Add test for sqliteconn.Raw()

Note that you can't call cgo from within a test file; so in order to
test the functionality, create new package, .../internal/sqlite3test,
to define the callback.  Then in the main sqlite3 package, add a test
file which includes this package and calls the callback.

As with the previous commit, the idea and the basic code was written
by @rittneje; I massaged it so that it compiled and passed with Go
1.9, and wrote this commit message and the comments.

NB that Golang 1.9 doesn't have sql.OpenDB, so we have to register a
new database driver.

Signed-off-by: George Dunlap <dunlapg@umich.edu>

v4:
- Refactor to get rid of sql.OpenDB call, which only appeared in Go
  1.10

v3:
- New

internal/sqlite3test/raw.go

@@ -0,0 +1,35 @@
+package sqlite3test
+
+/*
+#cgo CFLAGS: -DUSE_LIBSQLITE3
+#cgo linux LDFLAGS: -lsqlite3
+#cgo darwin LDFLAGS: -L/usr/local/opt/sqlite/lib -lsqlite3
+#cgo darwin CFLAGS: -I/usr/local/opt/sqlite/include
+#cgo openbsd LDFLAGS: -lsqlite3
+#cgo solaris LDFLAGS: -lsqlite3
+#include <stdlib.h>
+#include <sqlite3.h>
+
+static void one(sqlite3_context* ctx, int n, sqlite3_value** vals) {
+    sqlite3_result_int(ctx, 1);
+}
+
+static inline int _create_function(sqlite3* c) {
+    return sqlite3_create_function(c, "one", 0, SQLITE_UTF8|SQLITE_DETERMINISTIC, NULL, one, NULL, NULL);
+}
+*/
+import "C"
+import (
+	sqlite3 "github.com/mattn/go-sqlite3"
+	"unsafe"
+)
+
+func RegisterFunction(conn *sqlite3.SQLiteConn) error {
+	return conn.Raw(func(raw unsafe.Pointer) error {
+		rawConn := (*C.sqlite3)(raw)
+		if ret := C._create_function(rawConn); ret != C.SQLITE_OK {
+			return sqlite3.ErrNo(ret)
+		}
+		return nil
+	})
+}

raw_test.go

@@ -0,0 +1,37 @@
+package sqlite3_test
+
+import (
+	"database/sql"
+	"testing"
+
+	sqlite3 "github.com/mattn/go-sqlite3"
+	"github.com/mattn/go-sqlite3/internal/sqlite3test"
+)
+
+func TestRaw(t *testing.T) {
+	sql.Register("sqlite3_rawtest", &sqlite3.SQLiteDriver{
+		ConnectHook: func(c *sqlite3.SQLiteConn) error {
+			return sqlite3test.RegisterFunction(c)
+		},
+	})
+
+	db, err := sql.Open("sqlite3_rawtest", "...")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer db.Close()
+
+	if err := db.Ping(); err != nil {
+		t.Fatal(err)
+	}
+
+	var result int
+	if err := db.QueryRow(`SELECT one()`).Scan(&result); err != nil {
+		t.Fatal(err)
+	}
+
+	if result != 1 {
+		t.Errorf("expected custom one() function to return 1, but got %d", result)
+	}
+}