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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vw1sLrSjDksvH5zjU2RMYB
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Krzysztof Rudnicki 2026-07-18 16:15:54 +02:00
parent dbc3242c07
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<div class="wrap">
<h1>Never restart Steam mid-update</h1>
<p class="sub">How one guard fixed a deterministic game-crash bug in the Steam Backlog Enforcer · commit <code>dbc3242</code> · 2026-07-18</p>
<div class="toc">
<b>Contents</b>
<ol>
<li><a href="#background">Background — the enforcer and why it restarts Steam</a></li>
<li><a href="#intuition">Intuition — a restart mid-download is a shredder</a></li>
<li><a href="#code">Code — the guard, the scan, and one leaf module</a></li>
<li><a href="#quiz">Quiz</a></li>
</ol>
</div>
<h2 id="background">1 · Background</h2>
<p>
The <b>Steam Backlog Enforcer</b> is a small systemd daemon whose one job is discipline: out of a
library of ~1,370 games, exactly one — the <i>assigned</i> game — may be visible and installed.
Everything else is hidden and, optionally, uninstalled. It runs an enforce loop every 3 seconds.
</p>
<p>
To hide games it can't just edit a file — Steam's library is a live web app. So the enforcer drives
Steam over the <b>Chrome DevTools Protocol (CDP)</b>: it talks to the Steam client's embedded browser
on a debug port. That port only exists if Steam was launched with a special flag. So the enforcer has
a helper, <code>ensure_steam_debug_port()</code>, that guarantees the port is open — and if Steam is
running <i>without</i> it, the helper <b>restarts Steam</b>: <code>steam -shutdown</code>, then relaunch
with the debug flag.
</p>
<p>
Here is the piece that matters, before the change. If Steam is up but the port is closed, it is
bounced — unconditionally:
</p>
<pre><span class="kw">def</span> ensure_steam_debug_port() -&gt; <span class="kw">None</span>:
<span class="kw">if</span> _steam_has_debug_port():
<span class="kw">return</span> <span class="com"># port already open — nothing to do</span>
...
<span class="kw">if</span> _is_steam_running():
_shutdown_steam() <span class="com"># &lt;-- bounce Steam, whatever it's doing</span>
_launch_steam_with_debug()</pre>
<p>
The trouble: Steam is also a <b>download manager</b>. At any moment it may be pulling a multi-gigabyte
game update — first downloading chunks, then <i>committing</i> them to disk (the <code>staging</code>
phase). <code>steam -shutdown</code> suspends whatever transfer is in flight.
</p>
<div class="note">
A one-off suspend-and-resume is usually harmless — Steam is built to resume downloads. The danger is
<b>repetition</b>. A daemon that bounces Steam every time it wants the CDP port, while a big update is
trying to finish, interrupts that update over and over — including mid-commit.
</div>
<h2 id="intuition">2 · Intuition</h2>
<p>
Picture the assigned game (Age of Empires II DE) receiving a ~16&nbsp;GB update the same morning the
enforcer's restart logic started actually working. The two fight for hours. Steam's own log tells the
story — the update is cancelled and restarted again and again, several times <i>while committing files
to disk</i>:
</p>
<div class="tl">
<span class="t">06:29</span><span class="e">update canceled : <b>Disabled</b> (Suspended) &nbsp;<span class="arrow">←</span> steam -shutdown</span>
<span class="t">07:15</span><span class="e">update canceled : Disabled (Suspended)</span>
<span class="t">07:21</span><span class="e">App update changed : Running Update, <b>Committing</b> &nbsp;<span class="arrow">←</span> writing files</span>
<span class="t">09:22</span><span class="e">update canceled : Priority (Suspended)</span>
<span class="t">10:00</span><span class="e">update canceled : <b>Restart</b> (Suspended) &nbsp;<span class="arrow">←</span> Steam bounced again</span>
</div>
<p>
Interrupt a commit halfway and the install is left inconsistent — some files from the new build, some
from the old, some half-written. The cruel part: Steam's <b>"Verify integrity"</b> reported all 350k
files "OK", so the corruption was invisible. But the game ships with <b>Arxan anti-tamper</b>, which
checks the code against itself at startup, spots the mismatch, and kills the process — a deterministic
<code>0xc0000005</code> null-dereference crash at the exact same address, every launch. Environment,
Proton version, GPU, kernel, prefix — none of it mattered, because the broken bytes were the one
constant. Only a full reinstall fixed it.
