servuo-plugins/overlay/Scripts/Custom/Bridge/BridgeAssetValidator.cs

using System;
using System.IO;

using Ultima;

namespace Server.Custom.Bridge
{
    /// <summary>
    ///     **Validate before calling** (docs/link/v8.md §4.5) — the boundary between this protocol
    ///     and ServUO's vendored <c>Ultima</c> decoders. Phase 0 prototyped it in
    ///     <c>tools/scaffolding/BridgeAssetProbe.cs</c> and measured it both ways; phase 1 promoted
    ///     it here, into the overlay, and extended it to animations.
    ///
    ///     The principle: `Ultima`'s decoders take their bounds from the file they are reading, so
    ///     the extractor must decide whether a record is worth handing over *before* handing it
    ///     over. Every check below is against the index entry and the record header — cheap, and
    ///     enough to turn an uncatchable corrupted-state exception into a skipped asset.
    ///
    ///     **The failure this exists for is a wrong picture, not a crash.** `LoadStatic`,
    ///     `LoadLand` and `GetAnimation` all decode out of a shared <c>m_StreamBuffer</c> that is
    ///     reused, only ever grown, and filled by a <c>stream.Read</c> whose return value is
    ///     discarded. A record that is short, absent or out of bounds therefore renders **whatever
    ///     the previously-decoded asset left behind**, reports success, and is undetectable by
    ///     anything downstream. On the stock client on the machine phase 0 ran on that is 22,102
    ///     ids whose index entry reads <c>lookup 0, length 0</c>.
    ///
    ///     It cannot be complete and does not claim to be. It closes the shapes that reading the
    ///     source showed are reachable. What says the boundary is in the right place is the second
    ///     measurement rather than the first: against a client patched 21 ways it refused all eight
    ///     record-level defects, and against the **stock** client it refused **nothing** across
    ///     49,151 statics and 16,384 land tiles. A checker that refuses real art would be worse
    ///     than no checker.
    /// </summary>
    public static class BridgeAssetValidator
    {
        public enum Verdict
        {
            /// <summary>Nothing at this id, and the index says so honestly.</summary>
            Absent,

            /// <summary>The entry is self-consistent and inside its file.</summary>
            Ok,

            /// <summary>The entry claims something the file cannot support. Do not decode it.</summary>
            Refused
        }

        /// <summary>Land tiles decode a fixed 44×44 diamond: 2 × (2+4+…+44) ushorts.</summary>
        public const int LandRecordBytes = 2024;

        /// <summary>
        ///     A ceiling on decoded art dimensions. `LoadStatic` allocates
        ///     <c>new Bitmap(width, height)</c> straight from two ushorts in the record, so a
        ///     corrupt header asks for up to 65535×65535 — an 8 GB allocation, from a file. Real
        ///     art is a couple of hundred pixels at most.
        /// </summary>
        public const int MaxArtDimension = 1024;

        /// <summary>
        ///     Builds our own index over the same files, with the same constructor arguments
        ///     <c>Art</c> uses — including <c>hasExtra: false</c>, which is the whole reason the
        ///     art path is safe where the gump path is not (§4.1).
        /// </summary>
        public static FileIndex OpenArtIndex()
        {
            if (ArtDataPath() == null)
                return null;

            return new FileIndex("Artidx.mul", "Art.mul", "artLegacyMUL.uop", 0x10000, 4, ".tga", 0x13FDC, false);
        }

        /// <summary>
        ///     The file an art index entry's <c>lookup</c> is an offset **into** — which is not
        ///     <c>art.mul</c> on any current client.
        ///
        ///     This cost a whole probe run to learn and it is the single most important thing
        ///     phase 1 must not get wrong. <c>FileIndex</c>'s UOP constructor ends with a bare
        ///     <c>MulPath = uopPath</c>: **when <c>artLegacyMUL.uop</c> exists it wins outright**,
        ///     and <c>art.mul</c> / <c>artidx.mul</c> are never opened at all. A validator that
        ///     bounds offsets against <c>art.mul</c> while the index holds UOP offsets is not
        ///     merely approximate, it is nonsense — the first run of this probe refused 34,299
        ///     perfectly good statics for "declaring 10533x2085" because it was reading UOP
        ///     offsets into the wrong file.
        ///
        ///     So the resolution order here mirrors <c>FileIndex</c>'s exactly, and anything that
        ///     needs the bytes behind an entry must ask this rather than assume.
        /// </summary>
        public static string ArtDataPath()
        {
            var uop = Files.GetFilePath("artlegacymul.uop");

            if (uop != null)
                return uop;

            return Files.GetFilePath("art.mul");
        }

        public static long MulLength(string path)
        {
            if (path == null)
                return 0;

            try
            {
                return new FileInfo(path).Length;
            }
            catch
            {
                return 0;
            }
        }

