README.md

@@ -376,9 +376,10 @@ balancer, and CDN handles it correctly.
 - DMs are **fan-out to both sender and recipient** — the sender sees their own
   DM echoed back in their message queue, enabling multi-client consistency (your
   laptop sees DMs you sent from your phone).
-- DM history is stored in the `messages` table with the recipient nick as the
+- DM history is stored in the `messages` table with the recipient's session as
-  `msg_to` field. This means DM history is queryable per-nick, but if a user
+  the `to_session_id` field. A user's DM history therefore follows them across
-  changes their nick, old DMs are associated with the old nick.
+  nick changes, and a user who later takes a released nick sees none of the DMs
+  sent to its previous holder.
 - DMs are **not stored long-term** by default — they follow the same rotation
   policy as channel messages.
 
@@ -1839,18 +1840,21 @@ Fetch historical messages for a channel. Returns messages in chronological order
 
 **Query Parameters:**
 
-| Param    | Type    | Default    | Description                                                                                      |
+| Param    | Type    | Default    | Description                                                                                     |
-| -------- | ------- | ---------- | ------------------------------------------------------------------------------------------------ |
+| -------- | ------- | ---------- | ----------------------------------------------------------------------------------------------- |
-| `target` | string  | (required) | Channel name (e.g., `#general`)                                                                  |
+| `target` | string  | (required) | Channel name (e.g., `#general`)                                                                 |
-| `before` | string  | (none)     | The `id` of a message sent to `target`; return only messages older than it. Omit for the latest. |
+| `before` | string  | (none)     | The `id` of a message in this history; return only messages older than it. Omit for the latest. |
-| `limit`  | integer | `50`       | Maximum messages to return, 1–500; any other value gives 50.                                     |
+| `limit`  | integer | `50`       | Maximum messages to return, 1–500; any other value gives 50.                                    |
 
 To page back, pass the `id` of the first (oldest) message of a page as `before`
 for the next; an empty page means there is nothing older.
 
-`target` may also be your own nick, which returns the DMs sent to you. Errors:
+`target` may also be your own nick, which returns the DMs sent to your session,
-`400` `target required`, `400` `unknown before id` when no message with that
+including those sent under its earlier nicks. A channel created again after its
-`id` was sent to `target`, `404` `channel not found`, `403`
+last member left starts with no history, and a user who takes a released nick
+gets none of the DMs sent to its previous holder. Errors: `400`
+`target required`, `400` `unknown before id` when no message with that `id` is
+in this history, `404` `channel not found`, `403`
 `not a member of this channel`, and `403` `forbidden` for any other nick.
 
 **Response:** `200 OK`
@@ -2508,19 +2512,25 @@ Unique constraint on `(channel_id, session_id)`.
 
 #### `messages`
 
-| Column       | Type     | Description                                                     |
+| Column          | Type     | Description                                                                         |
-| ------------ | -------- | --------------------------------------------------------------- |
+| --------------- | -------- | ----------------------------------------------------------------------------------- |
-| `id`         | INTEGER  | Primary key (auto-increment). Internal ID for queue references. |
+| `id`            | INTEGER  | Primary key (auto-increment). Internal ID for queue references.                     |
-| `uuid`       | TEXT     | UUID v4, exposed to clients as the message `id`                 |
+| `uuid`          | TEXT     | UUID v4, exposed to clients as the message `id`                                     |
-| `command`    | TEXT     | IRC command (`PRIVMSG`, `JOIN`, etc.)                           |
+| `command`       | TEXT     | IRC command (`PRIVMSG`, `JOIN`, etc.)                                               |
-| `msg_from`   | TEXT     | Sender nick                                                     |
+| `msg_from`      | TEXT     | Sender nick                                                                         |
-| `msg_to`     | TEXT     | Target (`#channel` or nick)                                     |
+| `msg_to`        | TEXT     | Target (`#channel` or nick)                                                         |
-| `params`     | TEXT     | JSON-encoded IRC-style positional parameters                    |
+| `to_session_id` | INTEGER  | FK → sessions.id (set to null on delete): the session a DM or `INVITE` was sent to  |
-| `body`       | TEXT     | JSON-encoded body (array or object)                             |
+| `to_channel_id` | INTEGER  | FK → channels.id (set to null on delete): the channel a channel message was sent to |
-| `meta`       | TEXT     | JSON-encoded metadata                                           |
+| `params`        | TEXT     | JSON-encoded IRC-style positional parameters                                        |
-| `created_at` | DATETIME | Server timestamp                                                |
+| `body`          | TEXT     | JSON-encoded body (array or object)                                                 |
+| `meta`          | TEXT     | JSON-encoded metadata                                                               |
+| `created_at`    | DATETIME | Server timestamp                                                                    |
 
-Indexes on `(msg_to, id)` and `(created_at)`.
+Indexes on `(to_session_id, id)`, `(to_channel_id, id)` and `(created_at)`.
+`to_session_id` and `to_channel_id` are null on a message sent to neither, such
+as `QUIT`. `GET /api/v1/history` selects by one of these two columns rather than
+by `msg_to`, because nicks and channel names are reused once released, and
+session and channel ids are not.
 
