internal: add dlsym package for working with dynamically loaded symbols

The dlsym package provides LookupSymbol() which resolves a named symbol
(like "rbd_clone4") and returns a unsafe.Pointer to it. The caller is
expected to cast the symbol to function that can be called.

Signed-off-by: Niels de Vos <ndevos@ibm.com>

internal/dlsym/dlsym.go

@@ -0,0 +1,41 @@
+package dlsym
+
+// #cgo LDFLAGS: -ldl
+//
+// #include <stdlib.h>
+// #include <dlfcn.h>
+//
+// #ifndef RTLD_DEFAULT /* from dlfcn.h */
+// #define RTLD_DEFAULT ((void *) 0)
+// #endif
+import "C"
+
+import (
+	"errors"
+	"fmt"
+	"unsafe"
+)
+
+// ErrUndefinedSymbol is returned by LookupSymbol when the requested symbol
+// could not be found.
+var ErrUndefinedSymbol = errors.New("symbol not found")
+
+// LookupSymbol resolves the named symbol from the already dynamically loaded
+// libraries. If the symbol is found, a pointer to it is returned, in case of a
+// failure, the message provided by dlerror() is included in the error message.
+func LookupSymbol(symbol string) (unsafe.Pointer, error) {
+	cSymName := C.CString(symbol)
+	defer C.free(unsafe.Pointer(cSymName))
+
+	// clear dlerror before looking up the symbol
+	C.dlerror()
+	// resolve the address of the symbol
+	sym := C.dlsym(C.RTLD_DEFAULT, cSymName)
+	e := C.dlerror()
+	dlerr := C.GoString(e)
+	if dlerr != "" {
+		return nil, fmt.Errorf("%w: %s", ErrUndefinedSymbol, dlerr)
+	}
+
+	return sym, nil
+}

internal/dlsym/dlsym_test.go

@@ -0,0 +1,22 @@
+package dlsym
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestLookupSymbol(t *testing.T) {
+	t.Run("ValidSymbol", func(t *testing.T) {
+		sym, err := LookupSymbol("dlsym")
+		assert.NotNil(t, sym)
+		assert.NoError(t, err)
+	})
+
+	t.Run("InvalidSymbol", func(t *testing.T) {
+		sym, err := LookupSymbol("go_ceph_dlsym")
+		assert.Nil(t, sym)
+		assert.True(t, errors.Is(err, ErrUndefinedSymbol))
+	})
+}