        /// <summary>
        ///     Judges one index entry.
        ///
        ///     The check <c>FileIndex.Seek</c> is missing is the last one: it tests
        ///     <c>Stream.Length &lt; e.lookup</c> — that the record *starts* inside the file — and
        ///     never that it *ends* inside it. A record that begins two bytes before EOF and
        ///     declares a length of 4,000 passes, and <c>stream.Read</c> then returns a short count
        ///     that the decoders discard, leaving the previous asset's bytes in the shared buffer.
        /// </summary>
        public static Verdict CheckEntry(FileIndex index, int at, long mulLength, long verdataLength, out string reason)
        {
            reason = null;

            if (index == null || index.Index == null || at < 0 || at >= index.Index.Length)
            {
                reason = "index " + at + " out of range";
                return Verdict.Absent;
            }

            Entry3D e = index.Index[at];

            if (e.lookup < 0)
            {
                reason = "lookup " + e.lookup;
                return Verdict.Absent;
            }

            bool patched = (e.length & (1 << 31)) != 0;
            int length = e.length & 0x7FFFFFFF;

            if (!patched && e.length < 0)
            {
                reason = "length " + e.length;
                return Verdict.Absent;
            }

            if (length == 0)
            {
                reason = "lookup " + e.lookup + ", length 0";
                return Verdict.Absent;
            }

            long ceiling = patched ? verdataLength : mulLength;

            if (ceiling <= 0)
            {
                reason = (patched ? "verdata.mul" : "the art data file") + " has no length";
                return Verdict.Refused;
            }

            if (e.lookup >= ceiling)
            {
                reason = "lookup " + e.lookup + " past the end of "
                         + (patched ? "verdata.mul" : "the mul") + " (" + ceiling + ")";
                return Verdict.Refused;
            }

            // The missing check. A short read is silent, and its consequence is the PREVIOUS
            // asset's picture served under this id.
            if (e.lookup + (long)length > ceiling)
            {
                reason = "record runs " + (e.lookup + (long)length - ceiling) + " bytes past the end of "
                         + (patched ? "verdata.mul" : "the mul");
                return Verdict.Refused;
            }

            return Verdict.Ok;
        }

        /// <summary>
        ///     `LoadLand` reads 2,024 bytes regardless of the declared length, so a shorter record
        ///     reads past the end of a buffer sized from that length.
        /// </summary>
        public static bool LandLengthSane(FileIndex index, int at, out string reason)
        {
            reason = null;

            if (index == null || index.Index == null || at < 0 || at >= index.Index.Length)
                return true;

            int length = index.Index[at].length & 0x7FFFFFFF;

            if (length > 0 && length < LandRecordBytes)
            {
                reason = "land record is " + length + " bytes; LoadLand always reads " + LandRecordBytes;
                return false;
            }

            return true;
        }

        /// <summary>
        ///     Walks a static record's own row table the way <c>LoadStatic</c> will, and refuses
        ///     it if that walk would read outside the record.
        ///
        ///     This is the check with teeth. <c>LoadStatic</c>'s inner loop guards the write into
        ///     the bitmap (<c>xOffset &gt; delta</c>, <c>xOffset + xRun &gt; delta</c>) and does
        ///     nothing at all about the read cursor, which advances until it happens to find a
        ///     zero pair — potentially far outside a pinned array. Simulating the same walk with
        ///     a bound is the cheapest way to know whether handing the id over is safe.
        /// </summary>
        public static bool StaticRecordSane(byte[] record, int length, out string reason)
        {
            reason = null;

            if (length < 8)
            {
                reason = "record is " + length + " bytes; a static header needs 8";
                return false;
            }

            int words = length / 2;
            int width = ReadUInt16(record, 4);
            int height = ReadUInt16(record, 6);

