client: fix corner cases in secret declaration (#124)

1. Don't allow an empty secret name to be declared.  While in principle this
   could be fine, it is weird and not worth supporting.

2. Deduplicate secret names before initialization. I originally thought this
   was not needed, but because of the way we merge cached results with the
   initialization step, duplicates can "poison" the initialization state.

client/setec/store.go

@@ -14,6 +14,7 @@ import (
 	"log"
 	"math/rand"
 	"os"
+	"slices"
 	"sync"
 	"time"
 
@@ -855,10 +856,18 @@ func (c StoreConfig) secretNames() ([]string, []*Fields, error) {
 			return nil, nil, fmt.Errorf("parse struct fields: %w", err)
 		}
 
-		// We don't need to deduplicate here, the constructor merges all the
-		// names into a map.
 		sec = append(sec, fs.Secrets()...)
 		svs = append(svs, fs)
 	}
+	// Sort and compact (deduplicate) secret names.
+	// Although the constructor puts them into a map, duplicates can "poison"
+	// the initialization, so remove them up front.
+	slices.Sort(sec)
+	sec = slices.Compact(sec)
+	for _, name := range sec {
+		if name == "" {
+			return nil, nil, errors.New("empty secret name not allowed")
+		}
+	}
 	return sec, svs, nil
 }

client/setec/store_test.go

@@ -115,6 +115,32 @@ func TestStore(t *testing.T) {
 			t.Errorf("Lookup(bravo): got %q, want error", s.Get())
 		}
 	})
+
+	t.Run("NewStore_emptyName", func(t *testing.T) {
+		st, err := setec.NewStore(ctx, setec.StoreConfig{
+			Client:      cli,
+			Secrets:     []string{""},
+			Logf:        logger.Discard,
+			AllowLookup: true,
+		})
+		if err == nil {
+			t.Fatalf("NewStore(empty): got %+v, want error", st)
+		}
+	})
+
+	t.Run("NewStore_dupName", func(t *testing.T) {
+		// Regression: A duplicate name can poison the initialization map, so we
+		// need to deduplicate as part of collection.
+		st, err := setec.NewStore(ctx, setec.StoreConfig{
+			Client:  cli,
+			Secrets: []string{"alpha", "alpha"},
+			Logf:    logger.Discard,
+		})
+		if err != nil {
+			t.Fatalf("NewStore(dup): unexpected error: %v", err)
+		}
+		t.Logf("NewStore: got secret %q", st.Secret("alpha").GetString())
+	})
 }
 
 func TestCachedStore(t *testing.T) {