 #### `spent_hashcash`
 

internal/db/errors.go

@@ -17,3 +17,14 @@ func IsUniqueConstraintError(err error) bool {
 
 	return sqliteErr.Code() == sqlite3.SQLITE_CONSTRAINT_UNIQUE
 }
+
+// IsForeignKeyConstraintError reports whether err is a
+// SQLite foreign-key constraint violation.
+func IsForeignKeyConstraintError(err error) bool {
+	var sqliteErr *sqlite.Error
+	if !errors.As(err, &sqliteErr) {
+		return false
+	}
+
+	return sqliteErr.Code() == sqlite3.SQLITE_CONSTRAINT_FOREIGNKEY
+}

internal/db/queries.go

@@ -9,6 +9,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"math"
 	"strconv"
 	"strings"
 	"time"
@@ -940,9 +941,14 @@ func (database *Database) GetSessionChannelIDs(
 }
 
 // InsertMessage stores a message and returns its DB ID.
+// toSessionID is the session a DM or INVITE is sent to and
+// toChannelID the channel a channel message is sent to;
+// GetHistory selects by them. Pass 0 for one that does not
+// apply, which stores NULL.
 func (database *Database) InsertMessage(
 	ctx context.Context,
 	command, from, target string,
+	toSessionID, toChannelID int64,
 	params json.RawMessage,
 	body json.RawMessage,
 	meta json.RawMessage,
@@ -965,9 +971,12 @@ func (database *Database) InsertMessage(
 	res, err := database.conn.ExecContext(ctx,
 		`INSERT INTO messages
 		 (uuid, command, msg_from, msg_to,
+		  to_session_id, to_channel_id,
 		  params, body, meta, created_at)
-		 VALUES (?, ?, ?, ?, ?, ?, ?, ?)`,
+		 VALUES (?, ?, ?, ?, NULLIF(?, 0), NULLIF(?, 0),
+		  ?, ?, ?, ?)`,
 		msgUUID, command, from, target,
+		toSessionID, toChannelID,
 		string(params), string(body), string(meta), now)
 	if err != nil {
 		return 0, "", fmt.Errorf(
@@ -1058,26 +1067,31 @@ func (database *Database) PollMessages(
 	return msgs, lastQID, nil
 }
 
-// GetHistory returns message history for a target. A
+// GetHistory returns the PRIVMSG history of one recipient:
-// non-empty before is the id of a message sent to target,
+// the DMs sent to session toSessionID, or the messages sent
-// and only messages older than it are returned; an id with
+// to channel toChannelID. Pass 0 for the other. A non-empty
-// no such message gives an error wrapping sql.ErrNoRows.
+// before is the id of a PRIVMSG in that history, and only
+// messages older than it are returned; an id with no such
+// message gives an error wrapping sql.ErrNoRows.
 func (database *Database) GetHistory(
 	ctx context.Context,
-	target, before string,
+	toSessionID, toChannelID int64,
+	before string,
 	limit int,
 ) ([]IRCMessage, error) {
 	if limit <= 0 {
 		limit = defaultHistLimit
 	}
 
-	var beforeID int64
+	// With no before, every message is older than this.
+	beforeID := int64(math.MaxInt64)
 
 	if before != "" {
 		err := database.conn.QueryRowContext(ctx,
 			`SELECT id FROM messages
-			 WHERE uuid = ? AND msg_to = ?`,
+			 WHERE uuid = ? AND command = 'PRIVMSG'
-			before, target,
+			   AND (to_session_id = ? OR to_channel_id = ?)`,
+			before, toSessionID, toChannelID,
 		).Scan(&beforeID)
 		if err != nil {
 			return nil, fmt.Errorf(
@@ -1087,7 +1101,7 @@ func (database *Database) GetHistory(
 	}
 
 	rows, err := database.queryHistory(
-		ctx, target, beforeID, limit,
+		ctx, toSessionID, toChannelID, beforeID, limit,
 	)
 	if err != nil {
 		return nil, err
@@ -1107,21 +1121,23 @@ func (database *Database) GetHistory(
 	return msgs, nil
 }
 
+// queryHistory selects by to_session_id when toSessionID is
+// set and by to_channel_id otherwise, so that the column's
+// (column, id) index serves the order and the limit.
 func (database *Database) queryHistory(
 	ctx context.Context,
-	target string,
+	toSessionID, toChannelID, beforeID int64,
-	beforeID int64,
 	limit int,
 ) (*sql.Rows, error) {
-	if beforeID > 0 {
+	if toSessionID != 0 {
 		rows, err := database.conn.QueryContext(ctx,
 			`SELECT id, uuid, command, msg_from,
 			   msg_to, params, body, meta, created_at
 			 FROM messages
-			 WHERE msg_to = ? AND id < ?
+			 WHERE to_session_id = ? AND id < ?
 			   AND command = 'PRIVMSG'
 			 ORDER BY id DESC LIMIT ?`,
-			target, beforeID, limit)
+			toSessionID, beforeID, limit)
 		if err != nil {
 			return nil, fmt.Errorf(
 				"query history: %w", err,
@@ -1135,10 +1151,10 @@ func (database *Database) queryHistory(
 		`SELECT id, uuid, command, msg_from,
 		   msg_to, params, body, meta, created_at
 		 FROM messages
-		 WHERE msg_to = ?
+		 WHERE to_channel_id = ? AND id < ?
 		   AND command = 'PRIVMSG'
 		 ORDER BY id DESC LIMIT ?`,
-		target, limit)
+		toChannelID, beforeID, limit)
 	if err != nil {
 		return nil, fmt.Errorf("query history: %w", err)
 	}

internal/db/queries_test.go

@@ -619,7 +619,7 @@ func TestInsertMessage(t *testing.T) {
 	body := json.RawMessage(`["hello"]`)
 
 	dbID, msgUUID, err := database.InsertMessage(
-		ctx, "PRIVMSG", "poller", "#test", nil, body, nil,
+		ctx, "PRIVMSG", "poller", "#test", 0, 0, nil, body, nil,
 	)
 	if err != nil {
 		t.Fatal(err)
@@ -653,7 +653,7 @@ func TestPollMessages(t *testing.T) {
 	body := json.RawMessage(`["hello"]`)
 
 	dbID, _, err := database.InsertMessage(
-		ctx, "PRIVMSG", "poller", "#test", nil, body, nil,
+		ctx, "PRIVMSG", "poller", "#test", 0, 0, nil, body, nil,
 	)
 	if err != nil {
 		t.Fatal(err)
@@ -706,11 +706,16 @@ func TestGetHistory(t *testing.T) {
 	database := setupTestDB(t)
 	ctx := t.Context()
 