            // LoadStatic returns null for these rather than misbehaving, so it is not a refusal.
            if (width <= 0 || height <= 0)
                return true;

            if (width > MaxArtDimension || height > MaxArtDimension)
            {
                reason = "declares " + width + "x" + height + ", past the " + MaxArtDimension + "px ceiling";
                return false;
            }

            // The row-lookup table: height ushorts starting at word 4.
            if (4 + height > words)
            {
                reason = "row table (" + height + " entries) does not fit in a " + length + "-byte record";
                return false;
            }

            int start = height + 4;

            for (int y = 0; y < height; y++)
            {
                int cursor = start + ReadUInt16(record, (4 + y) * 2);

                while (true)
                {
                    // Two ushorts for the run header, and they must both be inside the record.
                    if (cursor < 0 || cursor + 1 >= words)
                    {
                        reason = "row " + y + " reads at word " + cursor + ", past the record's " + words;
                        return false;
                    }

                    int xOffset = ReadUInt16(record, cursor * 2);
                    int xRun = ReadUInt16(record, (cursor + 1) * 2);
                    cursor += 2;

                    if (xOffset + xRun == 0)
                        break;

                    // LoadStatic stops the row here, so the read cursor stops with it.
                    if (xOffset > width || xOffset + xRun > width)
                        break;

                    if (cursor + xRun > words)
                    {
                        reason = "row " + y + " declares a " + xRun + "-pixel run running past the record";
                        return false;
                    }

                    cursor += xRun;
                }
            }

            return true;
        }

        // ── animations (phase 1) ─────────────────────────────────────────────────────────────
        //
        // Phase 0 measured the art path and left this half unbuilt, and then proved it was
        // needed: the patched client's verdata entry for body 34 points past verdata.mul's own
        // end, and the wolf still "decoded" — counted among the 1,144 successes while rendering
        // something else entirely. `GetAnimation` has every weakness `LoadStatic` has and one
        // more, because the buffer it decodes from is longer than the record it read.

        /// <summary>The palette every animation record opens with: 0x100 ushorts.</summary>
        public const int AnimPaletteBytes = 0x100 * 2;

        /// <summary>
        ///     A ceiling on an animation's declared frame count. <c>GetAnimation</c> does
        ///     <c>new int[frameCount]</c> straight from four bytes in the file, before it has
        ///     looked at anything else. Real actions are tens of frames.
        /// </summary>
        public const int MaxAnimFrames = 1024;

        /// <summary>The xor <c>Frame</c> applies to every run header before decoding it.</summary>
        private const int DoubleXor = (0x200 << 22) | (0x200 << 12);

        /// <summary>
        ///     The <c>anim*.mul</c> an animation index entry's <c>lookup</c> is an offset into.
        ///
        ///     Unlike art (§4.6) there is no UOP precedence to get wrong here, and that is not
        ///     luck: <c>Animations</c> constructs its five <c>FileIndex</c>es with the four-argument
        ///     constructor, which passes <c>uopFile: null</c>. It never reads
        ///     <c>AnimationFrame*.uop</c> at all — which is the same fact that leaves six of the
        ///     twelve player-character bodies undecodable until §4.3's reader lands in phase 4.
        /// </summary>
        public static string AnimDataPath(int fileType)
        {
            switch (fileType)
            {
                case 1: return Files.GetFilePath("anim.mul");
                case 2: return Files.GetFilePath("anim2.mul");
                case 3: return Files.GetFilePath("anim3.mul");
                case 4: return Files.GetFilePath("anim4.mul");
                case 5: return Files.GetFilePath("anim5.mul");
                default: return null;
            }
        }

        /// <summary>
        ///     Builds our own index over one anim file, with the same constructor arguments
        ///     <c>Animations</c> uses — the entry lengths especially, since they decide how far
        ///     into the file an index runs.
        /// </summary>
        public static FileIndex OpenAnimIndex(int fileType)
        {
            if (AnimDataPath(fileType) == null)
                return null;

            switch (fileType)
            {
                case 1: return new FileIndex("Anim.idx", "Anim.mul", 0x40000, 6);
                case 2: return new FileIndex("Anim2.idx", "Anim2.mul", 0x10000, -1);
                case 3: return new FileIndex("Anim3.idx", "Anim3.mul", 0x20000, -1);
                case 4: return new FileIndex("Anim4.idx", "Anim4.mul", 0x20000, -1);
                case 5: return new FileIndex("Anim5.idx", "Anim5.mul", 0x20000, -1);
                default: return null;
            }
        }

