Introspection (part 4) #111
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achille-roussel
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Nov 18, 2023
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chriso
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Following on from #111, this PR adds framing to the custom serializer output so that an offline process can walk the object graph without access to the custom deserialization routine.
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chriso
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This is the final PR in the introspection series (#107, #108, #109, #111, #112, #113), allowing users to scan objects within regions. Each region encodes a primitive value, a compound array or struct, or a map. The scanner scans the tree using a preorder traversal, providing type information, primitive values (where applicable), access to functions/regions when references are encountered, and various other type-specific helpers to access information about an object. The scanner does not recurse when references to other regions are encountered. The user is responsible for following pointers (if desired) and keeping track of regions visited to prevent infinite loops when cycles are present in the object graph. I fixed two inconsistencies in order to build the scanner: * the root region now has a type of `interface{}`, since that's what `types.Serialize` actually serializes. Encoding this type (rather than the unboxed type) means we don't need a special case for the root region when scanning * we now avoid double indirection when there's a reference to a map <details> <summary>Here's an example:</summary> ```go c, err := types.Inspect(b) if err != nil { panic(err) } regions := []*types.Region{c.Root()} for i := 0; i < c.NumRegion(); i++ { regions = append(regions, c.Region(i)) } for i, r := range regions { fmt.Println("Reading region", i, "with type", r.Type(), "and size", r.Size()) s := r.Scan() for s.Next() { fmt.Println("=> reading from offset", s.Pos()) if s.Custom() { fmt.Println("=> read an object serialized by a custom serializer") } switch s.Kind() { case reflect.Bool: fmt.Println("=> read bool", s.Bool()) case reflect.Int: fmt.Println("=> read int", s.Int()) case reflect.Int8: fmt.Println("=> read int", s.Int8()) case reflect.Int16: fmt.Println("=> read int16", s.Int16()) case reflect.Int32: fmt.Println("=> read int32", s.Int32()) case reflect.Int64: fmt.Println("=> read int64", s.Int64()) case reflect.Uint: fmt.Println("=> read uint", s.Uint()) case reflect.Uint8: fmt.Println("=> read uint8", s.Uint8()) case reflect.Uint16: fmt.Println("=> read uint16", s.Uint16()) case reflect.Uint32: fmt.Println("=> read uint32", s.Uint32()) case reflect.Uint64: fmt.Println("=> read uint64", s.Uint64()) case reflect.Uintptr: fmt.Println("=> read uintptr", s.Uintptr()) case reflect.Float32: fmt.Println("=> read float32", s.Float32()) case reflect.Float64: fmt.Println("=> read float64", s.Float64()) case reflect.Complex64: fmt.Println("=> read complex64", s.Complex64()) case reflect.Complex128: fmt.Println("=> read complex128", s.Complex128()) case reflect.Array: fmt.Println("=> read array of type", s.Type(), "with length", s.Len()) case reflect.Interface: if s.Nil() { if t := s.Type(); t != nil { fmt.Println("=> read interface of type", t, "with nil value") } else { fmt.Println("=> read nil interface") } } else if r, off := s.Region(); r != nil { fmt.Println("=> read interface of type", s.Type(), "pointing to region", r, "offset", off) } else { fmt.Println("=> read interface of type", s.Type(), "with static data", s.Uint64()) } case reflect.String: if s.Len() == 0 { fmt.Println("=> read string with length 0") } else { r, off := s.Region() fmt.Println("=> read string with length", s.Len(), "pointing to region", r, "offset", off) } case reflect.Slice: if r, off := s.Region(); r != nil { fmt.Println("=> read slice of type", s.Type(), "with length", s.Len(), "and cap", s.Cap(), "pointing to region", r, "offset", off) } else { fmt.Println("=> read nil slice of type", s.Type(), "with length", s.Len(), "and cap", s.Cap()) } case reflect.Func: if s.Nil() { fmt.Println("=> read nil function with type", s.Type()) } else if ct := s.Function().ClosureType(); ct != nil { fmt.Println("=> read function with type", s.Type(), "and closure layout", ct) } else { fmt.Println("=> read function with type", s.Type()) } case reflect.Struct: fmt.Println("=> read struct of type", s.Type()) case reflect.Pointer: if r, off := s.Region(); r != nil { fmt.Println("=> read pointer of type", s.Type(), "pointing to region", r, off) } else { fmt.Println("=> read nil pointer of type", s.Type()) } case reflect.UnsafePointer: if r, off := s.Region(); r != nil { fmt.Println("=> read unsafe pointer pointing to region", r, "offset", off) } else { fmt.Println("=> read nil unsafe pointer") } default: panic(fmt.Sprintf("not implemented: %s", s.Kind())) } } if err := s.Close(); err != nil { panic(err) } } ``` </details>
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Following on from #109, this PR starts to break up the object graph into a form that's easier to work with.
The serialization layer scans pointers in order to find the set of disjoint memory regions (the underlying structs/arrays). When serializing the object graph, the serialization layer would encode these regions inline. This PR updates the layer to store regions in separate protobuf messages, along with their type. As with the previous PRs (#108, #109), regions are stored in an array where the index is used as a unique identifier. When a pointer is encountered in the graph, we store an ID + offset pair to reference a region and a particular offset within that region. We also store the "root" region which is the memory backing whatever object was passed to
types.Serialize. In the case of durable coroutine state, it'll be a*serializedCoroutinei.e. just a pointer to another region.