html: add option to set MaxBuf in Parse

[Jarcis-cy]

I encountered an issue when using html.Parse that triggers the following
call chain: html.Parse -> ParseWithOptions -> p.parse() -> p.tokenizer.Next()
-> readByte(). In the readByte() function, there's a logic block:

if z.maxBuf > 0 && z.raw.end-z.raw.start >= z.maxBuf {
z.err = ErrBufferExceeded
return 0
}

This logic only takes effect if maxBuf is set. However, when using html.Parse,
there is no way to use SetMaxBuf, nor is there any exported method to use
ParseWithOptions with SetMaxBuf. As a result, when parsing a very large HTML
document, such as this page: http://vod.culture.ihns.cas.cn, the memory usage
can increase significantly.

To solve this problem, I wrote a function using reflection:

func ParseOptionSetMaxBuf(maxBuf int) html.ParseOption {
funcValue := reflect.MakeFunc(
reflect.FuncOf([]reflect.Type{reflect.TypeOf((*html.ParseOption)(nil)).Elem().In(0)}, nil, false),
func(args []reflect.Value) (results []reflect.Value) {
parserValue := args[0].Elem()

        tokenizerField := parserValue.FieldByName("tokenizer")
        tokenizerPtr := reflect.NewAt(tokenizerField.Type(), unsafe.Pointer(tokenizerField.UnsafeAddr())).Elem().Interface()

        if tokenizer, ok := tokenizerPtr.(interface {
            SetMaxBuf(int)
        }); ok {
            tokenizer.SetMaxBuf(maxBuf)
        }

        return nil
    },
)
var option html.ParseOption
reflect.ValueOf(&option).Elem().Set(funcValue)
return option

}

And then used it as follows:

html.ParseWithOptions(bytes.NewReader(data), util.ParseOptionSetMaxBuf(len(data)*3))

Testing showed that setting maxBuf to at least 1.04 times the body length
ensures normal operation.

Therefore, would it be feasible to introduce a function similar to
ParseOptionEnableScripting that allows users to set MaxBuf?

Environment:

• Go version: 1.21
• OS: Tested on Ubuntu 22.04 and Windows 11

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Patch Set 1:

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 2: Hold+1

(1 comment)

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