Re: [Xen-devel] [PATCH v2 1/5] golang/xenlight: Create stub package

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 02/03/17 16:07, Ronald Rojas wrote:
> Create a basic Makefile to build and install libxenlight Golang
> bindings. Also add a stub package which only opens libxl context.
> 
> Include a global xenlight.Ctx variable which can be used as the
> default context by the entire program if desired.
> 
> For now, return simple errors. Proper error handling will be
> added in next patch.
> 
> Signed-off-by: Ronald Rojas <ronladred@xxxxxxxxx>
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>

Getting close!  A few more changes.

> ---
> 
> Changes:
> - Changed GPL Lisense to LGPL Lisense
> 
> - Initialized xentoollog_logger for storing error messages
> 
> - Moved manual-enable config option to tools/Rules.mk, use
>   CONFIG_GOLANG in tools/Makefile
> 
> - Added XEN_GOPATH, pointing to tools/golang
> 
> - Modified tools/golang/xenlight makefile to construct necessary $GOPATH
> 
> - Added tools/golang/Makefile, so we don't need special rules in tools
>   to make tools/golang/xenlight; and so we have a single place to remove the
>   $GOPATH build side-effects ($GOPATH/src and $GOPATH/pkg)
> 
> - Build of tools/golang/xenlight sets $GOPATH and does a 'go install'
> 
> - Use tree-provided CFLAGS_libxenlight and LDLIBS_libxenlight, rather
>   than hard-coding our own
> 
> - Made a PKGSOURCES variable to track dependencies of all target files
>   which need to be part of the output package (i.e., what's put in
>   $GOPATH/src).  At the moment this is one file, but it will probably
>   be more once we start using the IDL.

Good summary of changes, thanks.

> 
> CC: xen-devel@xxxxxxxxxxxxx
> CC: george.dunlap@xxxxxxxxxx
> CC: ian.jackson@xxxxxxxxxxxxx
> CC: wei.liu2@xxxxxxxxxx
> ---
> ---
>  tools/Makefile                    |  1 +
>  tools/Rules.mk                    |  6 +++
>  tools/golang/Makefile             | 27 +++++++++++++
>  tools/golang/xenlight/Makefile    | 49 ++++++++++++++++++++++
>  tools/golang/xenlight/xenlight.go | 85 
> +++++++++++++++++++++++++++++++++++++++
>  5 files changed, 168 insertions(+)
>  create mode 100644 tools/golang/Makefile
>  create mode 100644 tools/golang/xenlight/Makefile
>  create mode 100644 tools/golang/xenlight/xenlight.go
> 
> diff --git a/tools/Makefile b/tools/Makefile
> index 77e0723..df1fda1 100644
> --- a/tools/Makefile
> +++ b/tools/Makefile
> @@ -31,6 +31,7 @@ endif
>  
>  SUBDIRS-y += xenpmd
>  SUBDIRS-y += libxl
> +SUBDIRS-$(CONFIG_GOLANG) += golang
>  SUBDIRS-y += helpers
>  SUBDIRS-$(CONFIG_X86) += xenpaging
>  SUBDIRS-$(CONFIG_X86) += debugger/gdbsx
> diff --git a/tools/Rules.mk b/tools/Rules.mk
> index b35999b..24e5220 100644
> --- a/tools/Rules.mk
> +++ b/tools/Rules.mk
> @@ -30,6 +30,12 @@ XENSTORE_XENSTORED ?= y
>  debug ?= y
>  debug_symbols ?= $(debug)
>  
> +# Uncomment to compile with Go
> +CONFIG_GOLANG ?= y

You forgot to comment this out again before submitting it. :-)  (This is
something that can be fixed up on check-in if necessary.)

