From 00a1bbd109aa2a75834cb5cf7a4ef1f62bb362b4 Mon Sep 17 00:00:00 2001
From: Alan Wizemann <alan@wizemann.com>
Date: Tue, 5 May 2026 12:07:43 +0200
Subject: [PATCH] feat(scarfmon): split nous.readCache into
 fileExists/readFile/decode/bytes
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Last perf capture showed nous.readCache as a single 60-second
interval — but the function does three things (transport.fileExists,
transport.readFile, JSONDecoder). Splitting the measure points so
the next capture localizes which step actually owns the wall-clock.

Adds:
- nous.readCache.fileExists (interval) — SSH `test -e` round-trip
- nous.readCache.readFile (interval) — SSH `cat` round-trip
- nous.readCache.bytes (event) — payload size of the cache file
- nous.readCache.decode (interval) — JSON parsing cost

If the next 60-second beach ball localizes to readFile, we know
the cache file is somehow huge or the SSH read is hung; if it's
fileExists, the path resolution is the issue; if decode, we have
malformed JSON. All three wear the same outer wrapper so the
existing nous.readCache total stays for trend comparison.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 .../Services/NousModelCatalogService.swift    | 37 ++++++++++++++-----
 1 file changed, 27 insertions(+), 10 deletions(-)

diff --git a/scarf/Packages/ScarfCore/Sources/ScarfCore/Services/NousModelCatalogService.swift b/scarf/Packages/ScarfCore/Sources/ScarfCore/Services/NousModelCatalogService.swift
index 2913bc0..4a277b9 100644
--- a/scarf/Packages/ScarfCore/Sources/ScarfCore/Services/NousModelCatalogService.swift
+++ b/scarf/Packages/ScarfCore/Sources/ScarfCore/Services/NousModelCatalogService.swift
@@ -98,19 +98,36 @@ public struct NousModelCatalogService: Sendable {
     public func readCache() -> NousModelsCache? {
         ScarfMon.measure(.diskIO, "nous.readCache") {
             let transport = context.makeTransport()
-            guard transport.fileExists(cachePath) else { return nil }
+            // Split into separate measure points so the next perf
+            // capture localizes the 60-second observed beach ball
+            // — was it the fileExists probe, the read itself, or
+            // the JSON decode? Each on its own ScarfMon row.
+            let exists = ScarfMon.measure(.diskIO, "nous.readCache.fileExists") {
+                transport.fileExists(cachePath)
+            }
+            guard exists else { return nil }
             do {
-                let data = try transport.readFile(cachePath)
-                let decoder = JSONDecoder()
-                decoder.dateDecodingStrategy = .iso8601
-                let cache = try decoder.decode(NousModelsCache.self, from: data)
-                guard cache.version == NousModelsCache.currentVersion else {
-                    Self.logger.info("nous models cache schema mismatch (got v\(cache.version), expected v\(NousModelsCache.currentVersion)); ignoring")
-                    return nil
+                let data = try ScarfMon.measure(.diskIO, "nous.readCache.readFile") {
+                    try transport.readFile(cachePath)
+                }
+                ScarfMon.event(.diskIO, "nous.readCache.bytes", count: 1, bytes: data.count)
+                return ScarfMon.measure(.diskIO, "nous.readCache.decode") {
+                    let decoder = JSONDecoder()
+                    decoder.dateDecodingStrategy = .iso8601
+                    do {
+                        let cache = try decoder.decode(NousModelsCache.self, from: data)
+                        guard cache.version == NousModelsCache.currentVersion else {
+                            Self.logger.info("nous models cache schema mismatch (got v\(cache.version), expected v\(NousModelsCache.currentVersion)); ignoring")
+                            return Optional<NousModelsCache>.none
+                        }
+                        return cache
+                    } catch {
+                        Self.logger.warning("couldn't decode nous models cache: \(error.localizedDescription, privacy: .public)")
+                        return Optional<NousModelsCache>.none
+                    }
                 }
-                return cache
             } catch {
-                Self.logger.warning("couldn't decode nous models cache: \(error.localizedDescription, privacy: .public)")
+                Self.logger.warning("couldn't read nous models cache: \(error.localizedDescription, privacy: .public)")
                 return nil
             }
         }