        /// <summary>
        ///     Where a body's animation actually lives: which anim file, and which index in it.
        ///
        ///     **This is the never-sweep-file-types rule, written as code** (§4.3). It asks
        ///     <c>BodyConverter.Convert</c> once, takes its answer, and if that answer leads
        ///     nowhere it reports nowhere. There is deliberately no loop here and no fallback,
        ///     because asking the *other* anim files for an index they do not own does not fail —
        ///     it returns 175 decodable action/direction combinations of **a giant spider** for
        ///     gargoyle 666, and misaligned colour fragments for the other two. Every one of those
        ///     reads reports success, and nothing downstream can tell them from art.
        ///
        ///     A false return with <paramref name="reason"/> set is the ordinary, expected answer
        ///     for a body this client has no art for — the caller reports absent, not an error.
        /// </summary>
        public static bool ResolveAnimation(
            int body, int action, int direction, out int fileType, out int index, out string reason)
        {
            reason = null;
            fileType = 0;
            index = -1;

            if (body <= 0 || action < 0)
            {
                reason = "body " + body + " action " + action + " is not addressable";
                return false;
            }

            // Directions 5-7 are the client mirroring 1-3, and `Frame` decodes them through its
            // flip branch — different pointer arithmetic, which nothing below has checked.
            // §5.1 fixed this protocol at direction 0 or 1, so refusing the rest costs nothing
            // and keeps the validator honest about what it has actually verified.
            if (direction < 0 || direction > 4)
            {
                reason = "direction " + direction + " is mirrored; this protocol reads 0-4 only";
                return false;
            }

            int translated = body;
            int hue = 0;

            try
            {
                // Exactly what GetAnimation(..., preserveHue: false, ...) does first.
                Animations.Translate(ref translated, ref hue);
                fileType = BodyConverter.Convert(ref translated);
            }
            catch (Exception e)
            {
                reason = "body.def/bodyconv.def lookup failed: " + e.GetType().Name;
                return false;
            }

            if (AnimDataPath(fileType) == null)
            {
                // Gargoyle 666 lands here: Bodyconv.def maps it to anim5, and this client has no
                // anim5. Absent is the correct answer and the ONLY safe one.
                reason = "bodyconv sends body " + body + " to file type " + fileType
                         + ", which this client does not have";
                return false;
            }

            index = AnimIndexOf(translated, fileType) + (action * 5) + direction;

            return true;
        }

        /// <summary>
        ///     <c>Animations.GetFileIndex</c>'s own arithmetic, which is private. The banding is
        ///     per file type and the boundaries differ between them, so this is transcribed rather
        ///     than generalised — an index that disagrees with the library's by one is a picture
        ///     of the wrong creature, validated.
        /// </summary>
        private static int AnimIndexOf(int body, int fileType)
        {
            switch (fileType)
            {
                case 2:
                    return body < 200 ? body * 110 : 22000 + ((body - 200) * 65);

                case 3:
                    if (body < 300)
                        return body * 65;

                    return body < 400 ? 33000 + ((body - 300) * 110) : 35000 + ((body - 400) * 175);

                case 5:
                    // "looks strange, though it works" — the library's own comment. Body 34 is
                    // excluded from the first band here and nowhere else.
                    if (body < 200 && body != 34)
                        return body * 110;

                    return body < 400 ? 22000 + ((body - 200) * 65) : 35000 + ((body - 400) * 175);

                default: // 1 and 4 share their banding
                    if (body < 200)
                        return body * 110;

                    return body < 400 ? 22000 + ((body - 200) * 65) : 35000 + ((body - 400) * 175);
            }
        }

