improvements

- Consolidate status reads
- Pre-allocate descriptors
- Add read_only disk label
- Remove dead stdin field
- Guard nil pool maps
- Add error-path tests
- Add go vet to flake

CLAUDE.md

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+# pve-local-exporter
+
+Proxmox VE local metrics exporter for Prometheus, written in Go. Collects VM and storage metrics directly from the host (no PVE API) by reading /proc, /sys, /etc/pve, and running `qm monitor`.
+
+Reimplementation of the Python exporter at `../pvemon`.
+
+## Build & Test
+
+```sh
+nix develop             # shell with go + gopls
+nix develop -c go test ./...   # run tests (from src/)
+nix build               # build static binary
+nix flake check         # evaluate + check flake outputs
+```
+
+Go source lives in `src/`. The Nix build uses `buildGoModule` with `CGO_ENABLED=0`.
+
+After changing go.mod dependencies, run `go mod tidy` then update `vendorHash` in `default.nix` (set to a bogus hash, run `nix build`, copy the correct hash from the error).
+
+## Architecture
+
+All I/O is behind interfaces for testability. No test touches the real filesystem or runs real commands.
+
+```
+src/
+  main.go                          HTTP server, signal handling, log setup
+  internal/
+    config/                        CLI flag parsing -> Config struct
+    cache/                         TTLCache[K,V] (with jitter), MtimeCache[V] (file change detection)
+    procfs/                        /proc parsing: process discovery, CPU, IO, memory, threads, ctx switches
+    sysfs/                         /sys reading: NIC stats, block device sizes
+    qmmonitor/                     qm monitor pipe I/O + TTL cache, network/block output parsers
+    pveconfig/                     /etc/pve parsers: user.cfg (pools), storage.cfg (storage defs)
+    storage/                       statvfs wrapper, zpool list parser
+    collector/                     Prometheus Collector wiring all packages together
+```
+
+### Key interfaces (defined alongside their real implementations)
+
+- `procfs.ProcReader` -- process discovery and /proc/{pid}/* parsing
+- `sysfs.SysReader` -- /sys/class/net stats, /sys/block sizes
+- `qmmonitor.QMMonitor` -- `RunCommand(vmid, cmd)`, `InvalidateCache(vmid, cmd)`
+- `storage.StatFS` -- `Statfs(path)` for dir/nfs/cephfs sizing
+- `collector.CommandRunner` -- `Run(name, args...)` for zpool list
+- `collector.FileReaderIface` -- `ReadFile(path)` for config files
+
+### Concurrency
+
+Bounded goroutine pool (16 workers) for parallel NIC + disk collection per VM, matching the Python ThreadPoolExecutor(max_workers=16).
+
+## Conventions
+
+- Indent nix files with tabs
+- No emoji in code or text
+- Tests use fixture data / mock interfaces, never real /proc or /sys
+- Parser functions are exported and pure (take string, return parsed struct) so tests are straightforward
+- `storage.cfg` parser: section headers start at column 0, properties are indented (tab or space)
+- `user.cfg` pool format: `pool:name:comment:vmlist` (colon-separated, vmlist is comma-separated VM IDs)
+- Memory extended label keys are lowercase without trailing colon (e.g., `vmrss`, `vmpeak`)
+
+## CLI Flags
+
+| Flag | Default | Description |
+|------|---------|-------------|
+| `--port` | 9116 | HTTP listen port |
+| `--host` | 0.0.0.0 | Bind address |
+| `--collect-running-vms` | true | Collect KVM VM metrics |
+| `--collect-storage` | true | Collect storage pool metrics |
+| `--metrics-prefix` | pve | Prefix for all metric names |
+| `--loglevel` | INFO | DEBUG, INFO, WARNING, ERROR |
+| `--qm-terminal-timeout` | 10s | qm monitor command timeout |
+| `--qm-max-ttl` | 600s | TTL cache for qm monitor data |
+| `--qm-rand` | 60s | Jitter range for cache expiry |
+| `--qm-monitor-defer-close` | true | Defer closing unresponsive qm sessions |
+| `--version` | | Print version and exit |