feat(examples): add upgrade pattern d example (gnolang#2151)

Signed-off-by: moul <[email protected]>

examples/gno.land/r/x/manfred_upgrade_patterns/upgrade_d/README.md

@@ -0,0 +1,9 @@
+# Lazy Migration Example
+
+This example demonstrates lazy migrations from v1 to v2 of a data structure in Gno.
+
+## Notes
+
+Uses AVL trees, but storage can vary since public Get functions are used.
+
+v1 can be made pausable and readonly during migration.

examples/gno.land/r/x/manfred_upgrade_patterns/upgrade_d/v1/v1.gno

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+package v1
+
+import (
+	"strconv"
+
+	"gno.land/p/demo/avl"
+)
+
+var myTree avl.Tree
+
+type MyStruct struct {
+	FieldA string
+	FieldB int
+}
+
+func (s *MyStruct) String() string {
+	if s == nil {
+		return "nil"
+	}
+	return "v1:" + s.FieldA + ":" + strconv.Itoa(s.FieldB)
+}
+
+func Get(key string) *MyStruct {
+	ret, ok := myTree.Get(key)
+	if !ok {
+		return nil
+	}
+	return ret.(*MyStruct)
+}
+
+func init() {
+	myTree.Set("a", &MyStruct{FieldA: "a", FieldB: 1})
+	myTree.Set("b", &MyStruct{FieldA: "b", FieldB: 2})
+}

examples/gno.land/r/x/manfred_upgrade_patterns/upgrade_d/v1/z_filetest.gno

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+package main
+
+import (
+	"fmt"
+
+	"gno.land/r/x/manfred_upgrade_patterns/upgrade_d/v1"
+)
+
+func main() {
+	println("a", v1.Get("a"))
+	println("b", v1.Get("b"))
+	println("c", v1.Get("c"))
+}
+
+// Output:
+// a v1:a:1
+// b v1:b:2
+// c nil

examples/gno.land/r/x/manfred_upgrade_patterns/upgrade_d/v2/v2.gno

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+package v1
+
+import (
+	"gno.land/p/demo/avl"
+	"gno.land/p/demo/ufmt"
+	"gno.land/r/x/manfred_upgrade_patterns/upgrade_d/v1"
+)
+
+var myTree avl.Tree
+
+// MyStruct represents the structure with fields that may have been migrated from v1.
+type MyStruct struct {
+	FieldA string // Appending "imported-from-v1" if migrating from v1.
+	FieldB uint64 // Switching from int to uint64 and multiplying by 1000 if migrating from v1.
+	FieldC bool   // Adding a boolean field which is true by default for v1 objects.
+}
+
+// String returns a string representation of MyStruct.
+func (s *MyStruct) String() string {
+	if s == nil {
+		return "nil"
+	}
+	return ufmt.Sprintf("v2:%s:%d:%t", s.FieldA, s.FieldB, s.FieldC)
+}
+
+// Get retrieves a MyStruct from the tree by key. If the key does not exist in the current version,
+// it attempts to retrieve and migrate the structure from v1.
+func Get(key string) *MyStruct {
+	ret, ok := myTree.Get(key)
+	if !ok {
+		v1Struct := v1.Get(key)
+		if v1Struct == nil {
+			return nil
+		}
+		// Lazy migration code: convert v1 structure to v2 structure.
+		v2Struct := &MyStruct{
+			FieldA: v1Struct.FieldA + "-imported-from-v1",
+			FieldB: uint64(v1Struct.FieldB * 1000),
+			FieldC: true,
+		}
+		myTree.Set(key, v2Struct)
+		return v2Struct
+	}
+	return ret.(*MyStruct)
+}
+
+// init initializes the tree with some default values.
+func init() {
+	myTree.Set("c", &MyStruct{FieldA: "c", FieldB: 3, FieldC: true})
+	myTree.Set("d", &MyStruct{FieldA: "d", FieldB: 4, FieldC: false})
+}

examples/gno.land/r/x/manfred_upgrade_patterns/upgrade_d/v2/z_filetest.gno

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+package main
+
+import (
+	"fmt"
+
+	"gno.land/r/x/manfred_upgrade_patterns/upgrade_d/v2"
+)
+
+func main() {
+	println("a", v1.Get("a"))
+	println("b", v1.Get("b"))
+	println("c", v1.Get("c"))
+	println("d", v1.Get("d"))
+	println("e", v1.Get("e"))
+}
+
+// Output:
+// a v2:a-imported-from-v1:1000:true
+// b v2:b-imported-from-v1:2000:true
+// c v2:c:3:true
+// d v2:d:4:false
+// e nil