dlopen: make error handling thread safe

internal/dlopen/dlopen.go

@@ -23,6 +23,7 @@ import "C"
 import (
 	"errors"
 	"fmt"
+	"runtime"
 	"unsafe"
 )
 
@@ -56,6 +57,10 @@ func GetHandle(libs []string) (*LibHandle, error) {
 
 // GetSymbolPointer takes a symbol name and returns a pointer to the symbol.
 func (l *LibHandle) GetSymbolPointer(symbol string) (unsafe.Pointer, error) {
+	// Locking the thread is critical here as the dlerror() is thread local so
+	// go should not reschedule this onto another thread.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
 	sym := C.CString(symbol)
 	defer C.free(unsafe.Pointer(sym))
 
@@ -71,6 +76,10 @@ func (l *LibHandle) GetSymbolPointer(symbol string) (unsafe.Pointer, error) {
 
 // Close closes a LibHandle.
 func (l *LibHandle) Close() error {
+	// Locking the thread is critical here as the dlerror() is thread local so
+	// go should not reschedule this onto another thread.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
 	C.dlerror()
 	C.dlclose(l.Handle)
 	e := C.dlerror()

internal/dlopen/dlopen_test.go

@@ -16,6 +16,7 @@ package dlopen
 
 import (
 	"fmt"
+	"sync"
 	"testing"
 )
 
@@ -62,3 +63,42 @@ func TestDlopen(t *testing.T) {
 		}
 	}
 }
+
+// Note this is not a reliable reproducer for the problem.
+// It depends on the fact the it first generates some dlerror() errors
+// by using non existent libraries.
+func TestDlopenThreadSafety(t *testing.T) {
+	libs := []string{
+		"libstrange1.so",
+		"libstrange2.so",
+		"libstrange3.so",
+		"libstrange4.so",
+		"libstrange5.so",
+		"libstrange6.so",
+		"libstrange7.so",
+		"libstrange8.so",
+		"libc.so.6",
+		"libc.so",
+	}
+
+	// 10000 is the default golang thread limit, so adding more will likely fail
+	// but this number is enough to reproduce the issue most of the time for me.
+	count := 10000
+	wg := sync.WaitGroup{}
+	wg.Add(count)
+
+	for i := 0; i < count; i++ {
+		go func() {
+			defer wg.Done()
+			lib, err := GetHandle(libs)
+			if err != nil {
+				t.Errorf("GetHandle failed unexpectedly: %v", err)
+			}
+			_, err = lib.GetSymbolPointer("strlen")
+			if err != nil {
+				t.Errorf("GetSymbolPointer strlen failed unexpectedly: %v", err)
+			}
+		}()
+	}
+	wg.Wait()
+}