db: rework fileCache to use genericcache

We no longer intermingle the cache infrastructure logic with the file
cache specifics. This will help when we extend the file cache to
support blob files.

db_test.go

@@ -695,8 +695,10 @@ func TestIterLeak(t *testing.T) {
 		t.Run(fmt.Sprintf("leak=%t", leak), func(t *testing.T) {
 			for _, flush := range []bool{true, false} {
 				t.Run(fmt.Sprintf("flush=%t", flush), func(t *testing.T) {
+					fc := NewFileCache(10, 100)
 					d, err := Open("", testingRandomized(t, &Options{
-						FS: vfs.NewMem(),
+						FS:        vfs.NewMem(),
+						FileCache: fc,
 					}))
 					require.NoError(t, err)
 
@@ -710,15 +712,16 @@ func TestIterLeak(t *testing.T) {
 						require.NoError(t, iter.Close())
 						require.NoError(t, d.Close())
 					} else {
-						defer iter.Close()
 						if err := d.Close(); err == nil {
 							t.Fatalf("expected failure, but found success")
 						} else if !strings.HasPrefix(err.Error(), "leaked iterators:") {
 							t.Fatalf("expected leaked iterators, but found %+v", err)
 						} else {
 							t.Log(err.Error())
 						}
+						iter.Close()
 					}
+					fc.Unref()
 				})
 			}
 		})
@@ -732,13 +735,16 @@ func TestIterLeakSharedCache(t *testing.T) {
 		t.Run(fmt.Sprintf("leak=%t", leak), func(t *testing.T) {
 			for _, flush := range []bool{true, false} {
 				t.Run(fmt.Sprintf("flush=%t", flush), func(t *testing.T) {
+					fc := NewFileCache(10, 100)
 					d1, err := Open("", &Options{
-						FS: vfs.NewMem(),
+						FS:        vfs.NewMem(),
+						FileCache: fc,
 					})
 					require.NoError(t, err)
 
 					d2, err := Open("", &Options{
-						FS: vfs.NewMem(),
+						FS:        vfs.NewMem(),
+						FileCache: fc,
 					})
 					require.NoError(t, err)
 
@@ -789,6 +795,22 @@ func TestIterLeakSharedCache(t *testing.T) {
 						}
 					}
 
+					if !leak {
+						fc.Unref()
+					} else if flush {
+						require.Panics(t, func() {
+							fc.Unref()
+						})
+					} else {
+						// When we're not flushing and we leak an iterator, Unref might or
+						// might not panic, depending on whether there was a file involved.
+						func() {
+							defer func() {
+								recover()
+							}()
+							fc.Unref()
+						}()
+					}
 				})
 			}
 		})