rbd: return more useful error messages

Translate the return value (errno) from the C rbd_*() functions into the
standardized strerror() error message.

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

rbd/rbd.go

@@ -1,6 +1,9 @@
 package rbd
 
 // #cgo LDFLAGS: -lrbd
+// /* force XSI-complaint strerror_r() */
+// #define _POSIX_C_SOURCE 200112L
+// #undef _GNU_SOURCE
 // #include <errno.h>
 // #include <stdlib.h>
 // #include <rados/librados.h>
@@ -162,7 +165,19 @@ func (snapshot *Snapshot) validate(req uint32) error {
 
 //
 func (e RBDError) Error() string {
-	return fmt.Sprintf("rbd: ret=%d", e)
+	buf := make([]byte, 1024)
+	// strerror expects errno >= 0
+	errno := e
+	if errno < 0 {
+		errno = -errno
+	}
+
+	ret := C.strerror_r(C.int(errno), (*C.char)(unsafe.Pointer(&buf[0])), C.size_t(len(buf)))
+	if ret != 0 {
+		return fmt.Sprintf("rbd: ret=%d", e)
+	}
+
+	return fmt.Sprintf("rbd: ret=%d, %s", e, C.GoString((*C.char)(unsafe.Pointer(&buf[0]))))
 }
 
 //

rbd/rbd_test.go

@@ -22,6 +22,19 @@ func GetUUID() string {
 	return uuid.Must(uuid.NewV4()).String()
 }
 
+func TestRBDError(t *testing.T) {
+	err := rbd.GetError(0)
+	assert.NoError(t, err)
+
+	err = rbd.GetError(-39) // NOTEMPTY (image still has a snapshot)
+	assert.Error(t, err)
+	assert.Equal(t, err.Error(), "rbd: ret=-39, Directory not empty")
+
+	err = rbd.GetError(345) // no such errno
+	assert.Error(t, err)
+	assert.Equal(t, err.Error(), "rbd: ret=345")
+}
+
 func TestVersion(t *testing.T) {
 	var major, minor, patch = rbd.Version()
 	assert.False(t, major < 0 || major > 1000, "invalid major")