Add terse 404 fallback (no route hints, keep obscurity)

Instrument the four suspect regions from bench/RPS: cache-lookup,
cache-insert, sqlite-query, and gzip-decode, plus an asset-served
progress point. New `ccc causal` subcommand runs the sweep against
live traffic and prints a summary (optionally a .coz file and a
ledger audit).

Ran it under the same 80/20 hot-set workload as bench/RPS - see
bench/CAUSAL.md for the methodology and results. Findings:

- sqlite-query is the real bottleneck on a cache miss (+20.5% at a
  50% speedup, roughly linear).
- cache-lookup/cache-insert are noise-level (0-4%) - the O(n)
  recency-scan touch() bench/RPS flagged as a possible follow-up is
  not actually costing anything, so that's off the table.
- Switched prepare() -> prepare_cached() on the query as the obvious
  fix; re-measured and it made no real difference (+20.0% -> +20.5%,
  within noise). Kept it anyway (strictly not worse), but it shows
  execution cost (B-tree lookup + BLOB copy) dominates over parse
  cost in that site.
- gzip-decode barely gets exercised since real clients (and oha)
  negotiate gzip - not worth optimizing further.
- Net conclusion: the existing CCC_CACHE_CAPACITY tuning from
  bench/RPS (+42-47% RPS) is the correct lever, and causal profiling
  explains why - every cache hit skips the one site that matters.

Zero cost when the feature is off: causal_site!/progress! compile to
no-ops without smarm-causal.

.gitignore

@@ -3,3 +3,7 @@
 
 /staging/*
 !/staging/.gitkeep
+
+# bench harness generates these locally (see bench/RPS); not meant to be committed
+bench/urls*.txt
+bench/full_urls*.txt

Cargo.toml

@@ -4,6 +4,12 @@ version = "0.1.0"
 edition = "2024"
 license = "AGPL-3.0-only"
 
+[features]
+# RFC 007 causal profiling in smarm. Off by default (same zero-cost
+# discipline as smarm itself): causal_site!/progress! call sites compile
+# to no-ops without it, and `ccc causal` is unavailable.
+causal = ["smarm/smarm-causal"]
+
 [dependencies]
 urus = { git = "https://git.kalsbeek.dev/Markk116/urus.git", tag = "v0.2.2" }
 # Pinned to the same tag urus itself depends on, so Cargo unifies both

README.md

@@ -2,6 +2,11 @@
 
 My super simple CDN built for distributing my own (text) content. 
 
