cmd/link: skip zero values in fingerprint check

Normally, packages are loaded in dependency order, and if a
Library object is not nil, it is already loaded with the actual
fingerprint. In shared build mode, however, packages may be added
not in dependency order (e.g. go install -buildmode=shared std
adds all std packages before loading them), and it is possible
that a Library's fingerprint is not yet loaded. Skip the check
in this case (when the fingerprint is the zero value).

Fixes #39777.

Change-Id: I66208e92bf687c8778963ba8e33e9bd948f82f3a
Reviewed-on: https://go-review.googlesource.com/c/go/+/239517
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

misc/cgo/testshared/shared_test.go

@@ -1028,3 +1028,9 @@ func TestGeneratedHash(t *testing.T) {
 	goCmd(nil, "install", "-buildmode=shared", "-linkshared", "./issue30768/issue30768lib")
 	goCmd(nil, "test", "-linkshared", "./issue30768")
 }
+
+// Test that packages can be added not in dependency order (here a depends on b, and a adds
+// before b). This could happen with e.g. go build -buildmode=shared std. See issue 39777.
+func TestPackageOrder(t *testing.T) {
+	goCmd(t, "install", "-buildmode=shared", "-linkshared", "./issue39777/a", "./issue39777/b")
+}

misc/cgo/testshared/testdata/issue39777/a/a.go

@@ -0,0 +1,9 @@
+// Copyright 2020 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 a
+
+import "testshared/issue39777/b"
+
+func F() { b.F() }

misc/cgo/testshared/testdata/issue39777/b/b.go

@@ -0,0 +1,7 @@
+// Copyright 2020 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 b
+
+func F() {}

src/cmd/link/internal/ld/ld.go

@@ -160,7 +160,12 @@ func addlib(ctxt *Link, src, obj, lib string, fingerprint goobj2.FingerprintType
 	pkg := pkgname(ctxt, lib)
 
 	// already loaded?
-	if l := ctxt.LibraryByPkg[pkg]; l != nil {
+	if l := ctxt.LibraryByPkg[pkg]; l != nil && !l.Fingerprint.IsZero() {
+		// Normally, packages are loaded in dependency order, and if l != nil
+		// l is already loaded with the actual fingerprint. In shared build mode,
+		// however, packages may be added not in dependency order, and it is
+		// possible that l's fingerprint is not yet loaded -- exclude it in
+		// checking.
 		checkFingerprint(l, l.Fingerprint, src, fingerprint)
 		return l
 	}