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e2f8db27d0
The current "arg_string_list" and "free_string_list" implementations go
back to commit b6f01f3260 ("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-error-cgo-argument-has-go-pointer-to-go-pointer
[3] https://listman.redhat.com/archives/libguestfs/2021-September/msg00106.html
Cc: "Daniel P. Berrangé" <berrange@redhat.com>
Cc: "Richard W.M. Jones" <rjones@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210921192939.32468-1-lersek@redhat.com>
Tested-by: "Richard W.M. Jones" <rjones@redhat.com>
Acked-by: "Richard W.M. Jones" <rjones@redhat.com>
This program generates a large amount of code and documentation for all the daemon actions. To add a new action there are only two files you need to change, 'actions_*.ml' to describe the interface, and daemon/<somefile>.c to write the implementation. After editing these files, build it (make -C generator) to regenerate all the output files. 'make' will rerun this automatically when necessary. IMPORTANT: This program should NOT print any warnings at compile time or run time. If it prints warnings, you should treat them as errors. OCaml tips: (1) In emacs, install tuareg-mode to display and format OCaml code correctly. 'vim' comes with a good OCaml editing mode by default. (2) Read the resources at http://ocaml.org/learn/ (3) A module called 'Foo' is defined in one or two files called 'foo.mli' and 'foo.ml' (NB: lowercase first letter). The *.mli file, if present, defines the public interface for the module. The *.ml file is the implementation. If the *.mli file is missing then everything is exported. Some notable files in this directory: actions_*.ml The libguestfs API. proc_nr.ml Procedure numbers associated with each API. structs.ml Structures returned by the API. c.ml Generate C API. <lang>.ml Generate bindings for <lang>. main.ml The main generator program. Note about long descriptions: When referring to another action, use the format C<guestfs_other> (ie. the full name of the C function). This will be replaced as appropriate in other language bindings. Apart from that, long descriptions are just perldoc paragraphs. Note about extending functions: In general you cannot change the name, number of required arguments or type of required arguments of a function, since this would break backwards compatibility. You may add another optional argument, *if* the function has >= 1 optional arguments already. Add it at the end of the list. You may add optional arguments to a function that doesn't have any. However you *must* set the once_had_no_optargs flag to true, so that the relevant backwards compatibility bindings can be added.