sessions: persist to disk, prune, track client liveness, accept UE_LOGFMT (#1014)HEADmain

Branch started as a sessions-service overhaul (persistence, client liveness, UE_LOGFMT intake) and grew to pick up adjacent infrastructure work: an early-startup log backlog, a hardened `MemoryArena`, the `zen trace serve` viewer gaining a counter view + compact timeline + tabbed callsite panel, defensive fixes in the third-party `tourist` trace parser, a series of allocation reductions across the HTTP and compact-binary hot paths, and a new `zen sessions` CLI command tree.

## Sessions service

**Persistence.** Each session lives on disk under `<DataRoot>/sessions/<id>/` as `info.cb` (metadata) plus `log.bin` (length-prefixed CbObject log records). On startup the service scans that directory and loads prior sessions as ended sessions, preloading the tail of each log so historical views work after a restart. `SessionLog` is noexcept-constructed and falls back to a disabled state on disk errors, so a bad disk can't take down `RegisterSession`. `GetSession` falls back to the ended-sessions list (fixes historical log fetches over HTTP). `LoadTail` counts only successfully-parsed records.

**Pruning.** Periodic cleanup task drops ended sessions once any of three caps is exceeded: age (default 1 year), count (default 1000), or total on-disk footprint (default 50 MiB). Runs 30 s after startup, hourly thereafter. Active sessions never pruned; disk removal and directory stat happen outside the exclusive lock so a slow filesystem can't stall lookups.

**Client liveness.** Sessions carry a `ProcessHandle` for the client-reported pid, captured at registration time so Windows pid recycling can't produce false positives. A 30 s asio timer probes liveness and ends dead sessions through the normal remove path, producing a synthetic `Session ended: process exited (...)` line persisted to `log.bin`. Windows decodes common NTSTATUS exit codes to human names (Ctrl-C, access violation, stack overflow, ...); POSIX stays at plain `process exited`. Clients auto-fill `ClientPid` only for local targets (unix socket / loopback); the server defensively accepts pids only from `IsLocalMachineRequest()` peers. zenserver also reports its own pid when registering its self-session, so it shows up with a real pid in the dashboard and `zen sessions ls`.

**Synthetic end-of-session line.** `RemoveSession` takes an optional reason; before the session moves to the ended list it appends an Info-level `Session ended[: reason]` entry through the normal log path (released outside `m_Lock`). Current reasons: `client request` (HTTP DELETE), `server shutdown` (self-session), `process exited (...)` (liveness).

**UE_LOGFMT structured entries.** `POST /sessions/{id}/log` now accepts `{level, logger, format, fields}` alongside the existing `{level, logger, message}` shape. New `logtemplate.{h,cpp}` implements UE's `StructuredLog.cpp` template grammar (field paths with `.name` / `[N]`, `{{`/`}}` escapes, `$text` / `$format` / `$locformat` object conventions, bounded recursion). Renders to a displayable message at intake while persisting raw format + fields so a future UI can drill into fields without another schema bump. Hot path is zero-alloc — renders into `ExtendableStringBuilder<256>` using stack-buffered `Oid::ToString` / `IoHash::ToHexString` overloads. UI shows a `{…}` marker with the raw template + JSON-pretty fields on hover.

**Parent sessions.** `SessionInfo` gains `parent_session_id`; hub-managed storage server child processes inherit the hub's session id via `--parent-session=<id>`. `ZEN_SESSIONS_URL` env var becomes a fallback for `--sessions-url` / config when neither is provided. The in-process session log sink is disabled when a remote sessions target is configured (logs flow through `SessionsServiceClient` instead). The sessions UI groups child sessions under their parent (collapsible/expandable, sorts as a unit, supports nesting).

**Platform reporting.** `SessionInfo` gains `Platform`, flowed end-to-end: client auto-fills via `GetRuntimePlatformName()`, server persists in `info.cb` (`plat`) and emits on GET. UI renders as a SimpleIcons-style inline SVG (windows / macOS / iOS / linux / wine / android / playstation / xbox / nintendo) with case-insensitive alias resolution (Win32/Win64, PS4/PS5, XSX/XSS, NintendoSwitch, iPhone/iPad, Darwin/OSX). Unknown values fall back to text; sorting runs on the underlying string.

