go/internal/gcimporter: set underlying types in proper order; flatten imports

These are copied from go/internal, for processing
export data from unified IR compilation.

The underlying types order problem appeared in google-internal
testing. If the run-later functions are run in the wrong order,
type definitions won't resolve properly.

Flatten imports makes the unified IR export/import match the
behavior of export data from older compilers, though it's not
clear that this behavior was intended, it is now expected.

See https://go.dev/cl/419996 and https://go.dev/cl/419596

Change-Id: I4197fe9e93ee07eb7f24597ba9157ce083a1d086
Reviewed-on: https://go-review.googlesource.com/c/tools/+/420534
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: David Chase <drchase@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

go/internal/gcimporter/ureader_yes.go

@@ -36,6 +36,8 @@ type pkgReader struct {
 	// laterFns holds functions that need to be invoked at the end of
 	// import reading.
 	laterFns []func()
+	// laterFors is used in case of 'type A B' to ensure that B is processed before A.
+	laterFors map[types.Type]int
 }
 
 // later adds a function to be invoked at the end of import reading.
@@ -63,6 +65,15 @@ func UImportData(fset *token.FileSet, imports map[string]*types.Package, data []
 	return
 }
 
+// laterFor adds a function to be invoked at the end of import reading, and records the type that function is finishing.
+func (pr *pkgReader) laterFor(t types.Type, fn func()) {
+	if pr.laterFors == nil {
+		pr.laterFors = make(map[types.Type]int)
+	}
+	pr.laterFors[t] = len(pr.laterFns)
+	pr.laterFns = append(pr.laterFns, fn)
+}
+
 // readUnifiedPackage reads a package description from the given
 // unified IR export data decoder.
 func readUnifiedPackage(fset *token.FileSet, ctxt *types.Context, imports map[string]*types.Package, input pkgbits.PkgDecoder) *types.Package {
@@ -231,11 +242,35 @@ func (r *reader) doPkg() *types.Package {
 	for i := range imports {
 		imports[i] = r.pkg()
 	}
-	pkg.SetImports(imports)
+	pkg.SetImports(flattenImports(imports))
 
 	return pkg
 }
 
+// flattenImports returns the transitive closure of all imported
+// packages rooted from pkgs.
+func flattenImports(pkgs []*types.Package) []*types.Package {
+	var res []*types.Package
+
+	seen := make(map[*types.Package]bool)
+	var add func(pkg *types.Package)
+	add = func(pkg *types.Package) {
+		if seen[pkg] {
+			return
+		}
+		seen[pkg] = true
+		res = append(res, pkg)
+		for _, imp := range pkg.Imports() {
+			add(imp)
+		}
+	}
+
+	for _, pkg := range pkgs {
+		add(pkg)
+	}
+	return res
+}
+
 // @@@ Types
 
 func (r *reader) typ() types.Type {
@@ -482,7 +517,15 @@ func (pr *pkgReader) objIdx(idx pkgbits.Index) (*types.Package, string) {
 			// unit tests expected that), but cmd/compile doesn't care
 			// about it, so maybe we can avoid worrying about that here.
 			rhs := r.typ()
-			r.p.later(func() {
+			pk := r.p
+			pk.laterFor(named, func() {
+				// First be sure that the rhs is initialized, if it needs to be initialized.
+				delete(pk.laterFors, named) // prevent cycles
+				if i, ok := pk.laterFors[rhs]; ok {
+					f := pk.laterFns[i]
+					pk.laterFns[i] = func() {} // function is running now, so replace it with a no-op
+					f()                        // initialize RHS
+				}
 				underlying := rhs.Underlying()
 				named.SetUnderlying(underlying)
 			})