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79 lines
2.2 KiB
Go
79 lines
2.2 KiB
Go
package main
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import (
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"errors"
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"fmt"
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)
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const maxID = (1 << 63) - 1
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// Because of Go GC, there are restrictions on keeping Go pointers in C.
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// We cannot just pass *jsonnet.VM/JsonValue to C. So instead we use "handle" structs in C
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// which refer to JsonnetVM/JsonnetJsonValue by a numeric id.
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// The way it is implemented below is simple and has low overhead, but requires us to keep
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// a list of used IDs. This results in a permanent "leak". I don't expect it to ever
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// become a problem.
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// The Handle IDs start with 1, so 0 is never a valid ID and the Handle's index in the array is (ID - 1).
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// handlesTable is the set of active, valid Jsonnet allocated handles
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type handlesTable struct {
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objects []interface{}
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freedIDs []uint64
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}
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// errMaxNumberOfOpenHandles tells that there was an attempt to create more than maxID open handles
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var errMaxNumberOfOpenHandles = fmt.Errorf("maximum number of constructed Jsonnet handles exceeded (%d)", maxID)
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// errInvalidHandle tells that there was an attempt to dereference invalid handle ID
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var errInvalidHandle = errors.New("invalid handle ID was provided")
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// make registers the new object as a handle and returns the corresponding ID
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func (h *handlesTable) make(obj interface{}) (uint64, error) {
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var id uint64
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if len(h.freedIDs) > 0 {
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id, h.freedIDs = h.freedIDs[len(h.freedIDs)-1], h.freedIDs[:len(h.freedIDs)-1]
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h.objects[id-1] = obj
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} else {
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id = uint64(len(h.objects) + 1)
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if id > maxID {
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return 0, errMaxNumberOfOpenHandles
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}
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h.objects = append(h.objects, obj)
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}
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return id, nil
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}
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// free marks the given handle ID as unused
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func (h *handlesTable) free(id uint64) error {
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if err := h.ensureValidID(id); err != nil {
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return err
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}
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h.objects[id-1] = nil
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h.freedIDs = append(h.freedIDs, id)
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return nil
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}
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// get returns the corresponding object for the provided ID
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func (h *handlesTable) get(id uint64) (interface{}, error) {
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if err := h.ensureValidID(id); err != nil {
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return nil, err
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}
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return h.objects[id-1], nil
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}
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// ensureValidID returns an error if the given handle ID is invalid, otherwise returns nil
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func (h *handlesTable) ensureValidID(id uint64) error {
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if id == 0 || uint64(id) > uint64(len(h.objects)) {
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return errInvalidHandle
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}
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return nil
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}
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