HTTP API: a chat's recent messages, and sending a message (closes #5)

`GET /api/v1/chats/{id}/messages?count=N` returns a chat's recent messages, oldest first (`count` 1 to 100, default 20), and `POST` to the same path sends a text message and returns it once the chat client has taken it. Both answer `404` for any id the chats list does not show, looked up in the same contact list. The message record is defined once, for the webhooks to reuse.

Disclosures: items are read five at a time, because one item can be many times its text and 100 at once could pass the 16 MiB read limit; text too long gets `413`, a contact who deleted the chat `409` (unverified for one still connecting); a query the server cannot read gets its own `400`.

Model: opus-5-5

internal/simplex/client.go

@@ -14,6 +14,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -21,15 +22,33 @@ import (
 	"github.com/gorilla/websocket"
 )
 
-// maxMessageSize bounds one message from the chat client. The largest
-// thing it sends is a record carrying a contact's profile picture, well
-// under this.
+// maxMessageSize bounds one message from the chat client; a larger one
+// ends the connection. The largest it sends are the pages of chat items
+// ChatItems asks for, which chatItemsPage keeps under this.
 const maxMessageSize = 16 << 20
 
+// chatItemsPage is how many chat items ChatItems asks for at a time. An
+// item repeats a message's text and spells out its formatting, which for
+// text made of short mentions makes it 26 times as long as the text, and
+// a contact's message can hold 64 KiB: one item can take 1.7 MiB. Five
+// stay well under maxMessageSize.
+const chatItemsPage = 5
+
 var (
 	// ErrClosed is returned by Command once the connection has ended.
 	ErrClosed = errors.New("connection to the chat client closed")
 
+	// ErrNoContact is returned for a contact the user does not have.
+	ErrNoContact = errors.New("no such contact")
+
+	// ErrContactNotReady is returned for sending to a contact who cannot
+	// receive messages: one who has deleted their chat with the user, or
+	// who has not finished connecting.
+	ErrContactNotReady = errors.New("the contact cannot receive messages")
+
+	// ErrMessageTooLarge is returned for a message too large to send.
+	ErrMessageTooLarge = errors.New("the message is too large")
+
 	errUnexpected = errors.New("unexpected response")
 	errCommand    = errors.New("command failed")
 )
@@ -180,6 +199,43 @@ func (c *Client) Contacts(ctx context.Context, userID int64) ([]Contact, error)
 	return r.Contacts, err
 }
 
+// ChatItems returns the last count items of the chat with a contact,
+// oldest first, or all of them if the chat has fewer.
+func (c *Client) ChatItems(
+	ctx context.Context, contactID int64, count int,
+) ([]ChatItem, error) {
+	var (
+		items  []ChatItem
+		before int64 // 0 asks for the chat's last items
+	)
+
+	for len(items) < count {
+		page := min(count-len(items), chatItemsPage)
+
+		//nolint:tagliatelle // the chat client's wire format.
+		var r struct {
+			Chat struct {
+				ChatItems []ChatItem `json:"chatItems"`
+			} `json:"chat"`
+		}
+
+		err := c.command(ctx, cmdGetChat(contactID, before, page), TypeAPIChat, &r)
+		if err != nil {
+			return nil, err
+		}
+
+		items = slices.Concat(r.Chat.ChatItems, items)
+
+		if len(r.Chat.ChatItems) < page {
+			break
+		}
+
+		before = items[0].Meta.ItemID
+	}
+
+	return items, nil
+}
+
 // SendText sends a text message to a contact, as a reply to the message
 // quotedItemID (0 for none). It does not wait for the chat client to
 // accept it; a failure is logged when the client's answer arrives.
@@ -196,6 +252,32 @@ func (c *Client) SendText(contactID, quotedItemID int64, text string) error {
 	return c.write(id, cmd)
 }
 
+// SendMessage sends a text message to a contact and returns it as the
+// chat client recorded it. Unlike SendText, it waits for the chat
+// client's answer, so an EventHandler must never call it.
+func (c *Client) SendMessage(
+	ctx context.Context, contactID int64, text string,
+) (ChatItem, error) {
+	cmd, err := cmdSendText(contactID, 0, text)
+	if err != nil {
+		return ChatItem{}, err
+	}
+
+	var r NewChatItems
+
+	err = c.command(ctx, cmd, TypeNewChatItems, &r)
+	if err != nil {
+		return ChatItem{}, err
+	}
+
+	if len(r.ChatItems) != 1 {
+		return ChatItem{}, fmt.Errorf("%w to %q: %d chat items",
+			errUnexpected, cmdName(cmd), len(r.ChatItems))
+	}
+
+	return r.ChatItems[0].ChatItem, nil
+}
+
 // CommandError is a command the chat client refused. Type and Detail
 // are the discriminators of its chatError record, such as "errorStore"
 // and "userContactLinkNotFound".
@@ -341,6 +423,8 @@ func (c *Client) dispatch(data []byte) {
 	}
 }
 
+// commandError returns the error in a chatCmdError record, marked with
+// this package's error for the refusals that have one.
 func commandError(ev Event) error {
 	var r cmdError
 
@@ -359,7 +443,16 @@ func commandError(ev Event) error {
 		}
 	}
 
-	return e
+	switch e.Detail {
+	case "contactNotFound":
+		return fmt.Errorf("%w: %w", ErrNoContact, e)
+	case "contactNotReady":
+		return fmt.Errorf("%w: %w", ErrContactNotReady, e)
+	case "largeMsg":
+		return fmt.Errorf("%w: %w", ErrMessageTooLarge, e)
+	default:
+		return e
+	}
 }
 
 // cmdName is a command without its arguments, for error messages: the