</p>
<h3>Before vs after — the decision</h3>
<div class="flow">
<span class="box">enforcer wants CDP port</span>
<span class="arrow">→</span>
<span class="box bad">Steam bounced mid-update</span>
<span class="arrow">→</span>
<span class="box bad">install corrupted → crash</span>
</div>
<div class="flow">
<span class="box">enforcer wants CDP port</span>
<span class="arrow">→</span>
<span class="box warn">update in progress? → <b>defer</b></span>
<span class="arrow">→</span>
<span class="box good">retry in 3s, restart once idle</span>
</div>
<p>
The fix is a single question asked <i>before</i> the restart: <b>is any game mid-transfer right now?</b>
If yes, don't touch Steam — skip this pass and try again in 3 seconds. The library stays un-hidden a
little longer; the install stays intact. That trade is the whole change.
</p>
<h3>How "mid-transfer" is detected</h3>
<p>
Steam records live progress inside each <code>appmanifest_&lt;id&gt;.acf</code>. A game is still working
if it has bytes left to download <i>or</i> bytes left to stage (commit). When both totals equal their
progress, it's idle. That's the entire rule:
</p>
<div class="bars" id="demoBars"></div>
<p style="text-align:center"><span id="demoVerdict"></span></p>
<div class="w">
<h3 style="margin-top:0">▶ Play: the manifest scan</h3>
<p style="font-size:.9rem;margin-top:.2rem">Drag the progress values. The verdict updates with the exact
logic of <code>_manifest_transfer_active()</code>: active when <code>ToDownload&gt;Downloaded</code>
<b>or</b> <code>ToStage&gt;Staged</code>.</p>
<div class="sliders" id="sliders"></div>
<div class="verdict" id="scanVerdict"></div>
</div>
<div class="w">
<h3 style="margin-top:0">▶ Play: should the enforcer restart Steam?</h3>
<p style="font-size:.9rem;margin-top:.2rem">Toggle the world state. The verdict runs the new
<code>ensure_steam_debug_port()</code> decision tree.</p>
<div class="toggles" id="toggles"></div>
<div class="verdict" id="restartVerdict"></div>
</div>
<h2 id="code">3 · Code</h2>
<h3>A. The scan — a new leaf module</h3>
<p>
The detector needs the Steam library path, which lived in <code>game_install.py</code>. But the guard
needs to run inside <code>library_hider.py</code> — and <code>game_install</code> already imports
<code>library_hider</code>. Importing the other way would form a cycle. So the scan and the
<code>STEAMAPPS_PATH</code> constant moved to a dependency-free <b>leaf module</b>,
<code>_steam_state.py</code>, that both can import safely:
</p>
<pre><span class="com"># _steam_state.py — imports only the stdlib, so no import cycle</span>
STEAMAPPS_PATH = Path(<span class="str">"~/.local/share/Steam/steamapps"</span>).expanduser()
<span class="kw">def</span> _manifest_transfer_active(manifest: Path) -&gt; <span class="kw">bool</span>:
<span class="com"># BytesTo* are totals; *Downloaded/*Staged are progress.</span>
<span class="kw">return</span> (
_field(<span class="str">"BytesToDownload"</span>) &gt; _field(<span class="str">"BytesDownloaded"</span>)
<span class="kw">or</span> _field(<span class="str">"BytesToStage"</span>) &gt; _field(<span class="str">"BytesStaged"</span>)
)
<span class="kw">def</span> steam_update_in_progress() -&gt; <span class="kw">bool</span>:
<span class="kw">return</span> <span class="kw">any</span>(_manifest_transfer_active(m)
<span class="kw">for</span> m <span class="kw">in</span> STEAMAPPS_PATH.glob(<span class="str">"appmanifest_*.acf"</span>))</pre>
<p>
Using the byte totals (not <code>StateFlags</code>) is deliberate: <code>BytesToStage &gt; BytesStaged</code>
is exactly the <b>commit-to-disk</b> phase — the most dangerous moment to interrupt.
</p>
<h3>B. The guard — defer instead of bounce</h3>
<p>
In the daemon path, the check goes right before the shutdown. It raises a new
<code>SteamUpdateInProgressError</code> — deliberately a <b>subclass</b> of the existing
<code>SteamUnavailableError</code>, so the enforce loop's current handler already treats it as
"skip this pass, retry next tick." No new error-handling was needed anywhere.
</p>
<pre> <span class="kw">if</span> _is_steam_running():
<span class="kw">if</span> steam_update_in_progress():
logger.info(<span class="str">"Deferring Steam restart: a game update is in progress..."</span>)
<span class="kw">raise</span> SteamUpdateInProgressError(msg) <span class="com"># caught -&gt; skip + retry in 3s</span>
_shutdown_steam()
_launch_steam_with_debug()</pre>
<p>
The interactive "buy DLC → re-block store → restart Steam" path (<code>restart_steam()</code>) got the
same guard, but since it isn't inside the retry loop it simply <b>logs a warning and skips</b> the
restart rather than raising.
