reduce relaxedBodyCanonicalizer allocations

[pierrre]

In my application, nearly 50% of my memory allocations (in count) are caused by relaxedBodyCanonicalizer.

go-msgauth/dkim/canonical.go

Lines 130 to 158 in 6dd5b6c

	type relaxedBodyCanonicalizer struct {
	w io.Writer
	crlfBuf []byte
	wspBuf []byte
	written bool
	crlfFixer crlfFixer
	}
	func (c *relaxedBodyCanonicalizer) Write(b []byte) (int, error) {
	written := len(b)
	b = c.crlfFixer.Fix(b)
	canonical := make([]byte, 0, len(b))
	for _, ch := range b {
	if ch == ' ' || ch == '\t' {
	c.wspBuf = append(c.wspBuf, ch)
	} else if ch == '\r' || ch == '\n' {
	c.wspBuf = nil
	c.crlfBuf = append(c.crlfBuf, ch)
	} else {
	if len(c.crlfBuf) > 0 {
	canonical = append(canonical, c.crlfBuf...)
	c.crlfBuf = nil
	}
	if len(c.wspBuf) > 0 {
	canonical = append(canonical, ' ')
	c.wspBuf = nil
	}

Especially lines 146 and 149.

I think this code could be optimized.
Currently each call to append causes a new memory allocation and requires to copy the memory.
Because the slices are reset to nil.
It doesn't reuse the memory previously allocated.
The solution: do c.wspBuf = c.wspBuf[:0] instead of c.wspBuf = nil.
Same for crlfBuf.

WDYT ?

[emersion]

wspBuf could be replaced with a wsp bool field, since we never use the actual value. Your suggestion looks like a good idea for crlfBuf.

Patches welcome!

[pierrre]

wspBuf could be replaced with a wsp bool field

Yes I noticed that too 😅

I'll work on it tomorrow.

[pierrre]

Here is the PR #47