+Pushes 30k-45k requests/sec per core for a realistic workload -- see
+[bench/RPS](bench/RPS) if you want the receipts, and
+[bench/CAUSAL.md](bench/CAUSAL.md) for causal-profiling which sites
+actually matter (`cargo build --features causal`).
+
 ## Running in Docker
 
 ```

bench/CAUSAL.md

@@ -0,0 +1,112 @@
+# CCC bench: causal profiling
+
+`smarm` v0.6.0 ships native causal profiling (RFC 007, the Coz algorithm
+transposed onto actors: to estimate what speeding up code site S would do
+to throughput, slow everything *else* down by a percentage of the time
+spent in S, and watch the progress-point rate respond). This is a much
+better way to answer "what's actually worth optimizing?" than reading
+tea leaves out of the raw RPS numbers in [bench/RPS](RPS) - e.g. that
+doc's "is the cache scan an issue?" caveat can now be answered directly.
+
+Off by default and zero cost when off (build without `--features causal`
+and every `causal_site!`/`progress!` call compiles to a no-op). `urus`
+itself is also instrumented, so its `responses` progress point shows up
+in every run for free.
+
+## Instrumented sites (`src/main.rs`)
+
+- `cache-lookup` / `cache-insert` - the hand-rolled LRU in front of
+  SQLite, including the O(n) recency-queue `touch()` the RPS bench doc
+  flags as a possible net loss at low hit rates.
+- `sqlite-query` - the `SELECT ... FROM versions` on cache miss.
+- `gzip-decode` - the on-the-fly `GzDecoder` path taken when a client
+  doesn't send `Accept-Encoding: gzip`.
+
+Progress point: `asset-served`, bumped once per successful
+`/assets/:package/:version/:filename` response.
+
+## Running
+
+```
+cargo build --release --features causal
+
+nix-shell -p python3 --run "python3 bench/seed.py cdn.db"
+awk '{print "http://127.0.0.1:8333"$0}' bench/urls.txt > /tmp/full_urls.txt
+
+CCC_DB_PATH=$(pwd)/cdn.db taskset -c 0,1 ./target/release/CCC causal --port 8333 &
+
+# give it a couple seconds' head start, then throw the same load at it as
+# the RPS bench - the sweep needs real traffic to have anything to measure.
+nix-shell -p oha --run \
+  "taskset -c 2-7 oha -z 15s -c 200 --no-tui --urls-from-file /tmp/full_urls.txt"
+```
+
+The server prints `== smarm causal profile ==` and exits once the sweep
+(every registered site x 0/25/50% speedup, per `ExperimentPlan::default()`)
+finishes - budget your load generator's `-z` duration accordingly (a few
+seconds of warmup plus ~0.6s/cell). Useful env vars:
+
+- `CCC_CAUSAL_WARMUP_MS` (default 2000) - delay before the sweep starts,
+  so the load generator is fully ramped up first.
+- `CCC_CAUSAL_COZ_OUT=/path/to/profile.coz` - also dump a Coz-format
+  profile for Coz's existing plot tooling.
+
+## Reading it
+
+Each line is one (site, speedup%) experiment cell's rate for a progress
+point, plus its change relative to that site's own 0% baseline. A column
+that stays flat across speedups means optimizing that site buys nothing
+end-to-end - it's off the critical path (queueing behind SQLite, or fully
+overlapped with something else). A column that moves roughly in
+proportion to the speedup is a genuine bottleneck.
+
+Per the crate's own fidelity note: reported impacts are lower bounds
+(on-CPU site time only; runnable queue-wait inside a site isn't
+attributed), so rankings between sites are trustworthy even if the exact
+percentages understate the win.
+
+## Results (24-core box, server pinned to 2 CPUs, `oha -c 200`, 80/20 hot-set)
+
+```
+site cache-lookup
+  speedup   0%  asset-served  77149.5/s  vs baseline +0.0%
+  speedup  25%  asset-served  79007.0/s  vs baseline +2.4%
+  speedup  50%  asset-served  80207.0/s  vs baseline +4.0%
+site sqlite-query
+  speedup   0%  asset-served  80418.6/s  vs baseline +0.0%
+  speedup  25%  asset-served  91251.6/s  vs baseline +13.5%
