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io: refactor ReadAll to not check for realloc before first read #59702

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dolmen opened this issue Apr 19, 2023 · 3 comments
Closed

io: refactor ReadAll to not check for realloc before first read #59702

dolmen opened this issue Apr 19, 2023 · 3 comments
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@dolmen
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dolmen commented Apr 19, 2023

The current implementation of io.ReadAll checks for realloc before each read.

I propose instead to check for realloc after each successful read. This will avoid a check before the first read as we know the buffer is already allocated and empty.

Current implementation:

func ReadAll(r Reader) ([]byte, error) {
	b := make([]byte, 0, 512)
	for {
		if len(b) == cap(b) {
			// Add more capacity (let append pick how much).
			b = append(b, 0)[:len(b)]
		}
		n, err := r.Read(b[len(b):cap(b)])
		b = b[:len(b)+n]
		if err != nil {
			if err == EOF {
				err = nil
			}
			return b, err
		}
	}
}

My proposal:

func ReadAll(r Reader) ([]byte, error) {
	b := make([]byte, 0, 512)
	for {
		n, err := r.Read(b[len(b):cap(b)])
		b = b[:len(b)+n]
		if err != nil {
			if err == EOF {
				err = nil
			}
			return b, err
		}

		if len(b) == cap(b) {
			// Add more capacity (let append pick how much).
			b = append(b, 0)[:len(b)]
		}
	}
}
@dolmen
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dolmen commented Apr 19, 2023

CL: https://go-review.googlesource.com/c/go/+/486236

@ianlancetaylor
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Thanks, but no need to open an issue if you are going to send in a patch anyhow.

@gopherbot
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Change https://go.dev/cl/486236 mentions this issue: io.ReadAll: do not check for realloc in first round

@golang golang locked and limited conversation to collaborators Apr 19, 2024
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@dolmen @ianlancetaylor @gopherbot