Zero-copy buffer operations

GetToBuffer and SetFromBuffer read and write Redis values directly from/to pre-allocated byte buffers, eliminating the per-call payload allocation that Get / Set would do. Useful when working with values in the KiB–MiB range where GC pressure matters.

API

// Write: sends buf contents to Redis, no string conversion.
client.SetFromBuffer(ctx, key, buf)

// Read: reads response payload directly into buf, no intermediate allocation.
cmd := client.GetToBuffer(ctx, key, buf)
n   := cmd.Val()    // number of bytes read
data := cmd.Bytes() // buf[:n]

Both methods are available on *Client, *ClusterClient, *Ring, *Conn and Pipeliner.

How it works

Both paths rely on a property of Go's bufio package: when the data being read or written is larger than the internal buffer (32 KiB by default), bufio.Reader.Read and bufio.Writer.Write bypass the internal buffer and transfer data directly between the user's buffer and the underlying socket.

No special marker types, no raw socket access, no deferred writes — just the standard buffered I/O path with the optimization that bufio already provides.

Write path (SetFromBuffer)

 User buffer          bufio.Writer             net.Conn (socket)
┌──────────┐      ┌───────────────────┐      ┌──────────────┐
│  buf     │      │ 1. Write header   │      │              │
│ ([]byte) │      │    *3\r\n$3\r\n.. │      │  RESP header │  ← small, buffered
│          │      │                   │      │              │
│          │──────│ 2. Write(buf)     │──────│  <raw data>  │  ← large, direct
│          │      │    bypasses       │      │  \r\n        │
└──────────┘      │    internal buf   │      └──────────────┘
                  └───────────────────┘

1. The RESP command header (*3\r\n$3\r\nSET\r\n$<keylen>\r\n<key>\r\n$<len>\r\n) is written into bufio.Writer's internal buffer — it's small.
2. bufio.Writer.Write(buf) sees that buf is larger than its remaining buffer space, flushes the header, then writes buf directly to the socket. The data goes from the user's buffer to the kernel with no intermediate copy.

Read path (GetToBuffer)

 net.Conn (socket)    bufio.Reader            User buffer
┌──────────────┐    ┌───────────────────┐    ┌──────────┐
│  $<len>\r\n  │────│ 1. ReadLine()     │    │          │
│              │    │    parse RESP     │    │          │
│              │    │    header         │    │          │
│              │    │                   │    │          │
│  <raw data>  │────│ 2. io.ReadFull()  │────│  buf     │
│              │    │    drain internal │    │ ([]byte) │
│              │    │    buffer, then   │    │          │
│              │    │    read remaining │    │          │
│  \r\n        │────│ 3. discard CRLF   │    └──────────┘
└──────────────┘    └───────────────────┘

1. proto.Reader.ReadStringInto(buf) calls ReadLine() to parse the RESP header ($<len>\r\n) through the standard buffered reader. This means push notifications, errors and nil responses are all handled correctly through the normal code path.
2. io.ReadFull(bufio.Reader, buf[:n]) reads the bulk data into the user's buffer. bufio.Reader.Read() first drains any data already in its internal buffer, then for the remaining bytes reads directly from the underlying socket into buf.
3. The trailing \r\n is consumed and discarded.

Benchmarks

Headline numbers for 1 MiB payloads against the test stack (make docker.start + localhost:6379, Apple M4 Max, 5 samples, medians):

GetToBuffer matches a hand-written raw-socket implementation on both latency (within 3 µs at 1 MiB) and allocation footprint (within 100 B). The vanilla Get path allocates a full payload-sized slice every call; GetToBuffer removes that completely.

Limitations

• Buffer sizing: the caller must provide a buffer large enough for the value. If the value exceeds the buffer, cmd.Err() returns "buffer too small" and no extra bytes are consumed from the reader.
• No automatic retry for GetToBuffer: ZeroCopyStringCmd.NoRetry() is true because a retry after a partial read would corrupt the buffer. This also disables retry for any pipeline that contains a GetToBuffer.
• No expiration on SetFromBuffer: use Expire() separately if needed.
• Shared buffers across commands: a *ZeroCopyStringCmd holds a reference to the buffer passed in. Reusing that buffer for a later GetToBuffer will overwrite earlier results — Example 5 in main.go demonstrates this hazard.

Run

docker run --rm -p 6379:6379 redis        # or: make docker.start (from repo root)
go run .