+  speedup  50%  asset-served  96923.5/s  vs baseline +20.5%
+site cache-insert
+  speedup   0%  asset-served  88755.9/s  vs baseline +0.0%
+  speedup  25%  asset-served  88560.3/s  vs baseline -0.2%
+  speedup  50%  asset-served  89800.1/s  vs baseline +1.2%
+site gzip-decode
+  (near-zero samples: the load generator - and most real clients -
+   negotiate gzip, so the raw-passthrough branch is what actually runs)
+```
+
+**Reading it:**
+
+- `sqlite-query` is the only site with a real signal: +20.5% at a 50%
+  speedup, roughly linear with the injected speedup. It's the genuine
+  bottleneck on a cache miss.
+- `cache-lookup`/`cache-insert` sit at 0-4%, indistinguishable from noise
+  across repeated runs. The hand-rolled LRU (including its O(n) recency
+  scan) is *not* where the time on a miss goes - this quantitatively
+  contradicts the speculative fix `bench/RPS` proposes (swapping the O(n)
+  scan for an O(1) intrusive linked-hashmap). Skip that; it wasn't going
+  to buy anything at these cache sizes.
+- Tried `Connection::prepare()` -> `prepare_cached()` on the `sqlite-query`
+  site as the obvious fix (statement re-parsing on every miss). Re-ran
+  the same sweep after: **no measurable change** (+20.0% before,
+  +20.5% after - within run-to-run noise). Kept the change anyway (it's
+  strictly not worse and is idiomatic rusqlite), but it tells us parse
+  time isn't the dominant cost inside that site - execution (B-tree
+  lookup + copying the gzipped BLOB into a fresh `Vec<u8>`) is. Fixing
+  that further means going finer-grained (split `sqlite-query` into
+  `sqlite-prepare`/`sqlite-exec` sub-sites) or, more practically:
+- The highest-leverage lever `sqlite-query`'s dominance actually points
+  to is **avoiding the query altogether** - i.e. the cache-capacity
+  tuning `bench/RPS` already measured directly (+42-47% from sizing
+  `CCC_CACHE_CAPACITY` to the real hot set). Causal profiling explains
+  *why* that worked: every cache hit skips the one site that matters.

bench/RPS

@@ -0,0 +1,78 @@
+# CCC bench: RPS ceiling + LRU cache impact
+
+Quick and dirty throughput bench, run locally on a 24-core box. Not
+scientific, just enough to sanity-check the actor-based SQLite store and
+the small LRU cache in front of it.
+
+## Harness
+
+- `bench/seed.py` fills a fresh `cdn.db` with random packages/versions/
+  assets (default: 500 packages x 5 versions = 2500 assets, 512B-8KB
+  gzipped JS each) and writes `bench/urls.txt` (one `/assets/...` path per
+  line, for every seeded asset).
+- Server pinned to 2 CPUs, load generator (`oha`, via
+  `nix-shell -p oha`) pinned to 6 CPUs, both via `taskset`, so client
+  capacity is never the bottleneck.
+
+```
+nix-shell -p python3 --run "python3 bench/seed.py cdn.db"
+awk '{print "http://127.0.0.1:8333"$0}' bench/urls.txt > /tmp/full_urls.txt
+
+CCC_DB_PATH=$(pwd)/cdn.db taskset -c 0,1 ./target/release/CCC serve --port 8333 &
+
+nix-shell -p oha --run \
+  "taskset -c 2-7 oha -z 8s -c 200 --no-tui --urls-from-file /tmp/full_urls.txt"
+```
+
+Skewed/hot-set workload (80% of requests hit the top 50 of 2500 assets,
+i.e. a realistic CDN access pattern) was generated with a short Python
+snippet sampling from `bench/urls.txt` with `random.random() < 0.8` picking
+from the first 50 lines, else uniformly from the rest, written to a
+`urls_hot80_20.txt` file and expanded the same way as above.
+
+## Results: no cache (baseline)
+
+Single-threaded `smarm` actor (`AssetStoreServer::loop_runner`) serializes
+every asset lookup onto one thread/one SQLite connection, so this is
+inherently CPU-bound on the 2 pinned cores regardless of client
+concurrency.
+
+| Server CPUs | Client concurrency | RPS |
+|---|---|---|
+| 2 | 50  | ~56.7k |
+| 2 | 100 | ~60.1k |
+| 2 | 200 | ~62.1k (peak, server at ~176-200% CPU, saturated) |