+	chID, err := database.GetOrCreateChannel(ctx, "#hist")
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	const msgCount = 10
 
 	for range msgCount {
-		_, _, err := database.InsertMessage(
+		_, _, err = database.InsertMessage(
-			ctx, "PRIVMSG", "user", "#hist",
+			ctx, "PRIVMSG", "user", "#hist", 0, chID,
 			nil, json.RawMessage(`["msg"]`), nil,
 		)
 		if err != nil {
@@ -721,7 +726,7 @@ func TestGetHistory(t *testing.T) {
 	const histLimit = 5
 
 	msgs, err := database.GetHistory(
-		ctx, "#hist", "", histLimit,
+		ctx, 0, chID, "", histLimit,
 	)
 	if err != nil {
 		t.Fatal(err)
@@ -737,6 +742,90 @@ func TestGetHistory(t *testing.T) {
 	}
 }
 
+// TestGetHistoryReusedNames checks that history follows the
+// session and the channel row, not the nick or the channel
+// name: a session that takes a released nick, and a channel
+// created again under a deleted one's name, start with none.
+func TestGetHistoryReusedNames(t *testing.T) {
+	t.Parallel()
+
+	database := setupTestDB(t)
+	ctx := t.Context()
+
+	historyLen := func(toSessionID, toChannelID int64) int {
+		t.Helper()
+
+		msgs, err := database.GetHistory(
+			ctx, toSessionID, toChannelID, "", 0,
+		)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		return len(msgs)
+	}
+
+	oldSID, _, _, err := database.CreateSession(
+		ctx, "reused", "", "", "",
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	oldChID, err := database.GetOrCreateChannel(ctx, "#reused")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	body := json.RawMessage(`["hi"]`)
+
+	_, _, err = database.InsertMessage(
+		ctx, "PRIVMSG", "sender", "reused", oldSID, 0,
+		nil, body, nil,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, _, err = database.InsertMessage(
+		ctx, "PRIVMSG", "sender", "#reused", 0, oldChID,
+		nil, body, nil,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if historyLen(oldSID, 0) != 1 || historyLen(0, oldChID) != 1 {
+		t.Fatal("expected one DM and one channel message")
+	}
+
+	err = database.DeleteSession(ctx, oldSID)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = database.DeleteChannelIfEmpty(ctx, oldChID)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	newSID, _, _, err := database.CreateSession(
+		ctx, "reused", "", "", "",
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	newChID, err := database.GetOrCreateChannel(ctx, "#reused")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if historyLen(newSID, 0) != 0 || historyLen(0, newChID) != 0 {
+		t.Fatal("expected no history for the names' new holders")
+	}
+}
+
 func TestDeleteSession(t *testing.T) {
 	t.Parallel()
 
@@ -863,7 +952,7 @@ func TestEnqueueToClient(t *testing.T) {
 	body := json.RawMessage(`["test"]`)
 
 	dbID, _, err := database.InsertMessage(
-		ctx, "PRIVMSG", "sender", "#ch", nil, body, nil,
+		ctx, "PRIVMSG", "sender", "#ch", 0, 0, nil, body, nil,
 	)
 	if err != nil {
 		t.Fatal(err)

internal/db/schema/001_initial.sql

@@ -85,19 +85,25 @@ CREATE TABLE IF NOT EXISTS channel_members (
     UNIQUE(channel_id, session_id)
 );
 
--- Messages: IRC envelope format
+-- Messages: IRC envelope format. to_session_id is the session a DM
+-- or INVITE was sent to, to_channel_id the channel a channel message
+-- was sent to. History is selected by these ids because nicks and
+-- channel names are reused once released, and ids are not.
 CREATE TABLE IF NOT EXISTS messages (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     uuid TEXT NOT NULL UNIQUE,
     command TEXT NOT NULL DEFAULT 'PRIVMSG',
     msg_from TEXT NOT NULL DEFAULT '',
     msg_to TEXT NOT NULL DEFAULT '',
+    to_session_id INTEGER REFERENCES sessions(id) ON DELETE SET NULL,
+    to_channel_id INTEGER REFERENCES channels(id) ON DELETE SET NULL,
     params TEXT NOT NULL DEFAULT '[]',
     body TEXT NOT NULL DEFAULT '[]',
     meta TEXT NOT NULL DEFAULT '{}',
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP
 );
-CREATE INDEX IF NOT EXISTS idx_messages_to_id ON messages(msg_to, id);
+CREATE INDEX IF NOT EXISTS idx_messages_to_session ON messages(to_session_id, id);
+CREATE INDEX IF NOT EXISTS idx_messages_to_channel ON messages(to_channel_id, id);
 CREATE INDEX IF NOT EXISTS idx_messages_created ON messages(created_at);
 
