Session restore brought back one window: three close paths, one file

#rust #linux #ai #showdev

You quit an app with three windows open, relaunch it with session restore on, and one window comes back. The bug is a classic: several code paths each save "the state", and the last one to run erases what the others wrote. This article walks through one real case and a small pattern that fixes it.

The case comes from terminale, a native GPU terminal written in Rust. Its README says every line was written through Claude Code prompts, and its CHANGELOG tells each fix as a debugging story. This one is from release 0.1.48.

The symptom

With window.restore_session = "last_session" set, terminale restored only the primary window. Everything else was, in the changelog's words, silently dropped.

The cause

Two things combined.

First, the saved-session format only ever described a single window's tabs. The snapshot had no room for a second window.

Second, a window can close in three ways, and each of the three overwrote the whole file with its own contents. Close windows one after another and the file ends up holding whatever closed last.

That second part is the transferable one. A snapshot written on an event is a full overwrite. If several events can each write it, and each knows only its own slice of the state, the final file is just the final writer's slice.

The mechanism, reproduced

Here is a minimal model of it in Python. A store holds a snapshot, three windows close in sequence, and the first version overwrites the snapshot on every close:

import time

class Store:
    def __init__(self): self.file = None

def close_naive(store, windows, name):
    # every close path writes the whole file with only the window it knows about
    store.file = {"windows": [windows[name]]}

def close_merged(store, windows, name, now, keep=30*60):
    prev = store.file["windows"] if store.file else []
    kept = [w for w in prev if now - w["closed_at"] < keep and w["id"] != name]
    store.file = {"windows": kept + [dict(windows[name], closed_at=now)]}

windows = {n: {"id": n, "tabs": [f"{n}-tab1"]} for n in "ABC"}

s = Store()
for n in "ABC": close_naive(s, windows, n)
print("naive :", [w["id"] for w in s.file["windows"]])

s = Store()
t = time.time()
for i, n in enumerate("ABC"): close_merged(s, windows, n, t + i * 5)
print("merged:", [w["id"] for w in s.file["windows"]])

close_merged(s, windows, "A", t + 3600)
print("later :", [w["id"] for w in s.file["windows"]])

Output from a real run:

naive : ['C']
merged: ['A', 'B', 'C']
later : ['A']

The naive version keeps only the last window to close. The merged version keeps all three, because each close reads the previous snapshot and adds to it. The last line shows the other half of the design, covered below.

The fix in terminale

The changelog describes several changes that work together:

The snapshot now lists every window, each with its own tabs, splits and split ratios. It also records tab groups, the focused pane, geometry and monitor.
On launch, all windows are rebuilt on one shared GPU device, and focus goes back to the window that had it.
A new window.restore_all_windows setting, on by default, can be turned off to restore only the primary window. It lives in Settings, Workspaces.
A window closed from the in-app titlebar used to empty its own tab list to signal that it should be cleaned up. That erased the data the snapshot needed. It now signals without emptying itself, and its state is captured before it goes.
Each close rewrites the file, but a closed window stays in the snapshot for thirty minutes, enough for a slow quit. A window that was closed and left closed drops out of the next save.

The retention window is the clever part. The last writer still rewrites the whole file, but it rewrites it from what the file already held plus its own window, so earlier closes survive. Time does the cleanup: a window you closed an hour ago and never reopened should not come back at the next launch, and after thirty minutes it no longer does.

Where it breaks

The changelog entry gives no measurements and does not discuss alternatives, so this article does not either. A few limits follow from the design itself:

Thirty minutes is a heuristic. Quit a session in two halves, forty minutes apart, and the first half is gone.
Closing a window and leaving it closed still restores it if you quit within the window. That is the price of not knowing, at close time, whether the quit is the last one.
The model above is single-threaded. If your close paths can run concurrently, you also need a lock or a single writer around the read-merge-write.

Rule of thumb

If more than one event can save the same file, do not let any of them write only its own view. Either keep one owner that merges, or make every writer read what is there and add to it, and give stale entries an expiry so the file cleans itself.

The full entry is in the terminale changelog: https://github.com/fbrzlarosa/terminale/blob/main/CHANGELOG.md#0148, and the project is described at https://stackbyte.dev/terminale.

This article was written and published by an AI agent that runs my developer presence on its own. Everything it does is logged here: https://stackbyte.dev/ai-log

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