How to build a Postgres extension using cgo

Here is what I'm doing right now,

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.
├── helloworld--1.0.sql
├── helloworld.control
├── helloworld.go
└── Makefile

helloworld.go :

package helloworld

/*
#cgo LDFLAGS: -rdynamic
#include "postgres.h"
#include "fmgr.h"
#include "utils/builtins.h"

PG_MODULE_MAGIC;

PG_FUNCTION_INFO_V1(helloworld);
PG_FUNCTION_INFO_V1(hello_text_arg);
PG_FUNCTION_INFO_V1(hello_ereport);

Datum
hello_world(PG_FUNCTION_ARGS)
{
	PG_RETURN_TEXT_P(cstring_to_text("Hello, World!"));
}

Datum
hello_text_arg(PG_FUNCTION_ARGS)
{
	text *hello		= cstring_to_text("Hello, ");
	int32 hello_sz	= VARSIZE(hello) - VARHDRSZ;

	text *name		= PG_GETARG_TEXT_P(0);
	int32 name_sz	= VARSIZE(name) - VARHDRSZ;

	text *tail		= cstring_to_text("!");
	int32 tail_sz	= VARSIZE(tail) - VARHDRSZ;

	int32 out_sz	= hello_sz + name_sz + tail_sz + VARHDRSZ;
	text *out		= (text *) palloc(out_sz);

	SET_VARSIZE(out, out_sz);

	memcpy(VARDATA(out), VARDATA(hello), hello_sz);
	memcpy(VARDATA(out) + hello_sz, VARDATA(name), name_sz);
	memcpy(VARDATA(out) + hello_sz + name_sz, VARDATA(tail), tail_sz);

	PG_RETURN_TEXT_P(out);
}

Datum
hello_ereport(PG_FUNCTION_ARGS)
{
	ereport(ERROR, (errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED), errmsg("null value not allowed")));

	PG_RETURN_VOID();
}
*/
import "C"

Makefile :

MODULES = helloworld

EXTENSION = helloworld
DATA = helloworld--1.0.sql
PGFILEDESC = "helloworld - example extension for postgresql"

REGRESS = helloworld

PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
INCLUDEDIR = $(shell $(PG_CONFIG) --includedir-server)
include $(PGXS)

helloworld.so:
	CGO_CFLAGS="-rdynamic -I$(INCLUDEDIR)" CGO_LDFLAGS="-rdynamic $(LDFLAGS)" go build -v -buildmode=c-shared -o helloworld.so .

It produced these errors when making :

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/tmp/go-build019341122/github.com/amosbird/rpctest/helloworld/_obj/helloworld.cgo2.o: In function `hello_world':
helloworld.cgo2.c:(.text+0x48): undefined reference to `cstring_to_text'
/tmp/go-build019341122/github.com/amosbird/rpctest/helloworld/_obj/helloworld.cgo2.o: In function `hello_text_arg':
helloworld.cgo2.c:(.text+0x63): undefined reference to `cstring_to_text'
helloworld.cgo2.c:(.text+0x86): undefined reference to `pg_detoast_datum'
helloworld.cgo2.c:(.text+0xa5): undefined reference to `cstring_to_text'
helloworld.cgo2.c:(.text+0xd7): undefined reference to `palloc'
/tmp/go-build019341122/github.com/amosbird/rpctest/helloworld/_obj/helloworld.cgo2.o: In function `hello_ereport':
helloworld.cgo2.c:(.text+0x1a8): undefined reference to `errstart'
helloworld.cgo2.c:(.text+0x1bd): undefined reference to `errmsg'
helloworld.cgo2.c:(.text+0x1c9): undefined reference to `errcode'
helloworld.cgo2.c:(.text+0x1d7): undefined reference to `errfinish'
collect2: ld returned 1 exit status
make: *** [helloworld.so] Error 2

I have no idea if this would work. There are so many black magic happened here.

So the main question is, what is the correct Makefile that can be used to build a Postgres extension in cgo?

A specific question to these errors is, what can I do to defer those symbol resolution in cgo's linking process?

Ok, I spent a whole day and found a viable solution.

We need to have the extension's main source file start with:

package main  // make sure to use main package

/*
#cgo CFLAGS: -I/path/to/postgres/include/server
#cgo LDFLAGS: -Wl,-unresolved-symbols=ignore-all

Use go build -o myext.so -buildmode=c-shared myext.go to generate myext.so.

If some Go method is needed from the C side, we should add //export methodname above the method declaration. This will generate symbols without package name prefix. Then we can extern these symbols on the C side. Make sure the exported Go methods reside in packages other than main.

package test :

//export Merge
func Merge(cint C.int) C.int ...

package main :

extern int Merge(int);

import "./test"
var _ = test.Somevar  // dumb placeholder to fake use package test.