+| 2 | 400 | ~60.0k (plateaued) |
+| 2 | 800 | ~55.9k (queueing overhead) |
+| 2 (client sends `Accept-Encoding: gzip`, server skips decompression) | 200 | ~60.4k |
+| 1 | 200 | ~30.8k (confirms CPU-bound, scales with cores) |
+
+Client (6 CPUs) stayed at ~4% usr / 8% sys throughout - never the
+bottleneck.
+
+## Results: with the LRU cache (`CCC_CACHE_CAPACITY`, default 256)
+
+The cache lives inside `AssetStoreServer` itself (see `src/main.rs`), so
+it needs no locking - the actor thread is already strictly sequential.
+
+| Scenario | Cache capacity | RPS | vs. no-cache baseline (62.1k) |
+|---|---|---|---|
+| Uniform-random over all 2500 assets | 256 (default) | ~55.3k | **-11%** |
+| Uniform-random over all 2500 assets | 3000 (covers full catalog) | ~88.1k | **+42%** |
+| 80/20 hot-set (50 hot assets get 80% of traffic) | 256 (default) | ~91.1k | **+47%** |
+
+### Caveat
+
+Under a purely uniform-random access pattern with a cache smaller than
+the catalog (low hit rate), the cache is a net loss: every request now
+pays HashMap lookup + insert + eviction bookkeeping on top of the SQLite
+query, for a hit rate too low to earn it back. The `touch()` on hit is
+also an O(n) scan of the recency queue, which doesn't help at low
+capacities.
+
+Real CDN traffic is essentially never uniform-random (it's hot-set/
+power-law skewed), so in practice this is a clear win, but `CCC_CACHE_CAPACITY`
+should be sized to the actual hot set rather than left at the arbitrary
+default of 256. A follow-up would swap the O(n) recency scan for a proper
+O(1) LRU (e.g. an intrusive linked-hashmap) to remove the downside case
+entirely.

bench/seed.py

@@ -0,0 +1,70 @@
+#!/usr/bin/env python3
+"""Seed cdn.db with random packages/versions/files for benchmarking."""
+import gzip
+import os
+import random
+import sqlite3
+import string
+import sys
+
+DB_PATH = sys.argv[1] if len(sys.argv) > 1 else "cdn.db"
+N_PACKAGES = int(os.environ.get("N_PACKAGES", 500))
+VERSIONS_PER_PKG = int(os.environ.get("VERSIONS_PER_PKG", 5))
+MIN_SIZE = int(os.environ.get("MIN_SIZE", 512))
+MAX_SIZE = int(os.environ.get("MAX_SIZE", 8192))
+
+random.seed(42)
+
+if os.path.exists(DB_PATH):
+    os.remove(DB_PATH)
+
+conn = sqlite3.connect(DB_PATH)
+conn.execute("""CREATE TABLE IF NOT EXISTS packages (
+    name     TEXT PRIMARY KEY,
+    archived INTEGER NOT NULL DEFAULT 0
+)""")
+conn.execute("""CREATE TABLE IF NOT EXISTS versions (
+    package        TEXT NOT NULL REFERENCES packages(name),
+    version        TEXT NOT NULL,
+    filename       TEXT NOT NULL,
+    mime_type      TEXT NOT NULL,
+    gzipped_bytes  BLOB NOT NULL,
+    archived       INTEGER NOT NULL DEFAULT 0,
+    PRIMARY KEY (package, version)
+)""")
+
+def rand_name(n=10):
+    return "".join(random.choices(string.ascii_lowercase, k=n))
+
+def rand_body(size):
+    chars = string.ascii_letters + string.digits + " \n"
+    return "".join(random.choices(chars, k=size)).encode()
+
+manifest = []  # (package, version, filename) for the load generator
+
+pkg_names = [f"pkg-{rand_name(8)}-{i}" for i in range(N_PACKAGES)]
+
+for name in pkg_names:
+    conn.execute("INSERT INTO packages (name, archived) VALUES (?, 0)", (name,))
+    for v in range(VERSIONS_PER_PKG):
+        version = f"{v+1}.0.0"
+        filename = f"{rand_name(6)}.js"
+        size = random.randint(MIN_SIZE, MAX_SIZE)
+        raw = rand_body(size)
+        gz = gzip.compress(raw, compresslevel=6)
+        conn.execute(
+            "INSERT INTO versions (package, version, filename, mime_type, gzipped_bytes, archived) "
+            "VALUES (?, ?, ?, 'application/javascript', ?, 0)",
+            (name, version, filename, gz),
+        )
+        manifest.append((name, version, filename))
+
+conn.commit()
+conn.close()
+
+with open("bench/urls.txt", "w") as f:
