cmd/link: avoid deadcode of global map vars for programs using plugins

If a program imports the plugin package, the mechanisms in place for
detecting and deleting unused global map variables are no longer safe,
since it's possibly for a given global map var to be unreferenced in
the main program but referenced by a plugin. This patch changes the
linker to test for plugin use and to avoid removing any unused global
map variables if the main program could possibly load up a plugin.

Fixes #62430.

Change-Id: Ie00b18b681cb0d259e3c859ac947ade5778cd6c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/526115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

2 files changed, 46 insertions, 0 deletions

diff --git a/src/cmd/cgo/internal/testplugin/testdata/issue62430/main.go b/src/cmd/cgo/internal/testplugin/testdata/issue62430/main.go
new file mode 100644
index 0000000000..80108407c2
--- /dev/null
+++ b/src/cmd/cgo/internal/testplugin/testdata/issue62430/main.go
@@ -0,0 +1,35 @@
+// Copyright 2023 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.
+
+// Issue 62430: a program that uses plugins may appear
+// to have no references to an initialized global map variable defined
+// in some stdlib package (ex: unicode), however there
+// may be references to that map var from a plugin that
+// gets loaded.
+
+package main
+
+import (
+	"fmt"
+	"plugin"
+	"unicode"
+)
+
+func main() {
+	p, err := plugin.Open("issue62430.so")
+	if err != nil {
+		panic(err)
+	}
+	s, err := p.Lookup("F")
+	if err != nil {
+		panic(err)
+	}
+
+	f := s.(func(string) *unicode.RangeTable)
+	if f("C") == nil {
+		panic("unicode.Categories not properly initialized")
+	} else {
+		fmt.Println("unicode.Categories properly initialized")
+	}
+}
diff --git a/src/cmd/cgo/internal/testplugin/testdata/issue62430/plugin.go b/src/cmd/cgo/internal/testplugin/testdata/issue62430/plugin.go
new file mode 100644
index 0000000000..e42cd8bb77
--- /dev/null
+++ b/src/cmd/cgo/internal/testplugin/testdata/issue62430/plugin.go
@@ -0,0 +1,11 @@
+package main
+
+import (
+	"unicode"
+)
+
+func F(s string) *unicode.RangeTable {
+	return unicode.Categories[s]
+}
+
+func main() {}