</p>
<h3>C. Wiring &amp; tests</h3>
<p>
<code>game_install.py</code> now imports <code>STEAMAPPS_PATH</code> from the leaf (single source of
truth). The test <code>conftest.py</code> redirects <i>both</i> bindings of the path to a temp dir —
the repo's established pattern, since each module holds its own import. A new
<code>test_update_restart_guard.py</code> adds 13 tests (manifest active/idle/complete/unreadable, the
scan, and both guard paths), keeping the suite at 988 passing / 100% coverage.
</p>
<div class="note">
Verified live: restarting the real daemon while a runtime update was mid-flight produced
<code>Deferring Steam restart: a game update is in progress</code> in the journal — the enforcer chose
not to bounce Steam, exactly as designed.
</div>
<h2 id="quiz">4 · Quiz</h2>
<p class="sub">Five questions on the <i>reasoning</i>. Click to reveal.</p>
<div id="quiz"></div>
<div class="score" id="score">Score: 0 / 0</div>
</div>
<script>
/* ---------- manifest scan demo (static diagram, live) ---------- */
function activeCheck(s){ return (s.toDl > s.dled) || (s.toStg > s.stg); }
function renderBars(host, s){
const rows = [
["Downloaded", s.dled, s.toDl, "download"],
["Staged (commit)", s.stg, s.toStg, "stage"],
];
host.innerHTML = rows.map(function(r){
var pct = r[2] ? Math.round(100*r[1]/r[2]) : 100;
return '<div class="bar"><span>'+r[0]+'</span><div class="track"><div class="fill" style="width:'+pct+'%"></div></div></div>'
+ '<div class="bar"><span></span><span style="font-size:.75rem;color:var(--muted)">'+r[1].toLocaleString()+' / '+r[2].toLocaleString()+' bytes</span></div>';
}).join("");
}
var demoState = { toDl: 17324939056, dled: 9958785856, toStg: 28136876093, stg: 15748472761 };
(function(){
var host = document.getElementById("demoBars");
renderBars(host, demoState);
var act = activeCheck(demoState);
document.getElementById("demoVerdict").innerHTML = act
? '<span class="pill active">UPDATE IN PROGRESS — enforcer will defer</span>'
: '<span class="pill idle">idle — safe to restart</span>';
})();
/* ---------- interactive: manifest sliders ---------- */
var scan = { toDl: 100, dled: 40, toStg: 100, stg: 100 };
var defs = [["toDl","BytesToDownload"],["dled","BytesDownloaded"],["toStg","BytesToStage"],["stg","BytesStaged"]];
(function(){
var host = document.getElementById("sliders");
host.innerHTML = defs.map(function(d){
return '<label><span>'+d[1]+'</span><input type="range" min="0" max="100" value="'+scan[d[0]]+'" data-k="'+d[0]+'"><span class="val" data-v="'+d[0]+'">'+scan[d[0]]+'</span></label>';
}).join("");
function upd(){
var active = activeCheck(scan);
var v = document.getElementById("scanVerdict");
v.className = "verdict " + (active ? "v-defer" : "v-restart");
v.innerHTML = active
? '⏸ <b>_manifest_transfer_active → True</b><small>ToDownload&gt;Downloaded ('+(scan.toDl>scan.dled)+') or ToStage&gt;Staged ('+(scan.toStg>scan.stg)+'). Steam is mid-transfer — the enforcer defers.</small>'
: '✔ <b>_manifest_transfer_active → False</b><small>Both totals reached. Nothing in flight — safe to restart Steam.</small>';
}
host.addEventListener("input", function(e){
var k = e.target.getAttribute("data-k"); if(!k) return;
scan[k] = +e.target.value;
document.querySelector('[data-v="'+k+'"]').textContent = scan[k];
upd();
});
upd();
})();
/* ---------- interactive: restart decision tree ---------- */
var world = { cdpOpen: false, steamRunning: true, updating: true };
var tdefs = [["cdpOpen","CDP port already open"],["steamRunning","Steam is running"],["updating","a game update is in progress"]];
(function(){
var host = document.getElementById("toggles");
host.innerHTML = tdefs.map(function(d){
return '<button class="tg" data-k="'+d[0]+'" aria-pressed="'+world[d[0]]+'">'+d[1]+'</button>';
}).join("");
function decide(){
// mirrors ensure_steam_debug_port()
if (world.cdpOpen) return ["v-noop","· No action","Port already open — the helper returns immediately; Steam is never touched."];
if (world.steamRunning && world.updating) return ["v-defer","⏸ Defer","Steam is up, the port is closed, but a game is mid-update — raise SteamUpdateInProgressError, skip, retry in 3s."];
if (world.steamRunning) return ["v-restart","↻ Restart Steam","Port closed, nothing downloading — shutdown + relaunch with the debug flag. Safe."];