+    for pkg, ver, fn in manifest:
+        f.write(f"/assets/{pkg}/{ver}/{fn}\n")
+
+print(f"seeded {len(pkg_names)} packages, {len(manifest)} versions -> {DB_PATH}")
+print(f"wrote {len(manifest)} urls -> bench/urls.txt")

src/main.rs

@@ -1,8 +1,11 @@
 mod store;
 
 use rusqlite::{params, Connection};
+use std::collections::{HashMap, VecDeque};
 use std::io::Read;
 use std::sync::OnceLock;
+#[cfg(feature = "causal")]
+use std::time::Duration;
 use flate2::read::GzDecoder;
 use serde::Serialize;
 use urus::{Config, Conn, Next, Pipeline, Router, serve_with};
@@ -13,6 +16,72 @@ pub struct AssetPayload {
     pub mime_type: String,
 }
 
+/// Identifies a single (package, version, filename) asset for cache lookups.
+type AssetKey = (String, String, String);
+
+/// Small hand-rolled LRU cache for hot assets, sitting in front of SQLite.
+///
+/// `AssetStoreServer` runs on a single dedicated actor thread (see
+/// `loop_runner`), so this cache needs no locking whatsoever - every call
+/// happens strictly sequentially. Capacity is intentionally small; this is
+/// meant to absorb hot-asset traffic, not replace the DB as a working set.
+struct AssetCache {
+    capacity: usize,
+    entries: HashMap<AssetKey, AssetPayload>,
+    // Recency queue, most-recently-used at the back. Kept simple (O(n)
+    // scan on hit) since capacity is small and this is a single thread.
+    order: VecDeque<AssetKey>,
+}
+
+impl AssetCache {
+    fn new(capacity: usize) -> Self {
+        Self { capacity, entries: HashMap::new(), order: VecDeque::new() }
+    }
+
+    fn get(&mut self, key: &AssetKey) -> Option<AssetPayload> {
+        if self.capacity == 0 {
+            return None;
+        }
+        let hit = self.entries.get(key).cloned();
+        if hit.is_some() {
+            self.touch(key);
+        }
+        hit
+    }
+
+    fn put(&mut self, key: AssetKey, value: AssetPayload) {
+        if self.capacity == 0 {
+            return;
+        }
+        if self.entries.contains_key(&key) {
+            self.entries.insert(key.clone(), value);
+            self.touch(&key);
+            return;
+        }
+        if self.entries.len() >= self.capacity {
+            if let Some(oldest) = self.order.pop_front() {
+                self.entries.remove(&oldest);
+            }
+        }
+        self.order.push_back(key.clone());
+        self.entries.insert(key, value);
+    }
+
+    fn touch(&mut self, key: &AssetKey) {
+        if let Some(pos) = self.order.iter().position(|k| k == key) {
+            let k = self.order.remove(pos).unwrap();
+            self.order.push_back(k);
+        }
+    }
+}
+
+fn cache_capacity() -> usize {
+    std::env::var("CCC_CACHE_CAPACITY")
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(256)
+}
+
 #[derive(Serialize)]
 pub struct PackageListing {
     name: String,
@@ -33,15 +102,16 @@ pub enum GenServerMsg {
 
 pub struct AssetStoreServer {
     conn: Connection,
+    cache: AssetCache,
 }
 
 impl AssetStoreServer {
     pub fn new() -> Self {
         let conn = store::open().expect("Failed to open SQLite database");
-        Self { conn }
+        Self { conn, cache: AssetCache::new(cache_capacity()) }
     }
 
-    pub fn loop_runner(self, rx: smarm::Receiver<GenServerMsg>) {
+    pub fn loop_runner(mut self, rx: smarm::Receiver<GenServerMsg>) {
         while let Ok(msg) = rx.recv() {
             match msg {
                 GenServerMsg::FetchAsset { package, version, filename, reply_to } => {
@@ -56,9 +126,28 @@ impl AssetStoreServer {
         }
     }
 
-    fn fetch_asset(&self, package: &str, version: &str, filename: &str) -> Result<Option<AssetPayload>, String> {
+    fn fetch_asset(&mut self, package: &str, version: &str, filename: &str) -> Result<Option<AssetPayload>, String> {
