internal/config,watcher: add -config-dir (#873)

Over time the llama-swap configuration file can get really long and
challenging to work with. The -config-dir flag is used for a directory
of configuration YAML fragments.

These fragments are merged together and into a full configuration and
tested for validity. All previous configuration functionality remains
unchanged.

internal/watcher/dirwatcher.go

@@ -0,0 +1,137 @@
+package configwatcher
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"time"
+)
+
+// DirWatcher polls a directory for changes to its set of *.yml / *.yaml files.
+// It fires OnChange when a file is added, removed, or has its mod time/size
+// change. Like Watcher it is poll-based so it works in Docker bind-mounts and
+// k8s ConfigMap projections where inotify is unreliable.
+//
+// The baseline poll establishes initial state and does not fire OnChange.
+type DirWatcher struct {
+	Path     string
+	Interval time.Duration
+	OnChange func()
+}
+
+// dirSnapshot is an ordered map of file name -> file state. The ordering is
+// derived from sorted filenames so two snapshots compare deterministically
+// regardless of readdir order. exists reflects whether the directory was
+// readable at scan time; a missing directory yields exists=false.
+type dirSnapshot struct {
+	exists bool
+	names  []string
+	states map[string]snapshot
+}
+
+func newDirSnapshot() dirSnapshot {
+	return dirSnapshot{states: make(map[string]snapshot)}
+}
+
+// equal reports whether two snapshots describe the same file set and per-file
+// state. A missing directory (exists=false) is treated as equal to any other
+// missing directory regardless of cached names.
+func (s dirSnapshot) equal(other dirSnapshot) bool {
+	if !s.exists && !other.exists {
+		return true
+	}
+	if s.exists != other.exists {
+		return false
+	}
+	if len(s.names) != len(other.names) {
+		return false
+	}
+	for i, n := range s.names {
+		if other.names[i] != n {
+			return false
+		}
+	}
+	for _, n := range s.names {
+		a, b := s.states[n], other.states[n]
+		if a.exists != b.exists || a.size != b.size || !a.modTime.Equal(b.modTime) {
+			return false
+		}
+	}
+	return true
+}
+
+// Run blocks until ctx is canceled. It polls Path on Interval and invokes
+// OnChange whenever the directory's YAML file set changes.
+//
+// Policy mirrors the single-file Watcher: disappearance (directory missing or
+// empty) is treated as a transient rename-style write and stays quiet; the
+// transition back to present-with-content fires OnChange.
+func (w *DirWatcher) Run(ctx context.Context) {
+	interval := w.Interval
+	if interval <= 0 {
+		interval = DefaultInterval
+	}
+
+	prev := scanDir(w.Path)
+
+	ticker := time.NewTicker(interval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			cur := scanDir(w.Path)
+			// Suppress transitions involving an empty or missing directory —
+			// these are treated as transient rename-style writes, mirroring
+			// the single-file Watcher. Only present-with-content →
+			// present-with-content (changed) or no-content →
+			// present-with-content fires OnChange.
+			prevHasContent := prev.exists && len(prev.names) > 0
+			curHasContent := cur.exists && len(cur.names) > 0
+			if curHasContent && (!prevHasContent || !prev.equal(cur)) && w.OnChange != nil {
+				w.OnChange()
+			}
+			prev = cur
+		}
+	}
+}
+
+// scanDir returns a snapshot of the *.yml/*.yaml files in dir. If the
+// directory cannot be read (missing, permission denied) the snapshot reports
+// exists=false; the next successful scan will detect the recovery and fire
+// OnChange.
+func scanDir(dir string) dirSnapshot {
+	snap := newDirSnapshot()
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return snap // exists=false
+	}
+	snap.exists = true
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		name := e.Name()
+		if !strings.HasSuffix(name, ".yml") && !strings.HasSuffix(name, ".yaml") {
+			continue
+		}
+		fi, err := os.Stat(filepath.Join(dir, name))
+		if err != nil {
+			// File disappeared between ReadDir and Stat; skip it — the
+			// next poll will observe the removal cleanly.
+			continue
+		}
+		snap.names = append(snap.names, name)
+		snap.states[name] = snapshot{
+			exists:  true,
+			modTime: fi.ModTime(),
+			size:    fi.Size(),
+		}
+	}
+	sort.Strings(snap.names)
+	return snap
+}