dim/server.go

package main

import (
	"crypto/rand"
	"database/sql"
	"encoding/json"
	"fmt"
	"log"
	"net/http"
)

const (
	LevelFatal = `FATAL`
	LevelError = `ERROR`
	LevelInfo  = `INFO`
)

type (
	// Server is the central handler of all incoming requests. It generates
	// the context to call handlers, which then process the requests.
	Server struct {
		db     *sql.DB
		routes map[string]Handler
		debug  bool
	}

	// Handler is a function receiving a Context to process a request.
	// It is expected to investigate the Request and fill the Result.
	Handler func(c *Context, req Request, resp *Response) error

	// Context is the pre filled global context every handler receives.
	// It contains a prepared transaction for usage and important details like
	// the user account.
	Context struct {
		id    string
		req   *http.Request
		w     http.ResponseWriter
		debug bool // print debug output to the console

		username string
		tx       *sql.Tx
	}

	// Request contains the method name and parameters requested by the client.
	Request struct {
		// Method is the name to route to the correct function.
		Method string `json:"method"`
		// Params is the list of parameters in the request. These can be
		// read directly or parsed using any of the parse methods.
		Params []json.RawMessage `json:"params"`
	}

	// Response can have messages and/or a result to return the to client.
	Response struct {
		ID       string              `json:"id"`
		Messages map[string][]string `json:"messages,omitempty"`
		Result   interface{}         `json:"result,omitempty"`
	}

	ident []byte
)

// NewServer creates a new server handler.
func NewServer(db *sql.DB, debug bool) (*Server, error) {
	if db == nil {
		return nil, fmt.Errorf("database connection is not set")
	}
	return &Server{
		db:     db,
		routes: map[string]Handler{},
		debug:  debug,
	}, nil
}

// Register takes a new handler which will be called when the name is called.
func (s *Server) Register(name string, handler Handler) {
	if _, found := s.routes[name]; found {
		log.Fatalf("route with name %s already exists", name)
	}
	s.routes[name] = handler
}

// Handle implements http.HandleFunc to serve content for the standard http
// interface.
func (s *Server) Handle(w http.ResponseWriter, r *http.Request) {
	if r.Method != "POST" {
		w.WriteHeader(http.StatusMethodNotAllowed)
		w.Write([]byte("only POST requests allowed"))
		return
	}
	id, err := newIdent()
	if err != nil {
		log.Printf("could not generate request id: %s", err)
		w.WriteHeader(http.StatusInternalServerError)
		w.Write([]byte("sorry, can't currently process your request"))
		return
	}
	c := &Context{
		id:       id,
		req:      r,
		w:        w,
		debug:    s.debug,
		username: "unknown",
	}

	req := Request{}
	res := &Response{
		ID:       c.id,
		Messages: map[string][]string{},
		Result:   map[string]interface{}{},
	}

	tx, err := s.db.Begin()
	if err != nil {
		c.Logf(LevelError, "could not create transaction: %s", err)
		w.WriteHeader(http.StatusInternalServerError)
		res.AddMessage(LevelFatal, "database problems occured")
		c.render(res)
		return
	}
	defer tx.Rollback() // make a rollback, just in case something goes wrong
	c.tx = tx

	// set username for transaction
	// TODO check username to be ASCII and nothing else
	_, err = c.tx.Exec(
		fmt.Sprintf(`set local dim.username to '%s'; set local dim.transaction = '%s'`, c.username, c.id),
	)
	if err != nil {
		c.Logf(LevelError, "could not set transaction username: %s", err)
		w.WriteHeader(http.StatusInternalServerError)
		res.AddMessage(LevelFatal, "could not create transaction")
		c.render(res)
		return
	}

	dec := json.NewDecoder(r.Body)
	defer r.Body.Close()
	if err := dec.Decode(&req); err != nil {
		res.AddMessage(LevelError, fmt.Sprintf("could not parse payload: %s", err))
		c.w.WriteHeader(http.StatusBadRequest)
		c.render(res)
		return
	}