return ["v-restart","▶ Launch Steam","Steam isn't running — just launch it with the debug flag (a fresh launch resumes any paused update, it doesn't interrupt one)."];
}
function upd(){
var d = decide();
var v = document.getElementById("restartVerdict");
v.className = "verdict " + d[0];
v.innerHTML = "<b>"+d[1]+"</b><small>"+d[2]+"</small>";
}
host.addEventListener("click", function(e){
var k = e.target.getAttribute("data-k"); if(!k) return;
world[k] = !world[k];
e.target.setAttribute("aria-pressed", world[k]);
upd();
});
upd();
})();
/* ---------- quiz ---------- */
var QUIZ = [
{
q: "Why does bouncing Steam during an update corrupt the install rather than just pausing it?",
opts: [
"Shutting the client down cleanly always deletes partial download chunks",
"Repeated interruptions mid commit-to-disk leave files half-written",
"The CDP debug port holds a lock that blocks Steam's resume bookkeeping",
"A resumed download pulls chunks from a newer build than the paused one"
],
correct: 1,
rat: "A single suspend resumes fine; the damage is repeated interruption mid-commit, leaving an inconsistent mix of old, new, and half-written files."
},
{
q: "Why is SteamUpdateInProgressError made a subclass of SteamUnavailableError?",
opts: [
"It lets the two errors share one journal log-message string constant",
"The two errors happen to be raised from the very same line of code",
"The enforce loop's existing handler catches it and skips the pass",
"It makes the new exception appear in mypy's error-coverage output"
],
correct: 2,
rat: "The loop already treats SteamUnavailableError as 'no library to hide right now — skip this pass.' Subclassing means a deferral needs no new handling."
},
{
q: "How does the code decide that an update is in progress?",
opts: [
"It asks the already-running Steam client over its CDP debug port",
"It checks whether the steamwebhelper process is using the network",
"It reads StateFlags and calls any value other than 4 an update",
"It scans appmanifests for bytes still left to download or stage"
],
correct: 3,
rat: "It reads the live byte totals from each appmanifest. Bytes-left-to-stage is what catches the dangerous commit-to-disk phase."
},
{
q: "Why was the scan placed in a new _steam_state module instead of in game_install or library_hider?",
opts: [
"game_install and library_hider import each other; a shared leaf breaks the cycle",
"Leaf modules are exempt from the repository's strict 100% coverage requirement",
"The systemd service can then import the scan without loading the full enforcer package",
"The tests can monkeypatch the scan without importing either of the big modules"
],
correct: 0,
rat: "game_install already imports library_hider; importing back would cycle. A stdlib-only leaf that both import (and that owns STEAMAPPS_PATH) breaks the knot."
},
{
q: "An update is running and the enforcer finds the CDP port closed. What happens to library-hiding?",
opts: [
"Steam is force-killed and relaunched, and hiding proceeds right after",
"The enforcer blocks in place and waits until the whole update finishes",
"It's skipped this pass; games stay visible until a later idle tick",
"The daemon disables itself so it won't disturb the download any further"
],
correct: 2,
rat: "Deferring postpones the restart, and therefore the hide. The loop keeps running every 3s and hides once no transfer is in flight — 'visible a bit longer beats corrupted.'"
}
];
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6
docs/explanations/index.html
View File

@ -30,12 +30,16 @@
<div class="wrap">
<div class="kicker">steam-backlog-enforcer</div>
<h1>Change explanations</h1>
<p class="lead">1 explanation · interactive walkthroughs of notable changes, newest first.</p>
<p class="lead">2 explanations · interactive walkthroughs of notable changes, newest first.</p>
<div class="grid">
<a class="card" href="2026-07-18-explanation-steam-cdp-restart-fix.html">
<span class="date">2026-07-18</span>
<span class="title">Fix: Steam CDP restart never worked</span>
</a>
<a class="card" href="2026-07-18-explanation-defer-steam-restart-during-update.html">
<span class="date">2026-07-18</span>
<span class="title">Defer Steam restart while a game update is in progress</span>
</a>
</div>
<footer>Generated by the <code>explain-diff</code> skill. Open any card to replay its walkthrough.</footer>
</div>