**WebSocket log streaming.** Sessions UI moves from 2 s polling to a WebSocket push model. New `WsSubscriber` has a stable id + helper methods. UI caps the log-line DOM at 5 000 entries with a shared cursor-regression helper, factored out of two call sites. Per-broadcast allocations trimmed on the push path; fixed a stack overrun in the WS log broadcast hex-id buffer.

**Log memory.** `LogEntry::Level` is now `logging::LogLevel` (1 byte) instead of `std::string` (~32 B) — saves ~310 KB per full 10 k-entry deque and eliminates a per-message allocation in the in-proc sink. On-disk format writes an int32 and accepts either int or legacy string on read. `LogEntry` strings now live in a `MemoryArena`; logger names are interned across the deque. `SessionLog::Append` and `WriteSessionInfoFile` drop their `UniqueBuffer` round-trip and write `CbObject::GetView()` straight through `BasicFile` / `SafeWriteFile`. Multi-entry `POST /log` batched under one lock + one push.

**In-proc log timestamps.** `InProcSessionLogSink::TimePointToDateTime` previously preserved only whole seconds, so every in-proc entry rendered at `.000` ms in the dashboard and `zen sessions tail`. It now adds the sub-second part (nanoseconds → 100 ns ticks) to keep ms precision end-to-end.

**UI.** Side "Session Details" panel is gone — its info is inline in the table (appname, mode, platform, id, timestamps, this/log pills, active dot). Bottom panel is a tabbed `Log | Metadata` view with a right-side "Session Information" panel beside metadata; log-only controls (filter, newest-first, follow, log-level filter, expand/collapse) hide when Metadata is active, polling keeps running across tab switches. Wide-mode toggle fills the viewport edge-to-edge. Log lines show the logger category; timestamps render in 24 h with zero-padded fields regardless of locale. Sessions list defaults to All / 10 per page / created-desc, gains click-to-sort headers on the full dataset, a header filter box, and a pager aligned to the table's right edge. Duplicate auto-injected `<h1>Sessions</h1>` removed.

## `zen sessions` CLI

New command tree on the `zen` client for inspecting the sessions service from the terminal:

- **`zen sessions ls`** — lists sessions (active first, ended next; newest-first within each group) with id, status, app/mode, pid, created, duration, and log count. Supports `--status active|ended|all` (default `all`).
- **`zen sessions status`** — prints the sessions service summary: self id, active / ended counts, and the read/write/delete/list/request/bad-request counters from `/stats/sessions`.
- **`zen sessions tail [session]`** — tails a session's log. With no argument it tails zenserver's own session (resolved via `/sessions/list`'s `self_id`); an explicit 24-hex id targets any session, including ended ones (historical replay). `--lines N` (default 50, 0 = all buffered) trims the initial dump client-side. `--follow` prefers a WebSocket push subscription on `/sessions/ws` for sub-second latency; on upgrade failure (older server, blocked port, unix-socket transport) it falls back to HTTP cursor polling at `--interval-ms` (default 500), with sleeps chunked to 50 ms so Ctrl-C reacts quickly. Output matches `zen::logging::FullFormatter` (`[YY-MM-DD HH:MM:SS.mmm] [lvl] [logger] message`); on a TTY the level is colored and the logger is bold, with continuation lines indented under the message column using the *visible* prefix width. 404 surfaces as `(session ended)` and connection errors as `(server gone)` — both clean exits, so stopping the server mid-tail no longer prints a stack trace.
- **`zen sessions ui`** — opens `<host>/dashboard/?page=sessions` in the user's default browser. Rejects unix-socket hosts.

A small `ZenServiceClient::IsUnixSocket()` helper now wraps the unix-socket check used by `ui`, `sessions tail` (WS path), and `sessions ui`.

## Logging

`BacklogSink` captures early-startup log entries in a fixed-capacity ring so late-attached sinks (session sink, file sink) can replay them. Detaches from the broadcast list when disabled; backed by destructor-only cleanup (no `unique_ptr` indirection per entry). Tuned defaults so the backlog covers typical bring-up without unbounded growth.

