Fix host resolution and concurrent cache updates

internal/cache/cache.go

@@ -2,13 +2,17 @@ package cache
 
 import (
 	"encoding/json"
-	"git.zb-server.de/Sebi/ssh-netbox-wrapper/internal/storage"
+	"fmt"
 	"os"
 	"path/filepath"
 	"sort"
 	"strings"
 	"sync"
 	"time"
+
+	"golang.org/x/sys/unix"
+
+	"git.zb-server.de/Sebi/ssh-netbox-wrapper/internal/storage"
 )
 
 type Entry struct {
@@ -20,8 +24,18 @@ type Entry struct {
 	LastUsed time.Time `json:"last_used,omitempty"`
 }
 
+type changeKind uint8
+
+const (
+	upsertChange changeKind = iota
+	deleteChange
+	usageChange
+)
+
 type Cache struct {
 	mu           sync.RWMutex
+	dirty        map[string]changeKind
+	cleared      bool
 	entries      map[string]Entry
 	path         string
 	ttl          time.Duration
@@ -39,6 +53,7 @@ func New(path string, ttlSeconds int) *Cache {
 	}
 	return &Cache{
 		entries:      make(map[string]Entry),
+		dirty:        make(map[string]changeKind),
 		path:         path,
 		ttl:          time.Duration(ttlSeconds) * time.Second,
 		refreshStamp: stamp,
@@ -94,6 +109,8 @@ func (c *Cache) Load() error {
 		return err
 	}
 
+	c.dirty = make(map[string]changeKind)
+	c.cleared = false
 	c.entries = make(map[string]Entry, len(df.Entries))
 	for _, e := range df.Entries {
 		c.entries[e.Name] = e
@@ -101,23 +118,80 @@ func (c *Cache) Load() error {
 	return nil
 }
 
+// Save merges only local changes under a process-wide file lock. Atomic rename
+// alone prevents torn JSON but cannot prevent lost updates from another process.
 func (c *Cache) Save() error {
-	c.mu.RLock()
-	df := diskFormat{Entries: make([]Entry, 0, len(c.entries))}
-	for _, e := range c.entries {
-		df.Entries = append(df.Entries, e)
-	}
-	c.mu.RUnlock()
-
+	c.mu.Lock()
+	defer c.mu.Unlock()
 	if err := os.MkdirAll(filepath.Dir(c.path), 0o755); err != nil {
 		return err
 	}
+	lock, err := os.OpenFile(c.path+".lock", os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		return err
+	}
+	defer lock.Close()
+	if err := unix.Flock(int(lock.Fd()), unix.LOCK_EX); err != nil {
+		return err
+	}
+	defer unix.Flock(int(lock.Fd()), unix.LOCK_UN)
 
+	merged := make(map[string]Entry)
+	if !c.cleared {
+		data, err := os.ReadFile(c.path)
+		if err != nil && !os.IsNotExist(err) {
+			return err
+		}
+		if err == nil {
+			var df diskFormat
+			if err := json.Unmarshal(data, &df); err != nil {
+				return err
+			}
+			for _, e := range df.Entries {
+				merged[e.Name] = e
+			}
+		}
+	}
+	for name, change := range c.dirty {
+		if change == deleteChange {
+			delete(merged, name)
+			continue
+		}
+		e := c.entries[name]
+		old, exists := merged[name]
+		if change == usageChange && !exists {
+			// A connection finishing after a refresh or clear must not
+			// restore an entry that the other process removed.
+			continue
+		}
+		if exists {
+			lastUsed := e.LastUsed
+			if change == usageChange || old.CachedAt.After(e.CachedAt) {
+				e = old
+			}
+			if old.LastUsed.After(lastUsed) {
+				lastUsed = old.LastUsed
+			}
+			e.LastUsed = lastUsed
+		}
+		merged[name] = e
+	}
+	df := diskFormat{Entries: make([]Entry, 0, len(merged))}
+	for _, e := range merged {
+		df.Entries = append(df.Entries, e)
+	}
+	sort.Slice(df.Entries, func(i, j int) bool { return df.Entries[i].Name < df.Entries[j].Name })
 	data, err := json.MarshalIndent(df, "", "  ")
 	if err != nil {
 		return err
 	}
-	return storage.WriteFile(c.path, data, 0o600)
+	if err := storage.WriteFile(c.path, data, 0o600); err != nil {
+		return err
+	}
+	c.entries = merged
+	c.dirty = make(map[string]changeKind)
+	c.cleared = false
+	return nil
 }
 
 func (c *Cache) Upsert(e Entry) {
@@ -127,9 +201,18 @@ func (c *Cache) Upsert(e Entry) {
 		e.LastUsed = old.LastUsed
 	}
 	c.entries[e.Name] = e
+	c.dirty[e.Name] = upsertChange
 	c.mu.Unlock()
 }
 
+// Delete removes an entry when a refresh can no longer resolve it safely.
+func (c *Cache) Delete(name string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	delete(c.entries, name)
+	c.dirty[name] = deleteChange
+}
+
 // MarkUsed records the current time as LastUsed for the named entry.
 // It is a no-op if the entry does not exist.
 func (c *Cache) MarkUsed(name string) {
@@ -138,6 +221,9 @@ func (c *Cache) MarkUsed(name string) {
 	if e, ok := c.entries[name]; ok {
 		e.LastUsed = time.Now()
 		c.entries[name] = e
+		if _, changed := c.dirty[name]; !changed {
+			c.dirty[name] = usageChange
+		}
 	}
 }
 
@@ -179,19 +265,20 @@ func (c *Cache) Search(prefix string) []Entry {
 	return out
 }
 
-// GetByShortcut scans all entries and returns the first whose normalized name matches
-// the normalized shortcut. Returns (entry, found, fresh).
-func (c *Cache) GetByShortcut(shortcut string, normalize func(string) string) (entry Entry, found bool, fresh bool) {
+// GetByShortcut rejects collisions instead of choosing a random map entry.
+func (c *Cache) GetByShortcut(shortcut string, normalize func(string) string) (entry Entry, found bool, fresh bool, err error) {
 	c.mu.RLock()
 	defer c.mu.RUnlock()
 	norm := normalize(shortcut)
 	for _, e := range c.entries {
 		if normalize(e.Name) == norm {
-			isFresh := c.ttl > 0 && time.Since(e.CachedAt) < c.ttl
-			return e, true, isFresh
+			if found {
+				return Entry{}, false, false, fmt.Errorf("ambiguous shortcut %q; use a full hostname", shortcut)
+			}
+			entry, found = e, true
 		}
 	}
-	return Entry{}, false, false
+	return entry, found, found && c.ttl > 0 && time.Since(entry.CachedAt) < c.ttl, nil
 }
 
 // Get returns an entry and reports whether it is still within the TTL.
@@ -212,6 +299,8 @@ func (c *Cache) Get(name string) (entry Entry, fresh bool) {
 func (c *Cache) Clear() {
 	c.mu.Lock()
 	c.entries = make(map[string]Entry)
+	c.dirty = make(map[string]changeKind)
+	c.cleared = true
 	c.mu.Unlock()
 }