        /// <summary>
        ///     Walks an animation record the way <c>GetAnimation</c> and <c>Frame</c> will, and
        ///     refuses it if that walk would read outside the record or write outside the bitmap.
        ///
        ///     Two things make this stricter than the static walk, and both come from the library:
        ///
        ///     <c>GetAnimation</c> decodes through <c>new MemoryStream(m_StreamBuffer, false)</c> —
        ///     the whole shared buffer, not the <c>length</c> bytes it just read into it. So a
        ///     truncated record does not hit end-of-stream and throw; the reader sails on into the
        ///     **previous** animation's bytes and returns a plausible frame. Bounding against
        ///     <paramref name="length"/> rather than against the buffer is the entire point.
        ///
        ///     And <c>Frame</c>'s run loop is a *write* through a <c>LockBits</c> pointer whose
        ///     origin comes from two signed shorts in the file (<c>xCenter</c>, <c>yCenter</c>),
        ///     with no bound of any kind. <c>LoadStatic</c> at least guards its writes; this does
        ///     not, so the destination of every run is checked against the bitmap it locked.
        ///
        ///     <paramref name="maxFrames"/> is how many frames the caller will actually decode —
        ///     1 for the catalogue's thumbnail (<c>FirstFrame: true</c>), 0 for all of them.
        ///     Checking frames nobody decodes would invent refusals, which §4.5 costs more than
        ///     it saves.
        /// </summary>
        public static bool AnimationRecordSane(byte[] record, int length, int maxFrames, out string reason)
        {
            reason = null;

            if (length < AnimPaletteBytes + 4)
            {
                reason = "record is " + length + " bytes; an animation needs "
                         + (AnimPaletteBytes + 4) + " for its palette and frame count";
                return false;
            }

            int start = AnimPaletteBytes;
            int frameCount = ReadInt32(record, start);

            if (frameCount <= 0)
            {
                reason = "declares " + frameCount + " frames";
                return false;
            }

            if (frameCount > MaxAnimFrames)
            {
                reason = "declares " + frameCount + " frames, past the " + MaxAnimFrames + " ceiling";
                return false;
            }

            // The lookup table is read in full whatever FirstFrame says, so it is bounded in full.
            long tableEnd = (long)start + 4 + ((long)frameCount * 4);

            if (tableEnd > length)
            {
                reason = "frame table (" + frameCount + " entries) does not fit in a "
                         + length + "-byte record";
                return false;
            }

            int check = maxFrames > 0 && maxFrames < frameCount ? maxFrames : frameCount;

            for (int i = 0; i < check; i++)
            {
                int at = start + ReadInt32(record, start + 4 + (i * 4));

                if (!FrameSane(record, length, at, i, out reason))
                    return false;
            }

            return true;
        }

        private static bool FrameSane(byte[] record, int length, int at, int frame, out string reason)
        {
            reason = null;

            if (at < 0 || at + 8 > length)
            {
                reason = "frame " + frame + " starts at " + at + ", outside the "
                         + length + "-byte record";
                return false;
            }

            int xCenter = ReadInt16(record, at);
            int yCenter = ReadInt16(record, at + 2);
            int width = ReadUInt16(record, at + 4);
            int height = ReadUInt16(record, at + 6);

            // Frame's constructor returns before locking anything for these, so they are empty
            // rather than dangerous — and an empty frame is a real thing in this format.
            if (width == 0 || height == 0)
                return true;

            if (width > MaxArtDimension || height > MaxArtDimension)
            {
                reason = "frame " + frame + " declares " + width + "x" + height + ", past the "
                         + MaxArtDimension + "px ceiling";
                return false;
            }

            // Settings.PixelFormat is 16bpp and GDI+ pads each scanline to four bytes, so a row
            // is `delta` ushorts wide and the locked region is height*delta of them. This is the
            // same `bd.Stride >> 1` Frame computes.
            int delta = (((width * 2) + 3) & ~3) >> 1;
            long pixels = (long)height * delta;

            long origin = (xCenter - 0x200) + ((long)((yCenter + height) - 0x200) * delta);
            int cursor = at + 8;

            while (true)
            {
                if (cursor + 4 > length)
                {
                    reason = "frame " + frame
                             + " runs off the end of the record looking for its terminator";
                    return false;
                }

                int header = ReadInt32(record, cursor);
                cursor += 4;

                if (header == 0x7FFF7FFF)
                    break;

                header ^= DoubleXor;

                long dy = (header >> 12) & 0x3FF;
                long dx = (header >> 22) & 0x3FF;
                int run = header & 0xFFF;

                long first = origin + (dy * delta) + dx;

                if (first < 0 || first + run > pixels)
                {
                    reason = "frame " + frame + " writes pixels " + first + ".." + (first + run)
                             + " outside its own " + pixels + "-pixel bitmap";
                    return false;
                }