 -- Spent hashcash tokens for replay prevention (1-year TTL)

internal/handlers/api.go

@@ -566,7 +566,7 @@ func (hdlr *Handlers) enqueueNumeric(
 	}
 
 	dbID, _, insertErr := hdlr.params.Database.InsertMessage(
-		ctx, command, hdlr.serverName(), nick,
+		ctx, command, hdlr.serverName(), nick, 0, 0,
 		paramsJSON, json.RawMessage(body), nil,
 	)
 	if insertErr != nil {
@@ -652,7 +652,7 @@ func (hdlr *Handlers) initChannelState(
 		dbID, _, insErr := hdlr.params.Database.
 			InsertMessage(
 				ctx, "JOIN", nick, chanInfo.Name,
-				nil, nil, nil,
+				0, chanInfo.ID, nil, nil, nil,
 			)
 		if insErr != nil {
 			hdlr.log.Error(
@@ -2404,7 +2404,7 @@ func (hdlr *Handlers) executeBanChange(
 	)
 	if mErr == nil {
 		_ = hdlr.fanOutSilent(
-			request, irc.CmdMode, nick, channel,
+			request, irc.CmdMode, nick, channel, 0, chID,
 			json.RawMessage(modeBody), memberIDs,
 		)
 	}
@@ -2522,7 +2522,7 @@ func (hdlr *Handlers) setChannelKeyMode(
 	modeBody, mErr := json.Marshal([]string{"+k", key})
 	if mErr == nil {
 		_ = hdlr.fanOutSilent(
-			request, irc.CmdMode, nick, channel,
+			request, irc.CmdMode, nick, channel, 0, chID,
 			json.RawMessage(modeBody), memberIDs,
 		)
 	}
@@ -2571,7 +2571,7 @@ func (hdlr *Handlers) clearChannelKeyMode(
 	modeBody, mErr := json.Marshal([]string{"-k", "*"})
 	if mErr == nil {
 		_ = hdlr.fanOutSilent(
-			request, irc.CmdMode, nick, channel,
+			request, irc.CmdMode, nick, channel, 0, chID,
 			json.RawMessage(modeBody), memberIDs,
 		)
 	}
@@ -2646,7 +2646,7 @@ func (hdlr *Handlers) setChannelLimitMode(
 	)
 	if mErr == nil {
 		_ = hdlr.fanOutSilent(
-			request, irc.CmdMode, nick, channel,
+			request, irc.CmdMode, nick, channel, 0, chID,
 			json.RawMessage(modeBody), memberIDs,
 		)
 	}
@@ -2696,7 +2696,7 @@ func (hdlr *Handlers) clearChannelLimitMode(
 	modeBody, mErr := json.Marshal([]string{"-l"})
 	if mErr == nil {
 		_ = hdlr.fanOutSilent(
-			request, irc.CmdMode, nick, channel,
+			request, irc.CmdMode, nick, channel, 0, chID,
 			json.RawMessage(modeBody), memberIDs,
 		)
 	}
@@ -2755,7 +2755,7 @@ func (hdlr *Handlers) handleInvite(
 	if mErr == nil {
 		_ = hdlr.fanOutSilent(
 			request, irc.CmdInvite, nick, targetNick,
-			json.RawMessage(invBody),
+			targetSID, 0, json.RawMessage(invBody),
 			[]int64{targetSID},
 		)
 	}
@@ -3250,9 +3250,10 @@ func (hdlr *Handlers) HandleGetHistory() http.HandlerFunc {
 			return
 		}
 
-		if !hdlr.canAccessHistory(
+		toSessionID, toChannelID, ok := hdlr.canAccessHistory(
 			writer, request, sessionID, nick, target,
-		) {
+		)
+		if !ok {
 			return
 		}
 
@@ -3264,7 +3265,7 @@ func (hdlr *Handlers) HandleGetHistory() http.HandlerFunc {
 		}
 
 		msgs, err := hdlr.params.Database.GetHistory(
-			request.Context(), target,
+			request.Context(), toSessionID, toChannelID,
 			request.URL.Query().Get("before"), limit,
 		)
 		if errors.Is(err, sql.ErrNoRows) {
@@ -3297,17 +3298,23 @@ func (hdlr *Handlers) HandleGetHistory() http.HandlerFunc {
 }
 
 // canAccessHistory verifies the user can read history
-// for the given target (channel or DM participant).
+// for the given target (channel or DM participant) and
+// returns the session id and channel id to pass to
+// GetHistory: the user's own session for its nick, or the
+// channel's id, with 0 for the other. On false it has
+// written the error response.
 func (hdlr *Handlers) canAccessHistory(
 	writer http.ResponseWriter,
 	request *http.Request,
 	sessionID int64,
 	nick, target string,
-) bool {
+) (int64, int64, bool) {
 	if strings.HasPrefix(target, "#") {
-		return hdlr.canAccessChannelHistory(
+		chID, ok := hdlr.canAccessChannelHistory(
 			writer, request, sessionID, target,
 		)
+
+		return 0, chID, ok
 	}
 
 	// DM history: only allow if the target is the
@@ -3319,18 +3326,20 @@ func (hdlr *Handlers) canAccessHistory(
 			http.StatusForbidden,
 		)
 
-		return false
+		return 0, 0, false
 	}
 
-	return true
+	return sessionID, 0, true
 }
 
+// canAccessChannelHistory returns the id of the channel
+// named target if the user is a member of it.
 func (hdlr *Handlers) canAccessChannelHistory(
 	writer http.ResponseWriter,
 	request *http.Request,
 	sessionID int64,
 	target string,
-) bool {
+) (int64, bool) {
 	chID, err := hdlr.params.Database.GetChannelByName(
 		request.Context(), target,
 	)
@@ -3341,7 +3350,7 @@ func (hdlr *Handlers) canAccessChannelHistory(
 			http.StatusNotFound,
 		)
 
-		return false
+		return 0, false
 	}
 
 	isMember, err := hdlr.params.Database.IsChannelMember(
@@ -3357,7 +3366,7 @@ func (hdlr *Handlers) canAccessChannelHistory(
 			http.StatusInternalServerError,
 		)
 
