reports: Add basic lua infrastructure

Of course, it doesn't do anything other than print "Hello World", but
that's okay.

main.go

@@ -74,6 +74,7 @@ func main() {
 	servemux.HandleFunc("/account/", AccountHandler)
 	servemux.HandleFunc("/transaction/", TransactionHandler)
 	servemux.HandleFunc("/import/gnucash", GnucashImportHandler)
+	servemux.HandleFunc("/report/", ReportHandler)
 
 	listener, err := net.Listen("tcp", ":"+strconv.Itoa(port))
 	if err != nil {

reports.go

@@ -0,0 +1,59 @@
+package main
+
+import (
+	"context"
+	"github.com/yuin/gopher-lua"
+	"net/http"
+	"time"
+)
+
+//type and value to store user in lua's Context
+type key int
+
+const userContextKey key = 0
+
+const luaTimeoutSeconds uint = 5 // maximum time a lua request can run for
+
+func ReportHandler(w http.ResponseWriter, r *http.Request) {
+	user, err := GetUserFromSession(r)
+	if err != nil {
+		WriteError(w, 1 /*Not Signed In*/)
+		return
+	}
+
+	if r.Method == "GET" {
+		// Create a new LState without opening the default libs for security
+		L := lua.NewState(lua.Options{SkipOpenLibs: true})
+		defer L.Close()
+
+		// Create a new context holding the current user with a timeout
+		ctx := context.WithValue(context.Background(), userContextKey, user)
+		ctx, cancel := context.WithTimeout(ctx, luaTimeoutSeconds*time.Second)
+		defer cancel()
+		L.SetContext(ctx)
+
+		for _, pair := range []struct {
+			n string
+			f lua.LGFunction
+		}{
+			{lua.LoadLibName, lua.OpenPackage}, // Must be first
+			{lua.BaseLibName, lua.OpenBase},
+			{lua.TabLibName, lua.OpenTable},
+			{lua.StringLibName, lua.OpenString},
+			{lua.MathLibName, lua.OpenMath},
+		} {
+			if err := L.CallByParam(lua.P{
+				Fn:      L.NewFunction(pair.f),
+				NRet:    0,
+				Protect: true,
+			}, lua.LString(pair.n)); err != nil {
+				panic(err)
+			}
+		}
+		err := L.DoString(`print("Hello World")`)
+
+		if err != nil {
+			panic(err)
+		}
+	}
+}