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|
package rrdformat
import (
"encoding/binary"
"fmt"
"math"
"git.lukeshu.com/go/librrd/rrdformat/rrdbinary"
)
func SniffArchitecture(data []byte) (rrdbinary.Architecture, error) {
var arch rrdbinary.Architecture
var header struct {
Cookie rrdbinary.String `rrdbinary:"size=4" xml:"-"`
Version rrdbinary.String `rrdbinary:"size=5" xml:"version"`
}
if err := rrdbinary.NewDecoder(arch, data).Decode(&header, ""); err != nil {
return rrdbinary.Architecture{}, err
}
// 1. File header magic number
if header.Cookie != "RRD" {
return rrdbinary.Architecture{}, rrdbinary.NewBinError("not an RRD file: wrong magic number", data, 0, 4)
}
// 1. File format version string
switch header.Version {
case "0001", "0002", "0003", "0004", "0005":
// do nothing
default:
return rrdbinary.Architecture{}, rrdbinary.NewBinError(fmt.Sprintf("can't handle RRD file version %q", header.Version), data, 4, 5)
}
// 3. float cookie
//
// Assume IEEE 754 doubles. C doesn't assume 754 doubles, but anything that doesn't use 754 doubles is exotic
// enough that I'm OK saying "you're going to need to use `rrdtool dump`". This lets us assume that:
// - a 'double' is 8 bytes wide
// - the value will be exactly equal, and we don't need to worry about weird rounding.
arch.DoubleWidth = 8
magicFloat := float64(8.642135e130)
floatAddrPacked := 9
floatAddr32 := ((floatAddrPacked + 3) / 4) * 4
floatAddr64 := ((floatAddrPacked + 7) / 8) * 8
var restOffset int
switch {
case len(data) < floatAddr32+arch.DoubleWidth:
return rrdbinary.Architecture{}, rrdbinary.NewBinError("unexpected end-of-file", data, floatAddrPacked, floatAddr64+arch.DoubleWidth-floatAddrPacked)
case math.Float64frombits(binary.LittleEndian.Uint64(data[floatAddr32:])) == magicFloat:
arch.ByteOrder = binary.LittleEndian
arch.DoubleAlign = 4
restOffset = floatAddr32 + arch.DoubleWidth
case math.Float64frombits(binary.BigEndian.Uint64(data[floatAddr32:])) == magicFloat:
arch.ByteOrder = binary.BigEndian
arch.DoubleAlign = 4
restOffset = floatAddr32 + arch.DoubleWidth
case len(data) < floatAddr64+arch.DoubleWidth:
return rrdbinary.Architecture{}, rrdbinary.NewBinError("unexpected end-of-file", data, floatAddrPacked, floatAddr64+arch.DoubleWidth-floatAddrPacked)
case math.Float64frombits(binary.LittleEndian.Uint64(data[floatAddr64:])) == magicFloat:
arch.ByteOrder = binary.LittleEndian
arch.DoubleAlign = 8
restOffset = floatAddr64 + arch.DoubleWidth
case math.Float64frombits(binary.BigEndian.Uint64(data[floatAddr64:])) == magicFloat:
arch.ByteOrder = binary.BigEndian
arch.DoubleAlign = 8
restOffset = floatAddr64 + arch.DoubleWidth
default:
return rrdbinary.Architecture{}, rrdbinary.NewBinError("failed to sniff byte-order and float-alignment",
data, floatAddrPacked, floatAddr64+arch.DoubleWidth-floatAddrPacked)
}
// 5, 6. ds_cnt, rra_cnt
switch arch.DoubleAlign {
case 4:
// Assume that if floats are only 32-bit aligned, then everything is 32-bit
arch.LongWidth = 4
arch.LongAlign = 4
case 8:
// If floats are 64-bit aligned, then this might be all-in on 64-bit, or it might 32-bit ints.
// (The following heuristic is borrowed from javascriptRRD, and adjusted to also work with big-endian.)
//
// The next 2 things after the float_cookie are ds_cnt and rra_cnt (both 'unsigned long'--which may be
// either 32 or 64 bit). We'll inspect the bytes a bit to guess how long a long is.
//
// By assuming
// 1. ds_cnt <= math.MaxUint32
// 2l. rra_cnt > 0 (relevant if little-endian)
// 2b. ds_cnt > 0 (relevant if big-endian)
// we can inspect the 4 bytes (marked "big" or "little" below) that are either
// - the most significant bits of 64-bit ds_cnt, or
// - the entirety of 32-bit rra_cnt (if littlen-endian) or 32-bit ds_cnt (if big-endian)
// If we see that those 4 bytes are all 0, then we assume that it's part of a 64-bit ds_cnt.
//
// | | | | | | | big | little |
// |00|01|02|03|04|05|06|07|08|09|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|27|28|29|30|31|
// 32 | R| R| D|\0| 0| 0| 0| 3|\0| |<----doublecookie----->|<--ds_cnt->|<-rra_cnt->|
// 64le | R| R| D|\0| 0| 0| 0| 3|\0| |<----doublecookie----->|<1111----ds_cnt---0000>|
// 64be | R| R| D|\0| 0| 0| 0| 3|\0| |<----doublecookie----->|<0000----ds_cnt---1111>|
if len(data) < restOffset+8 {
return rrdbinary.Architecture{}, rrdbinary.NewBinError("unexpected end of file", data, restOffset, 8)
}
offset := map[binary.ByteOrder]int{
binary.BigEndian: restOffset, // 24 in the above diagram
binary.LittleEndian: restOffset + 4, // 28 in the above diagram
}[arch.ByteOrder]
if arch.ByteOrder.Uint32(data[offset:]) == 0 {
arch.LongWidth = 8
arch.LongAlign = 8
} else {
arch.LongWidth = 4
arch.LongAlign = 4
}
}
// The above just os happens that DoubleXXX >= LongXXX, so we
// can just set the Unival stuff to the Double Stuff.
arch.UnivalWidth = arch.DoubleWidth // max(DoubleWidth, LongWidth)
arch.UnivalAlign = arch.DoubleAlign // max(DoubleAlign, LongAlign)
// FIXME: Figure out how to sniff the sizeof(time_t).
//
// javascriptRRD assumes that it's the same as sizeof(long),
// which his historically been true, but
// - isn't true of proprietary 32-bit Unixen that are 2038-safe
// - isn't true of the Linux kernel with the x32 ABI
arch.TimeWidth = arch.LongWidth
arch.TimeAlign = arch.LongAlign
return arch, nil
}
|