-		return false
+		return 0, false
 	}
 
 	if !isMember {
@@ -3367,10 +3376,10 @@ func (hdlr *Handlers) canAccessChannelHistory(
 			http.StatusForbidden,
 		)
 
-		return false
+		return 0, false
 	}
 
-	return true
+	return chID, true
 }
 
 // HandleLogout deletes the authenticated client's token
@@ -3679,17 +3688,19 @@ func (hdlr *Handlers) deliverWhoisIdle(
 }
 
 // fanOut inserts a message and enqueues it to all given
-// sessions.
+// sessions. toSessionID and toChannelID are the recipient
+// InsertMessage records for history, 0 when there is none.
 func (hdlr *Handlers) fanOut(
 	request *http.Request,
 	command, from, target string,
+	toSessionID, toChannelID int64,
 	body json.RawMessage,
 	meta json.RawMessage,
 	sessionIDs []int64,
 ) (string, error) {
 	dbID, msgUUID, err := hdlr.params.Database.InsertMessage(
 		request.Context(), command, from, target,
-		nil, body, meta,
+		toSessionID, toChannelID, nil, body, meta,
 	)
 	if err != nil {
 		return "", fmt.Errorf("insert message: %w", err)
@@ -3714,12 +3725,13 @@ func (hdlr *Handlers) fanOut(
 func (hdlr *Handlers) fanOutSilent(
 	request *http.Request,
 	command, from, target string,
+	toSessionID, toChannelID int64,
 	body json.RawMessage,
 	sessionIDs []int64,
 ) error {
 	_, err := hdlr.fanOut(
 		request, command, from, target,
-		body, nil, sessionIDs,
+		toSessionID, toChannelID, body, nil, sessionIDs,
 	)
 
 	return err

internal/handlers/api_test.go

@@ -1551,6 +1551,60 @@ func (tserver *testServer) getHistory(
 	return resp.StatusCode, msgs
 }
 
+// getAllHistory pages back through target's history
+// pageSize messages at a time and returns the message texts,
+// oldest first, and the number of pages that held any. It
+// stops after maxPages pages, so a before that is ignored
+// cannot loop forever.
+func (tserver *testServer) getAllHistory(
+	token, target string,
+	pageSize, maxPages int,
+) ([]string, int) {
+	tserver.t.Helper()
+
+	var (
+		got   []string
+		pages int
+	)
+
+	// The first page has no before; each later one passes the
+	// id of the oldest message on the page fetched before it.
+	before := ""
+
+	for range maxPages {
+		status, page := tserver.getHistory(
+			token, target, before, pageSize,
+		)
+		if status != http.StatusOK {
+			tserver.t.Fatalf("expected 200, got %d", status)
+		}
+
+		if len(page) == 0 {
+			break
+		}
+
+		pages++
+
+		texts := make([]string, 0, len(page))
+
+		for _, msg := range page {
+			var body []string
+
+			err := json.Unmarshal(msg.Body, &body)
+			if err != nil {
+				tserver.t.Fatalf("decode body: %v", err)
+			}
+
+			texts = append(texts, body...)
+		}
+
+		got = slices.Insert(got, 0, texts...)
+		before = page[0].ID
+	}
+
+	return got, pages
+}
+
 func TestHistoryPaging(t *testing.T) {
 	const (
 		channel   = "#paging"
@@ -1579,46 +1633,10 @@ func TestHistoryPaging(t *testing.T) {
 		})
 	}
 
-	var (
+	got, pages := tserver.getAllHistory(
-		got   []string
+		token, channel, pageSize, msgCount,
-		pages int
 	)
 
-	// The first page has no before; each later one passes the
-	// id of the oldest message on the page fetched before it.
-	before := ""
-
-	for range msgCount {
-		status, page := tserver.getHistory(
-			token, channel, before, pageSize,
-		)
-		if status != http.StatusOK {
-			t.Fatalf("expected 200, got %d", status)
-		}
-
-		if len(page) == 0 {
-			break
-		}
-
-		pages++
-
-		texts := make([]string, 0, len(page))
-
-		for _, msg := range page {
-			var body []string
-
-			err := json.Unmarshal(msg.Body, &body)
-			if err != nil {
-				t.Fatalf("decode body: %v", err)
-			}
-
-			texts = append(texts, body...)
-		}
-
-		got = slices.Insert(got, 0, texts...)
-		before = page[0].ID
-	}
-
 	if pages != wantPages {
 		t.Fatalf(
 			"expected %d pages, got %d", wantPages, pages,
@@ -1630,6 +1648,81 @@ func TestHistoryPaging(t *testing.T) {
 	}
 }
 
