Golang DB.Get方法代码示例

// GetTableDescriptor retrieves a table descriptor directly from the KV layer.
func GetTableDescriptor(kvDB *client.DB, database string, table string) *TableDescriptor {
	dbNameKey := MakeNameMetadataKey(keys.RootNamespaceID, database)
	gr, err := kvDB.Get(context.TODO(), dbNameKey)
	if err != nil {
		panic(err)
	}
	if !gr.Exists() {
		panic("database missing")
	}
	dbDescID := ID(gr.ValueInt())

	tableNameKey := MakeNameMetadataKey(dbDescID, table)
	gr, err = kvDB.Get(context.TODO(), tableNameKey)
	if err != nil {
		panic(err)
	}
	if !gr.Exists() {
		panic("table missing")
	}

	descKey := MakeDescMetadataKey(ID(gr.ValueInt()))
	desc := &Descriptor{}
	if err := kvDB.GetProto(context.TODO(), descKey, desc); err != nil {
		panic("proto missing")
	}
	return desc.GetTable()
}

// concurrentIncrements starts two Goroutines in parallel, both of which
// read the integers stored at the other's key and add it onto their own.
// It is checked that the outcome is serializable, i.e. exactly one of the
// two Goroutines (the later write) sees the previous write by the other.
func concurrentIncrements(db *client.DB, t *testing.T) {
	// wgStart waits for all transactions to line up, wgEnd has the main
	// function wait for them to finish.
	var wgStart, wgEnd sync.WaitGroup
	wgStart.Add(2 + 1)
	wgEnd.Add(2)

	for i := 0; i < 2; i++ {
		go func(i int) {
			// Read the other key, write key i.
			readKey := []byte(fmt.Sprintf(testUser+"/value-%d", (i+1)%2))
			writeKey := []byte(fmt.Sprintf(testUser+"/value-%d", i))
			defer wgEnd.Done()
			wgStart.Done()
			// Wait until the other goroutines are running.
			wgStart.Wait()

			if err := db.Txn(context.TODO(), func(txn *client.Txn) error {
				txn.SetDebugName(fmt.Sprintf("test-%d", i), 0)

				// Retrieve the other key.
				gr, err := txn.Get(readKey)
				if err != nil {
					return err
				}

				otherValue := int64(0)
				if gr.Value != nil {
					otherValue = gr.ValueInt()
				}

				_, err = txn.Inc(writeKey, 1+otherValue)
				return err
			}); err != nil {
				t.Error(err)
			}
		}(i)
	}

	// Kick the goroutines loose.
	wgStart.Done()
	// Wait for the goroutines to finish.
	wgEnd.Wait()
	// Verify that both keys contain something and, more importantly, that
	// one key actually contains the value of the first writer and not only
	// its own.
	total := int64(0)
	results := []int64(nil)
	for i := 0; i < 2; i++ {
		readKey := []byte(fmt.Sprintf(testUser+"/value-%d", i))
		gr, err := db.Get(context.TODO(), readKey)
		if err != nil {
			t.Fatal(err)
		}
		if gr.Value == nil {
			t.Fatalf("unexpected empty key: %s=%v", readKey, gr.Value)
		}
		total += gr.ValueInt()
		results = append(results, gr.ValueInt())
	}

	// First writer should have 1, second one 2
	if total != 3 {
		t.Fatalf("got unserializable values %v", results)
	}
}