> +ifeq ($(CONFIG_GOLANG),y)
> +XEN_GOPATH        = $(XEN_ROOT)/tools/golang
> +endif
> +
>  ifeq ($(debug_symbols),y)
>  CFLAGS += -g3
>  endif
> diff --git a/tools/golang/Makefile b/tools/golang/Makefile
> new file mode 100644
> index 0000000..47a9235
> --- /dev/null
> +++ b/tools/golang/Makefile
> @@ -0,0 +1,27 @@
> +XEN_ROOT=$(CURDIR)/../..
> +include $(XEN_ROOT)/tools/Rules.mk
> +
> +# In order to link against a package in Go, the package must live in a
> +# directory tree in the way that Go expects.  To make this possible,
> +# there must be a directory such that we can set GOPATH=${dir}, and
> +# the package will be under $GOPATH/src/${full-package-path}.
> +
> +# So we set XEN_GOPATH to $XEN_ROOT/tools/golang.  The xenlight
> +# "package build" directory ($PWD/xenlight) will create the "package
> +# source" directory in the proper place.  Go programs can use this
> +# package by setting GOPATH=$(XEN_GOPATH).
> +
> +SUBDIRS-y = xenlight
> +
> +.PHONY: build all
> +all build: subdirs-all
> +
> +.PHONY: install
> +install: subdirs-install
> +
> +.PHONY: clean
> +clean: subdirs-clean
> +     $(RM) -r src pkg
> +
> +.PHONY: distclean
> +distclean: clean
> diff --git a/tools/golang/xenlight/Makefile b/tools/golang/xenlight/Makefile
> new file mode 100644
> index 0000000..5d578f3
> --- /dev/null
> +++ b/tools/golang/xenlight/Makefile
> @@ -0,0 +1,49 @@
> +XEN_ROOT=$(CURDIR)/../../..
> +include $(XEN_ROOT)/tools/Rules.mk
> +
> +# Standing boldly against convention, we insist on installing the
> +# package source under $(prefix)/share/gocode
> +GOCODE_DIR ?= $(prefix)/share/gocode/
> +GOXL_PKG_DIR = /src/xenproject.org/xenlight/

So I had a chat with Ian Jackson about the logistics of setting up a
repo such that "go get" would Just Work (TM).  What we agreed (if I
remember right) was to use the hostname "golang.xenproject.org" for
those mirrored repos, to make sure there was never any name collisions
between web pages at xenproject.org

So we should probably make a variable (maybe XEN_GOCODE_URL), set it to
"golang.xenproject.org", and then replace all instances of
"xenproject.org" in this makefile with $(XEN_GOCODE_URL).

> +GOXL_INSTALL_DIR = $(GOCODE_DIR)$(GOXL_PKG_DIR)
> +
> +# PKGSOURCES: Files which comprise the distributed source package
> +PKGSOURCES = xenlight.go
> +
> +GO ?= go
> +
> +.PHONY: all
> +all: build
> +
> +.PHONY: package
> +package: $(XEN_GOPATH)$(GOXL_PKG_DIR)$(PKGSOURCES)
> +
> +$(XEN_GOPATH)/src/xenproject.org/xenlight/$(PKGSOURCES): $(PKGSOURCES)
> +     $(INSTALL_DIR) $(XEN_GOPATH)$(GOXL_PKG_DIR)
> +     $(INSTALL_DATA) $(PKGSOURCES) $(XEN_GOPATH)$(GOXL_PKG_DIR)
> +
> +# Go will do its own dependency checking, and not actuall go through
> +# with the build if none of the input files have changed.
> +#
> +# NB that because the users of this library need to be able to
> +# recompile the library from source, it needs to include '-lxenlight'
> +# in the LDFLAGS; and thus we need to add -L$(XEN_XENLIGHT) here
> +# so that it can find the actual library.
> +.PHONY: build
> +build: package
> +     CGO_CFLAGS="$(CFLAGS_libxenlight)" CGO_LDFLAGS="$(LDLIBS_libxenlight) 
> -L$(XEN_XENLIGHT)" GOPATH=$(XEN_GOPATH) $(GO) install -x 
> xenproject.org/xenlight

Now that we -lxentoollog, we also need to add the appropriate CFLAGS,
LDLIBS, and library path: "$(CFLAGS_libxentoollog)" and
"$(LDLIBS_libxentoolllog) -L$(XEN_LIBXENTOOLLOG)" respectively.

