The current "arg_string_list" and "free_string_list" implementations
go
back to commit b6f01f32606d ("Add Go (language) bindings.", 2013-07-03).
There are two problems with them:
- "free_string_list" doesn't actually free anything,
- at the *first* such g.Internal_test() call site that passes an
Ostringlist member inside the Optargs argument, namely:
g.Internal_test ("abc",
string_addr ("def"),
[]string{},
false,
0,
0,
"123",
"456",
[]byte{'a', 'b', 'c', '\000',
'a', 'b', 'c'},
&guestfs.OptargsInternal_test{Ostringlist_is_set: true,
Ostringlist: []string{}
}
)
the "golang/run-bindtests" case crashes:
panic: runtime error: cgo argument has Go pointer to Go pointer
goroutine 1 [running]:
libguestfs.org/guestfs.(*Guestfs).Internal_test.func7(0xc000018180,
0xadfb60, 0xadfb80, 0xc000010048, 0x0, 0x0, 0x0, 0xae3e10, 0xae3e30,
0xade3a0, ...)
golang/src/libguestfs.org/guestfs/guestfs.go:6729 +0xa9
libguestfs.org/guestfs.(*Guestfs).Internal_test(0xc000018180, 0x4ee3a5,
0x3, 0xc000061be8, 0xc000061af8, 0x0, 0x0, 0xc000061a00, 0x0, 0x0, ...)
golang/src/libguestfs.org/guestfs/guestfs.go:6729 +0x3c9
main.main()
golang/bindtests/bindtests.go:77 +0x151e
exit status 2
FAIL run-bindtests (exit status: 1)
In Daniel P. Berrangé's words [1],
You're allowed to pass a Go pointer to C via CGo, but the memory
that
points to is not allowed to contained further Go pointers. So the struct
fields must strictly use a C pointer.
One pattern to solve the problem has been shown on stackoverflow [2].
Thus, rewrite the "arg_string_list" and "free_string_list" functions
almost entirely in C, following that example.
While this approach is not the most idiomatic Go, as a solution exists
without C helper functions [3], it should still be acceptable, at least as
an incremental improvement, for letting "golang/run-bindtests" pass.
[1]
https://listman.redhat.com/archives/libguestfs/2021-September/msg00118.html
[2]
https://stackoverflow.com/questions/35924545/golang-cgo-panic-runtime-err...
[3]
https://listman.redhat.com/archives/libguestfs/2021-September/msg00106.html
Cc: "Daniel P. Berrangé" <berrange(a)redhat.com>
Cc: "Richard W.M. Jones" <rjones(a)redhat.com>
Signed-off-by: Laszlo Ersek <lersek(a)redhat.com>
---
Notes:
v2:
- update commit message [Daniel, Rich]
- update code comment [Daniel, Rich]
- no code changes
- retested
generator/golang.ml | 47 ++++++++++++++++++++++++++++++++++++---------
1 file changed, 38 insertions(+), 9 deletions(-)
diff --git a/generator/golang.ml b/generator/golang.ml
index d328abe4ed08..0d6a92367f9b 100644
--- a/generator/golang.ml
+++ b/generator/golang.ml
@@ -52,6 +52,40 @@ _go_guestfs_create_flags (unsigned flags)
{
return guestfs_create_flags (flags);
}
+
+// Passing a Go pointer to C via CGo is allowed, but the memory that points to
+// is not allowed to contain further Go pointers. The helper functions below
+// are one way to implement this, although the same can be achieved purely in
+// Go as well. See the discussion here:
+// <
https://listman.redhat.com/archives/libguestfs/2021-September/msg00101.ht...;.
+typedef char *pChar;
+
+pChar *allocStringArray (size_t nmemb)
+{
+ pChar *array;
+
+ array = malloc (sizeof (pChar) * (nmemb + 1));
+ array[nmemb] = NULL;
+ return array;
+}
+
+void storeString (pChar *array, size_t idx, pChar string)
+{
+ array[idx] = string;
+}
+
+void freeStringArray (pChar *array)
+{
+ pChar *position;
+ pChar string;
+
+ position = array;
+ while ((string = *position) != NULL) {
+ free (string);
+ position++;
+ }
+ free (array);
+}
*/
import \"C\"
@@ -148,12 +182,11 @@ func (g *Guestfs) Close () *GuestfsError {
* C strings and golang []string.
*/
func arg_string_list (xs []string) **C.char {
- r := make ([]*C.char, 1 + len (xs))
+ r := C.allocStringArray (C.size_t (len (xs)))
for i, x := range xs {
- r[i] = C.CString (x)
+ C.storeString (r, C.size_t (i), C.CString (x));
}
- r[len (xs)] = nil
- return &r[0]
+ return (**C.char) (r)
}
func count_string_list (argv **C.char) int {
@@ -167,11 +200,7 @@ func count_string_list (argv **C.char) int {
}
func free_string_list (argv **C.char) {
- for *argv != nil {
- //C.free (*argv)
- argv = (**C.char) (unsafe.Pointer (uintptr (unsafe.Pointer (argv)) +
- unsafe.Sizeof (*argv)))
- }
+ C.freeStringArray ((*C.pChar) (argv))
}
func return_string_list (argv **C.char) []string {
--
2.19.1.3.g30247aa5d201