	handler, found := s.routes[req.Method]
	if !found {
		res.AddMessage(LevelError, "method %s does not exist", req.Method)
		c.w.WriteHeader(http.StatusNotFound)
		c.render(res)
		return
	}
	c.Logf(LevelInfo, "method '%s' called with '%s'", req.Method, req.Params)
	if err := handler(c, req, res); err != nil {
		c.w.WriteHeader(http.StatusInternalServerError)
		c.Logf(LevelError, "method '%s' returned an error: %s", req.Method, err)
	}
	if err := tx.Commit(); err != nil {
		c.Logf(LevelFatal, "could not commit changes: %s", err)
		res.AddMessage(LevelError, "changes were not committed")
	}
	c.render(res)
}

// Render converts the Result to json and sends it back to the client.
func (c *Context) render(res *Response) {
	enc := json.NewEncoder(c.w)
	if err := enc.Encode(res); err != nil {
		c.Logf(LevelError, "%s - could not encode result: %s\n%s", c.id, err, res)
		return
	}
}

// Logf logs a message to stdout, outfitted with the request ID.
func (c *Context) Logf(level, msg string, args ...interface{}) {
	log.Printf("%s - %s - %s", c.id, level, fmt.Sprintf(msg, args...))
}

// Debugf logs output only when the server is set into debug mode.
func (c *Context) Debugf(level, msg string, args ...interface{}) {
	if c.debug {
		log.Printf("%s - %s - %s", c.id, level, fmt.Sprintf(msg, args...))
	}
}

// Generate a useable request ID, so that it can be found in the logs.
func newIdent() (string, error) {
	b := make([]byte, 16)
	_, err := rand.Read(b)
	if err != nil {
		return "", err
	}

	return fmt.Sprintf("%X-%X-%X-%X-%X", b[0:4], b[4:6], b[6:8], b[8:10], b[10:]), nil
}

// AddMessage adds a new message to the result.
// When Render is called, these messages will be sent to the client.
func (r *Response) AddMessage(level string, msg string, args ...interface{}) {
	r.Messages[level] = append(r.Messages[level], fmt.Sprintf(msg, args...))
}

// Len returns the length of the parameter list.
func (r Request) Len() int {
	return len(r.Params)
}

// Parse will parse the exact number of parameters into arguments.
// The boundaries are checked before.
func (r Request) Parse(args ...interface{}) error {
	num := len(args)
	if r.Len() != num {
		return fmt.Errorf("expected %d parameters, got %d", num, r.Len())
	}
	return r.ParseAtLeast(num, args...)
}

// ParseAtLeast parses at least *num* many parameters into arguments.
// This function checks the boundaries before beginning to parse and returns
// errors if any boundary does not match.
func (r Request) ParseAtLeast(num int, args ...interface{}) error {
	if len(args) < num {
		return fmt.Errorf("requested %d arguments to parse, but only %d arguments given", num, len(args))
	}
	if r.Len() < num {
		return fmt.Errorf("need %d parameters, but only got %d", num, r.Len())
	}
	if r.Len() > len(args) {
		return fmt.Errorf("found %d parameters, when only %d are required", r.Len(), len(args))
	}
	for i, param := range r.Params {
		if err := json.Unmarshal(param, args[i]); err != nil {
			return fmt.Errorf("argument at position %d can't be parsed: %v", i, err)
		}
	}
	return nil
}

// ParseAt unmarshalls the argument at *pos* into the container.
// If the position is outside the size of incoming arguments an error is raised.
func (r Request) ParseAt(pos int, container interface{}) error {
	if pos >= r.Len() {
		return fmt.Errorf("out of bounds") // TODO make generic ErrOutOfBounds
	}
	if err := json.Unmarshal(r.Params[pos], container); err != nil {
		return fmt.Errorf("could not unmarshal parameter '%d': %s", pos, err)
	}
	return nil
}