metamorphic: reuse single delete keys

Currently, the metamorphic tests randomly generate a set of operations
to perform on the Pebble instance. Support for single deletes exists,
though the keys that have been deleted are not considered for reuse.

Track keys that have been singly deleted, and randomly reuse these keys
when generating subsequent operations to perform.

Prior to this patch, the generation operation log would resemble the
following:

```
db.Set("foo", "bar")
...
batch54.SingleDelete("foo")
...
// No more operations on key "foo".
```

With this patch, the following seqeunce of operations is permissible:

```
db.Set("foo", "bar")
...
db.SingleDelete("foo")
...
db.Set("foo", "baz")
...
db.Merge("foo", "bam")
...
db.Set("foo", "boom")
...
// Subsequent operations on key "foo" are permissible.
```

Related to cockroachdb/cockroach#69414.

internal/metamorphic/generator.go

@@ -32,6 +32,8 @@ type generator struct {
 	writerToSingleSetKeys map[objID]singleSetKeysForBatch
 	// Ensures no duplication of single set keys for the duration of the test.
 	generatedWriteKeys map[string]struct{}
+	// Keys that have either been deleted via a SINGLEDEL.
+	singleDeleteKeys [][]byte
 
 	// Unordered sets of object IDs for live objects. Used to randomly select on
 	// object when generating an operation. There are 4 concrete objects: the DB
@@ -156,6 +158,10 @@ func (g *generator) randKey(newKey float64) []byte {
 
 func (g *generator) randKeyForWrite(newKey float64, singleSetKey float64, writerID objID) []byte {
 	if n := len(g.keys); n > 0 && g.rng.Float64() > newKey {
+		// 25% chance of a key that has been single deleted.
+		if m := len(g.singleDeleteKeys); m > 0 && g.rng.Float64() < 0.25 {
+			return g.randSingleDeleteKey()
+		}
 		return g.keys[g.rng.Intn(n)]
 	}
 	key := g.randValue(4, 12)
@@ -184,6 +190,14 @@ func (g *generator) randKeyToSingleDelete() []byte {
 	return g.singleSetKeysInDB.removeKey(g.rng.Intn(length))
 }
 
+func (g *generator) randSingleDeleteKey() []byte {
+	length := len(g.singleDeleteKeys)
+	if length == 0 {
+		return nil
+	}
+	return g.singleDeleteKeys[g.rng.Intn(length)]
+}
+
 // TODO(peter): make the value size configurable. See valueSizeDist in
 // config.go.
 func (g *generator) randValue(min, max int) []byte {
@@ -844,6 +858,7 @@ func (g *generator) writerSingleDelete() {
 		writerID: writerID,
 		key:      key,
 	})
+	g.singleDeleteKeys = append(g.singleDeleteKeys, key)
 	g.tryRepositionBatchIters(writerID)
 }