runtime/cgo: add Handle for managing (c)go pointers

A non-trivial Cgo program may need to use callbacks and interact with
go objects per goroutine. Because of the rules for passing pointers
between Go and C, such a program needs to store handles to associated
Go values. This often causes much extra effort to figure out a way to
correctly deal with: 1) map collision; 2) identifying leaks and 3)
concurrency.

This CL implements a Handle representation in runtime/cgo package, and
related methods such as Value, Delete, etc. which allows Go users can
use a standard way to handle the above difficulties.

In addition, the CL allows a Go value to have multiple handles, and the
NewHandle always returns a different handle compare to the previously
returned handles. In comparison, CL 294670 implements a different
behavior of NewHandle that returns a unique handle when the Go value is
referring to the same object.

Benchmark:
name                      time/op
Handle/non-concurrent-16  487ns ± 1%
Handle/concurrent-16      674ns ± 1%

Fixes #37033

Change-Id: I0eadb9d44332fffef8fb567c745246a49dd6d4c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/295369
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Cherry Zhang <cherryyz@google.com>

3 files changed, 35 insertions, 0 deletions

diff --git a/misc/cgo/test/cgo_test.go b/misc/cgo/test/cgo_test.go
index f7a76d047b..837307263a 100644
--- a/misc/cgo/test/cgo_test.go
+++ b/misc/cgo/test/cgo_test.go
@@ -80,6 +80,7 @@ func TestNamedEnum(t *testing.T)             { testNamedEnum(t) }
 func TestCastToEnum(t *testing.T)            { testCastToEnum(t) }
 func TestErrno(t *testing.T)                 { testErrno(t) }
 func TestFpVar(t *testing.T)                 { testFpVar(t) }
+func TestHandle(t *testing.T)                { testHandle(t) }
 func TestHelpers(t *testing.T)               { testHelpers(t) }
 func TestLibgcc(t *testing.T)                { testLibgcc(t) }
 func TestMultipleAssign(t *testing.T)        { testMultipleAssign(t) }
diff --git a/misc/cgo/test/test.go b/misc/cgo/test/test.go
index 65823b1ca0..76afa524c3 100644
--- a/misc/cgo/test/test.go
+++ b/misc/cgo/test/test.go
@@ -899,6 +899,10 @@ static uint16_t issue31093F(uint16_t v) { return v; }
 // issue 32579
 typedef struct S32579 { unsigned char data[1]; } S32579;
 
+// issue 37033, cgo.Handle
+extern void GoFunc37033(uintptr_t handle);
+void cFunc37033(uintptr_t handle) { GoFunc37033(handle); }
+
 // issue 38649
 // Test that #define'd type aliases work.
 #define netbsd_gid unsigned int
@@ -920,6 +924,7 @@ import (
 	"os/signal"
 	"reflect"
 	"runtime"
+	"runtime/cgo"
 	"sync"
 	"syscall"
 	"testing"
@@ -2230,6 +2235,23 @@ func test32579(t *testing.T) {
 	}
 }
 
+// issue 37033, check if cgo.Handle works properly
+
+func testHandle(t *testing.T) {
+	ch := make(chan int)
+
+	for i := 0; i < 42; i++ {
+		h := cgo.NewHandle(ch)
+		go func() {
+			C.cFunc37033(C.uintptr_t(h))
+		}()
+		if v := <-ch; issue37033 != v {
+			t.Fatalf("unexpected receiving value: got %d, want %d", v, issue37033)
+		}
+		h.Delete()
+	}
+}
+
 // issue 38649
 
 var issue38649 C.netbsd_gid = 42
diff --git a/misc/cgo/test/testx.go b/misc/cgo/test/testx.go
index 2b2e69ec00..044c5bceff 100644
--- a/misc/cgo/test/testx.go
+++ b/misc/cgo/test/testx.go
@@ -12,6 +12,7 @@ package cgotest
 
 import (
 	"runtime"
+	"runtime/cgo"
 	"runtime/debug"
 	"strings"
 	"sync"
@@ -558,6 +559,17 @@ func test31891(t *testing.T) {
 	C.callIssue31891()
 }
 
+// issue 37033, check if cgo.Handle works properly
+
+var issue37033 = 42
+
+//export GoFunc37033
+func GoFunc37033(handle C.uintptr_t) {
+	h := cgo.Handle(handle)
+	ch := h.Value().(chan int)
+	ch <- issue37033
+}
+
 // issue 38408
 // A typedef pointer can be used as the element type.
 // No runtime test; just make sure it compiles.