## `zen trace serve` viewer

- Compact timeline mode for high-density views.
- New `TRACE_INT_VALUE` / `TRACE_FLOAT_VALUE` counter trace points + a counters page in the viewer.
- Callsite tables collapsed into a single tabbed panel.
- Lossless `Oid <-> Guid` bridge for trace session ids; trace `SessionId` plumbed through.
- `tourist` parser hardening: bounds-check `BufferStream::read`, validate `Type::info_size` before `patch()`, convert `parse_important_aux` to a loop (avoids deep recursion), widen `ParserPool` index to `uint32`, bounds-check field offsets in the dispatcher, pin `Types::parse` buffer up-front.

## `MemoryArena`

Configurable chunk size, inline chunk list, oversize requests routed to truly-dedicated chunks (no slack waste, no fragmentation when one allocation is much larger than the chunk).

## Allocation cleanups across hot paths

- `zenhttp::HttpRequestRouter::HandleRequest` and `FormatPackageMessageInternal`: drop heap allocations.
- Compact-binary validation: `eastl::fixed_vector` + `eastl::sort`; eliminate `std::vector` churn.
- `zenserverprocess`: trim transient allocations in spawn paths.
- Sessions HTTP intake / broadcast: drop transient `std::string` allocs.

1 files changed, 57 insertions, 16 deletions

diff --git a/src/zen/frontend/html/memory.js b/src/zen/frontend/html/memory.js
index 6b9760439..6e4d51061 100644
--- a/src/zen/frontend/html/memory.js
+++ b/src/zen/frontend/html/memory.js
@@ -1,17 +1,9 @@
 // Copyright Epic Games, Inc. All Rights Reserved.
-// Interactive memory analysis view: summary cards, memory timeline, leak/churn/hot callsite tables.
+// Interactive memory analysis view: summary cards, memory timeline, and a
+// tabbed callsite panel (Leaky / Churn / Hot) sharing one slot below the chart.
 
 import { getAllocSummary, getMemoryTimeline, getCallstackStats, getChurnStats, getCallstack, getAllocSizeHistogram } from "./api.js";
-
-function escapeHtml(s) {
-	return String(s).replace(/[&<>"']/g, (c) => ({
-		"&": "&amp;",
-		"<": "&lt;",
-		">": "&gt;",
-		'"': "&quot;",
-		"'": "&#39;",
-	}[c]));
-}
+import { escapeHtml } from "./util.js";
 
 function formatNum(n) {
 	return Number(n || 0).toLocaleString();
@@ -187,10 +179,17 @@ export class MemoryView {
 			churn: { sortKey: "churn_allocs", desc: true, groupMode: "none", filterText: "" },
 			hot: { sortKey: "total_allocs", desc: true, groupMode: "none", filterText: "" },
 		};
+		this.activeTable = "leaks";
 		this.loadStateFromUrl();
 		this.buildLayout();
 	}
 
+	static TAB_DEFS = [
+		{ name: "leaks", label: "Leaky callsites" },
+		{ name: "churn", label: "Churn" },
+		{ name: "hot", label: "Hot callsites" },
+	];
+
 	buildLayout() {
 		this.container.innerHTML =
 			`<div class="memory-view">` +
@@ -222,6 +221,12 @@ export class MemoryView {
 			`</div>` +
 			`</div>` +
 			`<div class="memory-grid">` +
+			`<div class="memory-tabbed">` +
+			`<div class="memory-tab-bar" role="tablist">` +
+			MemoryView.TAB_DEFS.map(({ name, label }) =>
+				`<button type="button" class="memory-tab" role="tab" data-mem-tab="${name}" id="memory-tab-${name}" aria-controls="memory-tabpanel-${name}">${escapeHtml(label)}</button>`
+			).join("") +
+			`</div>` +
 			this.buildPanelMarkup("leaks", "Leaky callsites", "Top live allocation stacks", [
 				["live_bytes", "Live bytes"],
 				["live_count", "Live allocs"],
@@ -239,6 +244,7 @@ export class MemoryView {
 				["churn_allocs", "Churn allocs"],
 				["summary", "Summary"],
 			]) +
+			`</div>` +
 			`<div class="memory-panel memory-callstack-panel">` +
 			`<div class="memory-panel-header"><div class="memory-panel-title">Callstack details</div><div class="memory-panel-subtitle" id="memory-callstack-meta">Select a row to inspect its frames</div></div>` +
 			`<div class="memory-callstack-body" id="memory-callstack-body"><div class="memory-empty">No callstack selected.</div></div>` +
@@ -339,6 +345,17 @@ export class MemoryView {
 			this.updateFilterButton(name);
 		}
 