+// TestHistoryDMPaging pages through a nick's DM history,
+// which holds the DMs its session got under an earlier nick
+// and none sent to another session, and checks that the id
+// of a DM sent to another session is not accepted as before.
+func TestHistoryDMPaging(t *testing.T) {
+	const (
+		firstNick = "readerFirst"
+		laterNick = "readerLater"
+		otherNick = "otherReader"
+		// DMs sent to firstNick before the nick change.
+		sentEarly = 2
+		pageSize  = 2
+		wantPages = 3
+	)
+
+	tserver := newTestServer(t)
+	senderToken := tserver.createSession("dmPager")
+	readerToken := tserver.createSession(firstNick)
+	otherToken := tserver.createSession(otherNick)
+
+	sendDM := func(to, text string) {
+		tserver.sendCommand(senderToken, map[string]any{
+			commandKey: privmsgCmd,
+			toKey:      to,
+			bodyKey:    []string{text},
+		})
+	}
+
+	want := []string{"dm 1", "dm 2", "dm 3", "dm 4", "dm 5"}
+
+	for _, text := range want[:sentEarly] {
+		sendDM(firstNick, text)
+	}
+
+	_, result := tserver.sendCommand(readerToken, map[string]any{
+		commandKey: irc.CmdNick,
+		bodyKey:    []string{laterNick},
+	})
+	if result[nickKey] != laterNick {
+		t.Fatalf("expected nick %s, got %v", laterNick, result)
+	}
+
+	sendDM(otherNick, "not for the reader")
+
+	for _, text := range want[sentEarly:] {
+		sendDM(laterNick, text)
+	}
+
+	got, pages := tserver.getAllHistory(
+		readerToken, laterNick, pageSize, len(want),
+	)
+
+	if pages != wantPages {
+		t.Fatalf(
+			"expected %d pages, got %d", wantPages, pages,
+		)
+	}
+
+	if !slices.Equal(got, want) {
+		t.Fatalf("expected %q, got %q", want, got)
+	}
+
+	_, otherDMs := tserver.getHistory(otherToken, otherNick, "", 1)
+	if len(otherDMs) != 1 {
+		t.Fatalf("expected 1 DM, got %d", len(otherDMs))
+	}
+
+	status, _ := tserver.getHistory(
+		readerToken, laterNick, otherDMs[0].ID, pageSize,
+	)
+	if status != http.StatusBadRequest {
+		t.Fatalf("expected 400, got %d", status)
+	}
+}
+
 func TestHistoryUnknownBefore(t *testing.T) {
 	tserver := newTestServer(t)
 	token := tserver.createSession("lost")
@@ -1654,9 +1747,25 @@ func TestHistoryUnknownBefore(t *testing.T) {
 		)
 	}
 
+	// History holds only PRIVMSG, so the id of the JOIN to
+	// #here is not in it.
+	var joinID string
+
+	queued, _ := tserver.pollMessages(token, 0)
+	for _, msg := range queued {
+		if msg[commandKey] == joinCmd && msg[toKey] == "#here" {
+			joinID, _ = msg["id"].(string)
+		}
+	}
+
+	if joinID == "" {
+		t.Fatal("expected a JOIN to #here in the queue")
+	}
+
 	cases := []struct{ name, before string }{
 		{"made-up id", "no-such-message"},
 		{"id from another channel", elsewhere[0].ID},
+		{"id of a JOIN", joinID},
 	}
 
 	for _, tc := range cases {
@@ -1671,6 +1780,101 @@ func TestHistoryUnknownBefore(t *testing.T) {
 	}
 }
 
+// TestHistoryReusedNick checks that a user who takes a nick
+// released by QUIT does not get the DMs sent to its previous
+// holder.
+func TestHistoryReusedNick(t *testing.T) {
+	const victim = "victimV"
+
+	tserver := newTestServer(t)
+	senderToken := tserver.createSession("dmA")
+	victimToken := tserver.createSession(victim)
+	newcomerToken := tserver.createSession("newcomer")
+
+	tserver.sendCommand(senderToken, map[string]any{
+		commandKey: privmsgCmd,
+		toKey:      victim,
+		bodyKey:    []string{"for the first victimV only"},
+	})
+
+	_, msgs := tserver.getHistory(victimToken, victim, "", 10)
+	if len(msgs) != 1 {
+		t.Fatalf("expected 1 DM for %s, got %d", victim, len(msgs))
+	}
+
+	tserver.sendCommand(victimToken, map[string]any{
+		commandKey: irc.CmdQuit,
+	})
+
+	_, result := tserver.sendCommand(newcomerToken, map[string]any{
+		commandKey: irc.CmdNick,
+		bodyKey:    []string{victim},
+	})
+	if result[nickKey] != victim {
+		t.Fatalf("expected nick %s, got %v", victim, result)
+	}
+
+	status, msgs := tserver.getHistory(newcomerToken, victim, "", 10)
+	if status != http.StatusOK {
+		t.Fatalf("expected 200, got %d", status)
+	}
+
+	if len(msgs) != 0 {
+		t.Fatalf("expected no history, got %v", msgs)
+	}
+}
+
+// TestHistoryRecreatedChannel checks that a channel created
+// again under the name of a deleted one starts with no
+// history, even when the old one had a key.
+func TestHistoryRecreatedChannel(t *testing.T) {
+	const channel = "#priv"
+
+	tserver := newTestServer(t)
+	ownerToken := tserver.createSession("roomR")
+	strangerToken := tserver.createSession("stranger")
+
+	tserver.sendCommand(ownerToken, map[string]any{
+		commandKey: joinCmd, toKey: channel,
+	})
+	tserver.sendCommand(ownerToken, map[string]any{
+		commandKey: irc.CmdMode,
+		toKey:      channel,
+		bodyKey:    []string{"+k", "topsecret"},
+	})
+	tserver.sendCommand(ownerToken, map[string]any{
+		commandKey: privmsgCmd,
+		toKey:      channel,
+		bodyKey:    []string{"members only"},
+	})
+
+	_, msgs := tserver.getHistory(ownerToken, channel, "", 10)
+	if len(msgs) != 1 {
+		t.Fatalf("expected 1 message, got %d", len(msgs))
+	}
+
+	// The last member leaving deletes the channel.
+	tserver.sendCommand(ownerToken, map[string]any{
+		commandKey: irc.CmdPart, toKey: channel,
+	})
+
+	_, result := tserver.sendCommand(strangerToken, map[string]any{
+		commandKey: joinCmd, toKey: channel,
+	})
+	if result[statusKey] != joinedStatus {
+		t.Fatalf("expected the join to succeed, got %v", result)
+	}
+
+	status, msgs := tserver.getHistory(strangerToken, channel, "", 10)
+	if status != http.StatusOK {
+		t.Fatalf("expected 200, got %d", status)
+	}
+
+	if len(msgs) != 0 {
+		t.Fatalf("expected no history, got %v", msgs)
+	}
+}
+
 func TestChannelList(t *testing.T) {
 	tserver := newTestServer(t)
 	token := tserver.createSession("lister")

internal/ircserver/commands.go

@@ -1268,7 +1268,7 @@ func (c *Conn) handleInvite(
 	)
 
