[Libguestfs] [PATCH 4/4] Go bindings: fix "C array of strings" -- char** -- allocation

Richard W.M. Jones
2021-09-20T10:36:59Z

On Mon, Sep 20, 2021 at 07:23:35AM +0200, Laszlo Ersek
wrote:
> 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)
>
> It turns out that a C-language array containing C-language pointers can
> only be allocated in C:
>
>
https://stackoverflow.com/questions/35924545/golang-cgo-panic-runtime-error-cgo-argument-has-go-pointer-to-go-pointer
>
> Rewrite the "arg_string_list" and "free_string_list"
functions almost
> entirely in C.
>
> Signed-off-by: Laszlo Ersek <lersek at redhat.com>
> ---
>  generator/golang.ml | 46 ++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 37 insertions(+), 9 deletions(-)
>
> diff --git a/generator/golang.ml b/generator/golang.ml
> index d328abe4ed08..5db478e8a494 100644
> --- a/generator/golang.ml
> +++ b/generator/golang.ml
> @@ -52,6 +52,39 @@ _go_guestfs_create_flags (unsigned flags)
>  {
>      return guestfs_create_flags (flags);
>  }
> +
> +//
> +// A C-language array of C-language pointers needs to be allocated in C.
> +//
https://stackoverflow.com/questions/35924545/golang-cgo-panic-runtime-error-cgo-argument-has-go-pointer-to-go-pointer
> +//
> +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 +181,11 @@ 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 +199,7 @@ 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 {

I had to do something similar to this in libnbd recently.

Thanks, ACK series.

Rich.

--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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