core: return a common error on KeyValueReader.Get when not found

This allow to distinguish between not found and an actual read error. Also allow for
fancy layering on top of a KeyValueStore.

Also, document KeyValueWriter.Delete as idempotent.

ethdb/database.go

@@ -22,12 +22,19 @@ import (
 	"io"
 )
 
+var (
+	// ErrKVNotFound is returned if a key is requested that is not found in
+	// the KeyValueStore.
+	ErrKVNotFound = errors.New("not found")
+)
+
 // KeyValueReader wraps the Has and Get method of a backing data store.
 type KeyValueReader interface {
 	// Has retrieves if a key is present in the key-value data store.
 	Has(key []byte) (bool, error)
 
 	// Get retrieves the given key if it's present in the key-value data store.
+	// Returns ErrKVNotFound if the key is not found.
 	Get(key []byte) ([]byte, error)
 }
 
@@ -37,6 +44,7 @@ type KeyValueWriter interface {
 	Put(key []byte, value []byte) error
 
 	// Delete removes the key from the key-value data store.
+	// If the key doesn't exist, this method returns no error.
 	Delete(key []byte) error
 }
 

ethdb/dbtest/testsuite.go

@@ -216,6 +216,10 @@ func TestKeyValueStoreSuite(t *testing.T, New func() ethdb.KeyValueStore) {
 			t.Errorf("wrong value: %t", got)
 		}
 
+		if _, err := db.Get(key); err != ethdb.ErrKVNotFound {
+			t.Errorf("expected ethdb.ErrKVNotFound")
+		}
+
 		value := []byte("hello world")
 		if err := db.Put(key, value); err != nil {
 			t.Error(err)

ethdb/leveldb/leveldb.go

@@ -188,6 +188,9 @@ func (db *Database) Has(key []byte) (bool, error) {
 // Get retrieves the given key if it's present in the key-value store.
 func (db *Database) Get(key []byte) ([]byte, error) {
 	dat, err := db.db.Get(key, nil)
+	if err == leveldb.ErrNotFound {
+		return nil, ethdb.ErrKVNotFound
+	}
 	if err != nil {
 		return nil, err
 	}

ethdb/memorydb/memorydb.go

@@ -31,10 +31,6 @@ var (
 	// errMemorydbClosed is returned if a memory database was already closed at the
 	// invocation of a data access operation.
 	errMemorydbClosed = errors.New("database closed")
-
-	// errMemorydbNotFound is returned if a key is requested that is not found in
-	// the provided memory database.
-	errMemorydbNotFound = errors.New("not found")
 )
 
 // Database is an ephemeral key-value store. Apart from basic data storage
@@ -62,7 +58,7 @@ func NewWithCap(size int) *Database {
 }
 
 // Close deallocates the internal map and ensures any consecutive data access op
-// failes with an error.
+// fails with an error.
 func (db *Database) Close() error {
 	db.lock.Lock()
 	defer db.lock.Unlock()
@@ -94,7 +90,7 @@ func (db *Database) Get(key []byte) ([]byte, error) {
 	if entry, ok := db.db[string(key)]; ok {
 		return common.CopyBytes(entry), nil
 	}
-	return nil, errMemorydbNotFound
+	return nil, ethdb.ErrKVNotFound
 }
 
 // Put inserts the given value into the key-value store.