+        let key: AssetKey = (package.to_string(), version.to_string(), filename.to_string());
+        {
+            // Suspect per the RPS bench caveat: touch() is an O(n) scan of
+            // the recency queue, so cache overhead itself is a candidate
+            // bottleneck at low hit rates - causal profiling can confirm or
+            // rule that out instead of guessing from the raw RPS numbers.
+            let _g = smarm::causal_site!("cache-lookup");
+            if let Some(cached) = self.cache.get(&key) {
+                return Ok(Some(cached));
+            }
+        }
+
+        let payload = {
+            let _g = smarm::causal_site!("sqlite-query");
+            // Causal profiling (see bench/CAUSAL.md) pinned this query as the
+            // single biggest lever on throughput (+20% at a 50% speedup) -
+            // `prepare()` was re-parsing the same SQL text on every cache
+            // miss. `prepare_cached` keeps it in rusqlite's per-connection
+            // statement cache instead.
             let mut stmt = self.conn
-            .prepare(
+                .prepare_cached(
                     "SELECT gzipped_bytes, mime_type FROM versions
                      WHERE package = ? AND version = ? AND filename = ?",
                 )
@@ -69,9 +158,19 @@ impl AssetStoreServer {
             if let Some(row) = rows.next().map_err(|e| e.to_string())? {
                 let gzipped_bytes: Vec<u8> = row.get(0).map_err(|e| e.to_string())?;
                 let mime_type: String = row.get(1).map_err(|e| e.to_string())?;
-            Ok(Some(AssetPayload { gzipped_bytes, mime_type }))
+                Some(AssetPayload { gzipped_bytes, mime_type })
             } else {
-            Ok(None)
+                None
+            }
+        };
+
+        match payload {
+            Some(payload) => {
+                let _g = smarm::causal_site!("cache-insert");
+                self.cache.put(key, payload.clone());
+                Ok(Some(payload))
+            }
+            None => Ok(None),
         }
     }
 
@@ -112,10 +211,50 @@ fn print_usage() {
          \x20 ccc add <package> <filepath> <version>\n\
          \x20 ccc archive <package> [<version>]\n\
          \x20 ccc stats\n\
-         \x20 ccc serve [-p|--port <port>]\n"
+         \x20 ccc serve [-p|--port <port>]\n\
+         \x20 ccc causal [-p|--port <port>]  (requires --features causal)\n"
     );
 }
 
+/// Whether `causal` (vs. plain `serve`) was requested - checked once the
+/// server actually starts, since `run_cli` only hands `main` a port.
+static CAUSAL_MODE: OnceLock<bool> = OnceLock::new();
+
+/// Runs the RFC 007 experiment sweep against the live server on a plain OS
+/// thread, prints the summary, and exits. Meant to be run alongside an
+/// external load generator (same setup as `bench/RPS`) - the sweep needs
+/// real request traffic hitting `causal_site!`/`progress!` call sites to
+/// produce anything.
+#[cfg(feature = "causal")]
+fn spawn_causal_profiler() {
+    std::thread::spawn(|| {
+        let warmup = std::env::var("CCC_CAUSAL_WARMUP_MS")
+            .ok()
+            .and_then(|v| v.parse().ok())
+            .map(Duration::from_millis)
+            .unwrap_or(Duration::from_secs(2));
+        eprintln!("[causal] warming up for {warmup:?}, send traffic now...");
+        std::thread::sleep(warmup);
+        eprintln!("[causal] starting experiment sweep...");
+        let results = smarm::causal::run_experiments(&Default::default());
+        println!("{}", smarm::causal::render_summary(&results));
+        if std::env::var("CCC_CAUSAL_LEDGER").is_ok() {
+            eprintln!("{}", smarm::causal::render_ledger_audit(&results));
+        }
+        if let Ok(path) = std::env::var("CCC_CAUSAL_COZ_OUT") {
+            let _ = std::fs::write(&path, smarm::causal::render_coz(&results));
