forked from toolshed/abra
146 lines
4.6 KiB
Go
146 lines
4.6 KiB
Go
// Copyright (c) 2015-2016 Dave Collins <dave@davec.name>
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//
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// Permission to use, copy, modify, and distribute this software for any
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// purpose with or without fee is hereby granted, provided that the above
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// copyright notice and this permission notice appear in all copies.
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//
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// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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// NOTE: Due to the following build constraints, this file will only be compiled
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// when the code is not running on Google App Engine, compiled by GopherJS, and
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// "-tags safe" is not added to the go build command line. The "disableunsafe"
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// tag is deprecated and thus should not be used.
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// Go versions prior to 1.4 are disabled because they use a different layout
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// for interfaces which make the implementation of unsafeReflectValue more complex.
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// +build !js,!appengine,!safe,!disableunsafe,go1.4
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package spew
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import (
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"reflect"
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"unsafe"
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)
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const (
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// UnsafeDisabled is a build-time constant which specifies whether or
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// not access to the unsafe package is available.
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UnsafeDisabled = false
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// ptrSize is the size of a pointer on the current arch.
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ptrSize = unsafe.Sizeof((*byte)(nil))
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)
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type flag uintptr
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var (
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// flagRO indicates whether the value field of a reflect.Value
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// is read-only.
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flagRO flag
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// flagAddr indicates whether the address of the reflect.Value's
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// value may be taken.
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flagAddr flag
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)
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// flagKindMask holds the bits that make up the kind
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// part of the flags field. In all the supported versions,
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// it is in the lower 5 bits.
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const flagKindMask = flag(0x1f)
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// Different versions of Go have used different
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// bit layouts for the flags type. This table
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// records the known combinations.
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var okFlags = []struct {
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ro, addr flag
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}{{
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// From Go 1.4 to 1.5
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ro: 1 << 5,
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addr: 1 << 7,
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}, {
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// Up to Go tip.
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ro: 1<<5 | 1<<6,
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addr: 1 << 8,
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}}
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var flagValOffset = func() uintptr {
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field, ok := reflect.TypeOf(reflect.Value{}).FieldByName("flag")
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if !ok {
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panic("reflect.Value has no flag field")
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}
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return field.Offset
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}()
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// flagField returns a pointer to the flag field of a reflect.Value.
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func flagField(v *reflect.Value) *flag {
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return (*flag)(unsafe.Pointer(uintptr(unsafe.Pointer(v)) + flagValOffset))
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}
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// unsafeReflectValue converts the passed reflect.Value into a one that bypasses
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// the typical safety restrictions preventing access to unaddressable and
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// unexported data. It works by digging the raw pointer to the underlying
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// value out of the protected value and generating a new unprotected (unsafe)
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// reflect.Value to it.
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//
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// This allows us to check for implementations of the Stringer and error
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// interfaces to be used for pretty printing ordinarily unaddressable and
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// inaccessible values such as unexported struct fields.
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func unsafeReflectValue(v reflect.Value) reflect.Value {
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if !v.IsValid() || (v.CanInterface() && v.CanAddr()) {
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return v
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}
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flagFieldPtr := flagField(&v)
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*flagFieldPtr &^= flagRO
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*flagFieldPtr |= flagAddr
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return v
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}
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// Sanity checks against future reflect package changes
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// to the type or semantics of the Value.flag field.
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func init() {
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field, ok := reflect.TypeOf(reflect.Value{}).FieldByName("flag")
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if !ok {
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panic("reflect.Value has no flag field")
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}
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if field.Type.Kind() != reflect.TypeOf(flag(0)).Kind() {
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panic("reflect.Value flag field has changed kind")
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}
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type t0 int
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var t struct {
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A t0
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// t0 will have flagEmbedRO set.
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t0
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// a will have flagStickyRO set
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a t0
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}
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vA := reflect.ValueOf(t).FieldByName("A")
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va := reflect.ValueOf(t).FieldByName("a")
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vt0 := reflect.ValueOf(t).FieldByName("t0")
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// Infer flagRO from the difference between the flags
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// for the (otherwise identical) fields in t.
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flagPublic := *flagField(&vA)
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flagWithRO := *flagField(&va) | *flagField(&vt0)
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flagRO = flagPublic ^ flagWithRO
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// Infer flagAddr from the difference between a value
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// taken from a pointer and not.
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vPtrA := reflect.ValueOf(&t).Elem().FieldByName("A")
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flagNoPtr := *flagField(&vA)
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flagPtr := *flagField(&vPtrA)
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flagAddr = flagNoPtr ^ flagPtr
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// Check that the inferred flags tally with one of the known versions.
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for _, f := range okFlags {
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if flagRO == f.ro && flagAddr == f.addr {
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return
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}
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}
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panic("reflect.Value read-only flag has changed semantics")
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}
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