+		this.tabButtons = {};
+		this.tabPanels = {};
+		for (const { name } of MemoryView.TAB_DEFS) {
+			const button = this.container.querySelector(`[data-mem-tab="${name}"]`);
+			const panel = this.container.querySelector(`[data-mem-tabpanel="${name}"]`);
+			this.tabButtons[name] = button;
+			this.tabPanels[name] = panel;
+			button.addEventListener("click", () => this.setActiveTable(name));
+		}
+		this.setActiveTable(this.activeTable, /*save=*/ false);
+
 		this.container.addEventListener("keydown", (e) => {
 			if (e.key !== "/" || e.defaultPrevented) {
 				return;
@@ -352,20 +369,39 @@ export class MemoryView {
 			if (activeView && activeView.hidden) {
 				return;
 			}
-			const firstFilter = this.panelRefs.leaks.filter;
-			if (firstFilter) {
-				firstFilter.focus();
-				firstFilter.select();
+			const activeFilter = this.panelRefs[this.activeTable]?.filter;
+			if (activeFilter) {
+				activeFilter.focus();
+				activeFilter.select();
 			}
 		});
 		this.container.tabIndex = -1;
 		this.container.dataset.memoryView = "true";
 	}
 
+	setActiveTable(name, save = true) {
+		if (!this.tabButtons || !this.tabButtons[name]) {
+			return;
+		}
+		this.activeTable = name;
+		for (const { name: tabName } of MemoryView.TAB_DEFS) {
+			const isActive = tabName === name;
+			const button = this.tabButtons[tabName];
+			const panel = this.tabPanels[tabName];
+			button.classList.toggle("active", isActive);
+			button.setAttribute("aria-selected", isActive ? "true" : "false");
+			button.tabIndex = isActive ? 0 : -1;
+			panel.hidden = !isActive;
+		}
+		if (save) {
+			this.saveStateToUrl();
+		}
+	}
+
 	buildPanelMarkup(name, title, subtitle, sortOptions) {
 		const sortHtml = sortOptions.map(([value, label]) => `<option value="${value}">${escapeHtml(label)}</option>`).join("");
 		return `
-			<div class="memory-panel">
+			<div class="memory-panel memory-tabpanel" data-mem-tabpanel="${name}" id="memory-tabpanel-${name}" role="tabpanel" aria-labelledby="memory-tab-${name}" hidden>
 				<div class="memory-panel-header memory-panel-header-wrap">
 					<div>
 						<div class="memory-panel-title">${escapeHtml(title)}</div>
@@ -393,6 +429,10 @@ export class MemoryView {
 		if (histogramMetric === "count" || histogramMetric === "bytes") {
 			this.histogramMetric = histogramMetric;
 		}
+		const activeTable = params.get("mem_table");
+		if (activeTable && this.tableState[activeTable]) {
+			this.activeTable = activeTable;
+		}
 		for (const [name, state] of Object.entries(this.tableState)) {
 			const sortKey = params.get(`mem_${name}_sort`);
 			const groupMode = params.get(`mem_${name}_group`);
@@ -419,6 +459,7 @@ export class MemoryView {
 	saveStateToUrl() {
 		const url = new URL(window.location.href);
 		url.searchParams.set("mem_hist_metric", this.histogramMetric);
+		url.searchParams.set("mem_table", this.activeTable);
 		for (const [name, state] of Object.entries(this.tableState)) {
 			url.searchParams.set(`mem_${name}_sort`, state.sortKey);
 			url.searchParams.set(`mem_${name}_group`, state.groupMode);