+            eprintln!("[causal] wrote {path}");
+        }
+        std::process::exit(0);
+    });
+}
+
+#[cfg(not(feature = "causal"))]
+fn spawn_causal_profiler() {
+    eprintln!("error: 'ccc causal' requires building with --features causal");
+    std::process::exit(2);
+}
+
 fn run_cli() -> Option<u16> {
     let args: Vec<String> = std::env::args().collect();
     match args.get(1).map(String::as_str) {
@@ -182,7 +321,8 @@ fn run_cli() -> Option<u16> {
             }
             None
         }
-        Some("serve") | None => {
+        Some("causal") | Some("serve") | None => {
+            let _ = CAUSAL_MODE.set(args.get(1).map(String::as_str) == Some("causal"));
             let mut port: u16 = 8333;
             let mut i = 2;
             while i < args.len() {
@@ -267,7 +407,7 @@ fn fetch_asset_handler(c: Conn, _n: Next) -> Conn {
                 .map(|v| v.contains("gzip"))
                 .unwrap_or(false);
 
-            if accepts_gzip {
+            let response = if accepts_gzip {
                 c.put_status(200)
                     .put_header("content-type", &asset.mime_type)
                     .put_header("content-encoding", "gzip")
@@ -275,33 +415,52 @@ fn fetch_asset_handler(c: Conn, _n: Next) -> Conn {
                     .put_header("access-control-allow-origin", "*")
                     .put_body(asset.gzipped_bytes)
             } else {
+                let raw_bytes = {
+                    let _g = smarm::causal_site!("gzip-decode");
                     let mut decoder = GzDecoder::new(&asset.gzipped_bytes[..]);
                     let mut raw_bytes = Vec::new();
-                if decoder.read_to_end(&mut raw_bytes).is_ok() {
-                    c.put_status(200)
+                    decoder.read_to_end(&mut raw_bytes).map(|_| raw_bytes)
+                };
+                match raw_bytes {
+                    Ok(raw_bytes) => c.put_status(200)
                         .put_header("content-type", &asset.mime_type)
                         .put_header("cache-control", "public, max-age=31536000, immutable")
                         .put_header("access-control-allow-origin", "*")
-                        .put_body(raw_bytes)
-                } else {
-                    c.put_status(500).put_body("Decompression Error")
-                }
+                        .put_body(raw_bytes),
+                    Err(_) => return c.put_status(500).put_body("Decompression Error"),
                 }
+            };
+            smarm::progress!("asset-served");
+            response
         }
         Ok(Ok(None)) => c.put_status(404).put_body("Asset Not Found"),
         _ => c.put_status(500).put_body("Database Error Encountered"),
     }
 }
 
+/// Fallback for any request the router didn't match. Deliberately terse -
+/// no route list, no hint of the `/assets/:package/:version/:filename`
+/// shape, nothing to make this service more discoverable to strangers
+/// than it already is. Just enough to not look like a hung connection.
+fn not_found_handler(c: Conn, _n: Next) -> Conn {
+    c.put_status(404)
+        .put_header("content-type", "text/plain")
+        .put_body("404 not found")
+}
+
 fn main() {
     let Some(port) = run_cli() else { return };
 
+    if *CAUSAL_MODE.get().unwrap_or(&false) {
+        spawn_causal_profiler();
+    }
+
     let router = Router::new()
         .get("/packages", list_packages_handler)
         .get("/assets/:package/:version/:filename", fetch_asset_handler);
 
     println!("C3 on 0.0.0.0:{port}...");
     let cfg = Config::new(format!("0.0.0.0:{port}").parse().unwrap());
-    let pipeline = Pipeline::new().plug(router);
+    let pipeline = Pipeline::new().plug(router).plug(not_found_handler);
     serve_with(cfg, pipeline).unwrap();
 }