> +
> +.PHONY: install
> +install: build
> +     $(INSTALL_DIR) $(DESTDIR)$(GOXL_INSTALL_DIR)
> +     $(INSTALL_DATA) $(XEN_GOPATH)$(GOXL_PKG_DIR)$(PKGSOURCES) 
> $(DESTDIR)$(GOXL_INSTALL_DIR)
> +
> +.PHONY: clean
> +clean:
> +     $(RM) -r $(XEN_GOPATH)$(GOXL_PKG_DIR)
> +     $(RM) $(XEN_GOPATH)/pkg/*/xenproject.org/xenlight.a
> +
> +.PHONY: distclean
> +distclean: clean
> +
> +-include $(DEPS)
> diff --git a/tools/golang/xenlight/xenlight.go 
> b/tools/golang/xenlight/xenlight.go
> new file mode 100644
> index 0000000..0a0cea2
> --- /dev/null
> +++ b/tools/golang/xenlight/xenlight.go
> @@ -0,0 +1,85 @@
> +/*
> + * Copyright (C) 2016 George W. Dunlap, Citrix Systems UK Ltd
> + *
> + * This library is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation;
> + * version 2.1 of the License.
> + *
> + * This library is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with this library; If not, see 
> <http://www.gnu.org/licenses/>.
> + */
> +package xenlight
> +
> +/*
> +#cgo LDFLAGS: -lxenlight -lyajl -lxentoollog
> +#include <stdlib.h>
> +#include <libxl.h>
> +*/
> +import "C"
> +
> +/*
> + * Other flags that may be needed at some point:
> + *  -lnl-route-3 -lnl-3
> + *
> + * To get back to static linking:
> + * #cgo LDFLAGS: -lxenlight -lyajl_s -lxengnttab -lxenstore -lxenguest 
> -lxentoollog -lxenevtchn -lxenctrl -lblktapctl -lxenforeignmemory -lxencall 
> -lz -luuid -lutil
> +*/
> +
> +import (
> +     "fmt"
> +     "unsafe"
> +)
> +
> +
> +/*
> + * Types: Builtins
> + */
> +type Context struct {
> +     ctx *C.libxl_ctx
> +}
> +
> +/*
> + * Context
> + */
> +var Ctx Context
> +
> +func (Ctx *Context) IsOpen() bool {
> +     return Ctx.ctx != nil
> +}
> +
> +func (Ctx *Context) Open() (err error) {
> +     if Ctx.ctx != nil {
> +             return
> +     }
> +
> +     logger := C.xtl_createlogger_stdiostream(C.stderr, C.XTL_ERROR, 0);

You don't check the return value of logger to make sure that it succeeds.

Also -- xtl_create_logger() will allocate a struct; and libxl_ctx_free()
will not free that struct; which means that repeated calls to
Context.Open() and Context.Close() will leak C memory.  (Yay for no
garbage collection!)

So you need to add the logger to the Context struct and call
C.xtl_logger_destroy(Ctx.logger) on it in Context.Close().

Also -- you forgot to re-run `go fmt`, and so there's a stray semicolon
at the end of the line. :-)

> +     ret := C.libxl_ctx_alloc(unsafe.Pointer(&Ctx.ctx), C.LIBXL_VERSION, 0, 
> unsafe.Pointer(logger))

Actually, I realized looking at your code that a lot of the
unsafe.Pointer() casts (which you inherited from the code I wrote
originally) are unnecessary.  We need to use it for logger because it's
doing C's version of polymorphism.  But for libxl_ctx_alloc, it's
expecting the first argument to be a pointer to C.libxl_ctx, so we don't
need any casts.  Same with libxl_ctx_free().

Finally, in other parts of the tree we limit line lenghth to 80
characters unless there's a good reason to do otherwise.  It seems like
in this case breaking it after the '0,' would be a good idea.

(It will be nice once this is checked in and I can just post a patch to
fix a lot of these types of issues, rather than asking you to do it.)

Thanks,
 -George


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