go/importer: make source importer more tolerant in presence of errors

If the source importer only encounters "soft" type checking errors
it can safely return the type-checked package because it will be
completely set up. This makes the source importer slightly more
robust in the presence of errors.

Fixes #20855.

Change-Id: I5af9ccdb30eee6bca7a0fab872f6057bde521bf3
Reviewed-on: https://go-review.googlesource.com/55730
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/internal/srcimporter/srcimporter.go

@@ -128,19 +128,33 @@ func (p *Importer) ImportFrom(path, srcDir string, mode types.ImportMode) (*type
 	}
 
 	// type-check package files
+	var firstHardErr error
 	conf := types.Config{
 		IgnoreFuncBodies: true,
 		FakeImportC:      true,
+		// continue type-checking after the first error
+		Error: func(err error) {
+			if firstHardErr == nil && !err.(types.Error).Soft {
+				firstHardErr = err
+			}
+		},
 		Importer: p,
 		Sizes:    p.sizes,
 	}
 	pkg, err = conf.Check(bp.ImportPath, p.fset, files, nil)
 	if err != nil {
-		// Type-checking stops after the first error (types.Config.Error is not set),
-		// so the returned package is very likely incomplete. Don't return it since
-		// we don't know its condition: It's very likely unsafe to use and it's also
-		// not added to p.packages which may cause further problems (issue #20837).
-		return nil, fmt.Errorf("type-checking package %q failed (%v)", bp.ImportPath, err)
+		// If there was a hard error it is possibly unsafe
+		// to use the package as it may not be fully populated.
+		// Do not return it (see also #20837, #20855).
+		if firstHardErr != nil {
+			pkg = nil
+			err = firstHardErr // give preference to first hard error over any soft error
+		}
+		return pkg, fmt.Errorf("type-checking package %q failed (%v)", bp.ImportPath, err)
+	}
+	if firstHardErr != nil {
+		// this can only happen if we have a bug in go/types
+		panic("package is not safe yet no error was returned")
 	}
 
 	p.packages[bp.ImportPath] = pkg

src/go/internal/srcimporter/srcimporter_test.go

@@ -148,3 +148,17 @@ func TestReimport(t *testing.T) {
 		t.Errorf("got %v; want reimport error", err)
 	}
 }
+
+func TestIssue20855(t *testing.T) {
+	if !testenv.HasSrc() {
+		t.Skip("no source code available")
+	}
+
+	pkg, err := importer.ImportFrom("go/internal/srcimporter/testdata/issue20855", ".", 0)
+	if err == nil || !strings.Contains(err.Error(), "missing function body") {
+		t.Fatalf("got unexpected or no error: %v", err)
+	}
+	if pkg == nil {
+		t.Error("got no package despite no hard errors")
+	}
+}

src/go/internal/srcimporter/testdata/issue20855/issue20855.go

@@ -0,0 +1,7 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue20855
+
+func init() // "missing function body" is a soft error