 	_, _, _ = c.svc.FanOut( //nolint:dogsled // fire-and-forget broadcast
-		ctx, "INVITE", c.nick, targetNick,
+		ctx, "INVITE", c.nick, targetNick, targetID, 0,
 		nil, body, nil, []int64{targetID},
 	)
 }

internal/ircserver/relay_test.go

@@ -28,7 +28,7 @@ func (env *testEnv) queueMessage(
 	}
 
 	msgID, _, err := env.database.InsertMessage(
-		ctx, command, from, to, nil, bodyJSON, nil,
+		ctx, command, from, to, 0, 0, nil, bodyJSON, nil,
 	)
 	if err != nil {
 		t.Fatalf("insert message: %v", err)

internal/service/service.go

@@ -311,7 +311,7 @@ func (s *Service) SendWallops(
 	}
 
 	_, _, err = s.FanOut(
-		ctx, irc.CmdWallops, nick, "*",
+		ctx, irc.CmdWallops, nick, "*", 0, 0,
 		nil, body, nil, recipients,
 	)
 	if err != nil {
@@ -344,15 +344,19 @@ type DirectMsgResult struct {
 }
 
 // FanOut inserts a message and enqueues it to all given
-// session IDs, notifying each via the broker.
+// session IDs, notifying each via the broker. toSessionID
+// and toChannelID are the recipient InsertMessage records
+// for history, 0 when there is none.
 func (s *Service) FanOut(
 	ctx context.Context,
 	command, from, to string,
+	toSessionID, toChannelID int64,
 	params, body, meta json.RawMessage,
 	sessionIDs []int64,
 ) (int64, string, error) {
 	dbID, msgUUID, err := s.db.InsertMessage(
-		ctx, command, from, to, params, body, meta,
+		ctx, command, from, to,
+		toSessionID, toChannelID, params, body, meta,
 	)
 	if err != nil {
 		return 0, "", fmt.Errorf("insert message: %w", err)
@@ -396,13 +400,13 @@ func (s *Service) SendChannelMessage(
 	nick, command, channel string,
 	body, meta json.RawMessage,
 ) (int64, string, error) {
+	noSuchChannel := &IRCError{
+		irc.ErrNoSuchChannel, []string{channel}, msgNoSuchChannel,
+	}
+
 	chID, err := s.db.GetChannelByName(ctx, channel)
 	if err != nil {
-		return 0, "", &IRCError{
+		return 0, "", noSuchChannel
-			irc.ErrNoSuchChannel,
-			[]string{channel},
-			msgNoSuchChannel,
-		}
 	}
 
 	if command == irc.CmdPrivmsg {
@@ -459,9 +463,14 @@ func (s *Service) SendChannelMessage(
 	recipients := excludeSession(memberIDs, sessionID)
 
 	dbID, uuid, fanErr := s.FanOut(
-		ctx, command, nick, channel,
+		ctx, command, nick, channel, 0, chID,
 		nil, body, meta, recipients,
 	)
+	// The channel was deleted after the lookup above.
+	if db.IsForeignKeyConstraintError(fanErr) {
+		return 0, "", noSuchChannel
+	}
+
 	if fanErr != nil {
 		return 0, "", fanErr
 	}
@@ -478,13 +487,13 @@ func (s *Service) SendDirectMessage(
 	nick, command, target string,
 	body, meta json.RawMessage,
 ) (*DirectMsgResult, error) {
+	noSuchNick := &IRCError{
+		irc.ErrNoSuchNick, []string{target}, "No such nick",
+	}
+
 	targetSID, err := s.db.GetSessionByNick(ctx, target)
 	if err != nil {
-		return nil, &IRCError{
+		return nil, noSuchNick
-			irc.ErrNoSuchNick,
-			[]string{target},
-			"No such nick",
-		}
 	}
 
 	away, _ := s.db.GetAway(ctx, targetSID)
@@ -495,9 +504,14 @@ func (s *Service) SendDirectMessage(
 	}
 
 	_, uuid, fanErr := s.FanOut(
-		ctx, command, nick, target,
+		ctx, command, nick, target, targetSID, 0,
 		nil, body, meta, recipients,
 	)
+	// The target's session was deleted after the lookup above.
+	if db.IsForeignKeyConstraintError(fanErr) {
+		return nil, noSuchNick
+	}
+
 	if fanErr != nil {
 		return nil, fanErr
 	}
@@ -561,7 +575,7 @@ func (s *Service) JoinChannel(
 	body, _ := json.Marshal([]string{channel}) //nolint:errchkjson
 
 	_, _, _ = s.FanOut( //nolint:dogsled // fire-and-forget broadcast
-		ctx, irc.CmdJoin, nick, channel,
+		ctx, irc.CmdJoin, nick, channel, 0, chID,
 		nil, body, nil, memberIDs,
 	)
 
@@ -604,7 +618,7 @@ func (s *Service) PartChannel(
 	body, _ := json.Marshal([]string{reason}) //nolint:errchkjson
 
 	_, _, _ = s.FanOut( //nolint:dogsled // fire-and-forget broadcast
-		ctx, irc.CmdPart, nick, channel,
+		ctx, irc.CmdPart, nick, channel, 0, chID,
 		nil, body, nil, recipients,
 	)
 
