Fix Upgrader.Upgrade never reusing hijacked write buffer

[veeceey]

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update
• Go Version Update
• Dependency Update

Description

bufio.Writer.AvailableBuffer() returns a slice with len == 0 but cap equal to the available buffer space:

// From bufio package:
func (b *Writer) AvailableBuffer() []byte {
    return b.buf[b.n:][:0]
}

The existing code in Upgrader.Upgrade checked len(buf) which was always 0, making the write buffer reuse path dead code:

buf := brw.Writer.AvailableBuffer()
// len(buf) is always 0 here, so this condition is never true:
if u.WriteBufferPool == nil && u.WriteBufferSize == 0 && len(buf) >= maxFrameHeaderSize+256 {
    writeBuf = buf  // never reached
}

This caused an unnecessary memory allocation on every Upgrade call even when the hijacked write buffer was large enough to be reused.

Changes

1. server.go - Fix buffer capacity check: Use cap(buf) instead of len(buf) to check the hijacked buffer size, and extend the slice to its full capacity with buf[:cap(buf)] so downstream code using len(c.writeBuf) works correctly.

2. server.go - Fix response header buffer selection: Use cap(p) instead of len(p) when comparing the hijacked buffer against the connection write buffer for constructing the response header. Without this fix, len(p) is always 0, so the comparison len(c.writeBuf) > len(p) always selects the connection buffer even when the hijacked buffer is larger.

3. conn.go / server.go - Fix typos: "effor" -> "effort" (2 occurrences in conn.go), "buferred" -> "buffered" (1 occurrence in server.go).

4. server_test.go - Add TestWriteBufferReuse: Validates that large enough hijacked buffers are reused and small buffers are not. This test fails on the old code and passes with the fix.

Related Tickets & Documents

• Closes [BUG] fix: len(buf) => cap(buf) #973

Added/updated tests?

• Yes

Run verifications and test

• go vet ./... is passing
• go test ./... is passing

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.

[veeceey]

hey, circling back on this. would love to get it merged if it looks good to you