}}
 	for i, tc := range testCases {
 		t.Run(strconv.Itoa(i)+"/"+strings.Join(tc.args, ":"), func(t *testing.T) {

cue/load/testdata/testmod/cue.mod/pkg/mod.test/cycle/bar/bar.cue

@@ -0,0 +1,6 @@
+package bar
+
+import "mod.test/cycle/foo"
+
+#Bar1: foo.#Foo1 + 8
+#Bar2: foo.#Foo2 + 16

cue/load/testdata/testmod/cue.mod/pkg/mod.test/cycle/foo/foo.cue

@@ -0,0 +1,7 @@
+package foo
+
+import "mod.test/cycle/bar"
+
+#Foo1: 1
+#Foo2: bar.#Bar1 + 2
+#Foo:  bar.#Bar2 + 4

cue/load/testdata/testmod/cycle/cycle.cue

@@ -0,0 +1,5 @@
+package cycle
+
+import "mod.test/cycle/foo"
+
+Foo: foo.#Foo