@@ -666,7 +680,7 @@ func (s *Service) SetTopic(
 	body, _ := json.Marshal([]string{topic}) //nolint:errchkjson
 
 	_, _, _ = s.FanOut( //nolint:dogsled // fire-and-forget broadcast
-		ctx, irc.CmdTopic, nick, channel,
+		ctx, irc.CmdTopic, nick, channel, 0, chID,
 		nil, body, nil, memberIDs,
 	)
 
@@ -730,7 +744,7 @@ func (s *Service) KickUser(
 	)
 
 	_, _, _ = s.FanOut( //nolint:dogsled // fire-and-forget broadcast
-		ctx, irc.CmdKick, nick, channel,
+		ctx, irc.CmdKick, nick, channel, 0, chID,
 		params, body, nil, memberIDs,
 	)
 
@@ -886,7 +900,7 @@ func (s *Service) BroadcastQuit(
 		body, _ := json.Marshal([]string{reason}) //nolint:errchkjson
 
 		_, _, _ = s.FanOut(
-			ctx, irc.CmdQuit, nick, "",
+			ctx, irc.CmdQuit, nick, "", 0, 0,
 			nil, body, nil, recipients,
 		)
 	}
@@ -1074,7 +1088,7 @@ func (s *Service) BroadcastMode(
 	body, _ := json.Marshal([]string{modeText}) //nolint:errchkjson
 
 	_, _, _ = s.FanOut( //nolint:dogsled // fire-and-forget broadcast
-		ctx, irc.CmdMode, nick, channel,
+		ctx, irc.CmdMode, nick, channel, 0, chID,
 		nil, body, nil, memberIDs,
 	)
 }
@@ -1351,7 +1365,7 @@ func (s *Service) broadcastNickChange(
 	notified := make(map[int64]bool)
 
 	dbID, _, insErr := s.db.InsertMessage(
-		ctx, irc.CmdNick, oldNick, "",
+		ctx, irc.CmdNick, oldNick, "", 0, 0,
 		nil, body, nil,
 	)
 	if insErr != nil {

internal/service/service_test.go

@@ -164,7 +164,7 @@ func TestFanOut(t *testing.T) {
 	body, _ := json.Marshal([]string{"hello"}) //nolint:errchkjson
 
 	dbID, uuid, err := env.svc.FanOut(
-		ctx, irc.CmdPrivmsg, "alice", "#test",
+		ctx, irc.CmdPrivmsg, "alice", "#test", 0, 0,
 		nil, body, nil,
 		[]int64{sid1, sid2},
 	)
@@ -339,6 +339,78 @@ func TestBroadcastQuit(t *testing.T) {
 	}
 }
 
+// TestSendDirectMessageTargetQuits checks that a DM whose
+// target quits after its nick is looked up gets the same
+// error as a DM to an unknown nick. The trigger deletes the
+// target's session just before the DM is stored.
+func TestSendDirectMessageTargetQuits(t *testing.T) {
+	env := newTestEnv(t)
+	ctx := t.Context()
+
+	sid := createSession(ctx, t, env.db, "alice")
+	createSession(ctx, t, env.db, "bob")
+
+	_, err := env.db.GetDB().ExecContext(ctx,
+		`CREATE TRIGGER delete_recipient
+		 BEFORE INSERT ON messages
+		 BEGIN
+		   DELETE FROM sessions WHERE id = NEW.to_session_id;
+		 END`)
+	if err != nil {
+		t.Fatalf("create trigger: %v", err)
+	}
+
+	body, _ := json.Marshal([]string{"hi"}) //nolint:errchkjson
+
+	_, err = env.svc.SendDirectMessage(
+		ctx, sid, "alice", irc.CmdPrivmsg, "bob", body, nil,
+	)
+
+	var ircErr *service.IRCError
+	if !errors.As(err, &ircErr) || ircErr.Code != irc.ErrNoSuchNick {
+		t.Fatalf("want ERR_NOSUCHNICK, got %v", err)
+	}
+}
+
+// TestSendChannelMessageChannelDeleted checks that a channel
+// message whose channel is deleted after it is looked up gets
+// the same error as a message to an unknown channel. The
+// trigger deletes the channel just before the message is
+// stored.
+func TestSendChannelMessageChannelDeleted(t *testing.T) {
+	env := newTestEnv(t)
+	ctx := t.Context()
+
+	sid := createSession(ctx, t, env.db, "alice")
+
+	_, err := env.svc.JoinChannel(ctx, sid, "alice", "#gone", "")
+	if err != nil {
+		t.Fatalf("join alice: %v", err)
+	}
+
+	_, err = env.db.GetDB().ExecContext(ctx,
+		`CREATE TRIGGER delete_recipient
+		 BEFORE INSERT ON messages
+		 BEGIN
+		   DELETE FROM channels WHERE id = NEW.to_channel_id;
+		 END`)
+	if err != nil {
+		t.Fatalf("create trigger: %v", err)
+	}
+
+	body, _ := json.Marshal([]string{"hi"}) //nolint:errchkjson
+
+	_, _, err = env.svc.SendChannelMessage(
+		ctx, sid, "alice", irc.CmdPrivmsg, "#gone", body, nil,
+	)
+
+	var ircErr *service.IRCError
+	if !errors.As(err, &ircErr) ||
+		ircErr.Code != irc.ErrNoSuchChannel {
+		t.Fatalf("want ERR_NOSUCHCHANNEL, got %v", err)
+	}
+}
+
 func TestSendChannelMessage_Moderated(t *testing.T) {
 	env := newTestEnv(t)
 	ctx := t.Context()