                // One palette byte per pixel, read straight out of the record.
                if (cursor + run > length)
                {
                    reason = "frame " + frame + " declares a " + run
                             + "-pixel run running past the record";
                    return false;
                }

                cursor += run;
            }

            return true;
        }

        private static int ReadUInt16(byte[] b, int at)
        {
            return b[at] | (b[at + 1] << 8);
        }

        private static int ReadInt16(byte[] b, int at)
        {
            return (short)(b[at] | (b[at + 1] << 8));
        }

        private static int ReadInt32(byte[] b, int at)
        {
            return b[at] | (b[at + 1] << 8) | (b[at + 2] << 16) | (b[at + 3] << 24);
        }

        /// <summary>
        ///     Reads a record's actual bytes so <see cref="StaticRecordSane"/> or
        ///     <see cref="AnimationRecordSane"/> can walk it.
        ///
        ///     Holds its own handles rather than borrowing the library's, because <c>FileIndex</c>
        ///     hands out the stream it decodes from and moving that stream's position underneath
        ///     the decoder would be its own bug. Opened <c>FileShare.ReadWrite</c> to match how
        ///     <c>FileIndex</c> opens the same files.
        ///
        ///     One reader serves one data file, so an animation sweep wants one per file type,
        ///     built from <see cref="AnimDataPath"/>.
        /// </summary>
        public sealed class RecordReader : IDisposable
        {
            private readonly FileStream _mul;
            private readonly FileStream _verdata;
            private byte[] _scratch = new byte[64 * 1024];

            public RecordReader(string mulPath, string verdataPath)
            {
                _mul = Open(mulPath);
                _verdata = Open(verdataPath);
            }

            private static FileStream Open(string path)
            {
                if (path == null || !File.Exists(path))
                    return null;

                try
                {
                    return new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
                }
                catch
                {
                    return null;
                }
            }

            /// <summary>
            ///     True when the record at <paramref name="at"/> is safe to hand to
            ///     <c>Art.GetStatic</c>. A record that cannot be read at all is reported sane —
            ///     <see cref="CheckEntry"/> has already judged the entry, and this must not
            ///     invent a second reason to refuse.
            /// </summary>
            public bool StaticSane(FileIndex index, int at, out string reason)
            {
                int length = ReadRecord(index, at, out reason);

                if (length < 0)
                    return true;

                if (length == 0)
                    return false;

                return StaticRecordSane(_scratch, length, out reason);
            }

            /// <summary>
            ///     True when the record at <paramref name="at"/> is safe to hand to
            ///     <c>Animations.GetAnimation</c>. <paramref name="maxFrames"/> is how many frames
            ///     the caller will decode — 1 for a <c>FirstFrame</c> call, 0 for all of them.
            /// </summary>
            public bool AnimationSane(FileIndex index, int at, int maxFrames, out string reason)
            {
                int length = ReadRecord(index, at, out reason);

                if (length < 0)
                    return true;

                if (length == 0)
                    return false;

                return AnimationRecordSane(_scratch, length, maxFrames, out reason);
            }

            /// <summary>
            ///     Reads one record into <see cref="_scratch"/>. Returns its length, 0 for a
            ///     failure (with <paramref name="reason"/> set), or -1 when there is nothing to
            ///     read at all — <see cref="CheckEntry"/> has already judged the entry, and this
            ///     must not invent a second reason to refuse.
            /// </summary>
            private int ReadRecord(FileIndex index, int at, out string reason)
            {
                reason = null;

                if (index == null || index.Index == null || at < 0 || at >= index.Index.Length)
                    return -1;

                Entry3D e = index.Index[at];
                bool patched = (e.length & (1 << 31)) != 0;
                int length = e.length & 0x7FFFFFFF;

                var stream = patched ? _verdata : _mul;

                if (stream == null || length <= 0 || e.lookup < 0)
                    return -1;

                if (_scratch.Length < length)
                    _scratch = new byte[length];

                int read;

                try
                {
                    stream.Seek(e.lookup, SeekOrigin.Begin);
                    read = stream.Read(_scratch, 0, length);
                }
                catch (Exception ex)
                {
                    reason = "cannot read the record: " + ex.GetType().Name;
                    return 0;
                }

                // The short read the decoders discard. Refusing here is the whole point: the
                // library would decode whatever the shared buffer happened to hold.
                if (read < length)
                {
                    reason = "short read — " + read + " of " + length + " bytes available";
                    return 0;
                }

                return length;
            }

            public void Dispose()
            {
                if (_mul != null)
                    _mul.Dispose();

                if (_verdata != null)
                    _verdata.Dispose();
            }
        }
    }
}