[7.x](backport #27782) LineReader: Reuse temporary buffer to reduce per-line allocation #27824
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) ## What does this PR do? Previously, the `LineReader` would allocate a []byte of size `config.BufferSize` before decoding each line. The underlying array's size allocation is fixed, so `outBuffer.Append` retains all of it even when the appended bytes are much shorter. With this change, we store a single `tempBuffer []byte` which is reused across lines anywhere we need temporary storage. Converting to `outBuffer.Write` forces the buffer to copy data out of tempBuffer, but is able to only allocate space for the written bytes. ## Why is it important? In our production environment, we run beats with k8s-enforced memory limits and are trying to resolve OOMs. The LineReader code path contributes a significant amount of memory allocation. The benchmarks added in bench_test.go show this reduces the memory profile with various line lengths: ``` goos: darwin goarch: amd64 pkg: github.com/elastic/beats/v7/libbeat/reader/readfile cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz name old time/op new time/op delta EncoderReader/buffer-sized_lines-16 125µs ± 3% 94µs ± 9% -24.55% (p=0.008 n=5+5) EncoderReader/short_lines-16 52.6µs ± 4% 36.3µs ±10% -30.88% (p=0.008 n=5+5) EncoderReader/long_lines-16 1.82ms ± 2% 1.70ms ±10% ~ (p=0.151 n=5+5) EncoderReader/skip_lines-16 133µs ± 3% 140µs ± 8% ~ (p=0.151 n=5+5) name old alloc/op new alloc/op delta EncoderReader/buffer-sized_lines-16 442kB ± 0% 239kB ± 0% -46.07% (p=0.000 n=4+5) EncoderReader/short_lines-16 118kB ± 0% 15kB ± 0% -87.27% (p=0.008 n=5+5) EncoderReader/long_lines-16 8.73MB ± 0% 7.63MB ± 0% -12.62% (p=0.000 n=4+5) EncoderReader/skip_lines-16 270kB ± 0% 220kB ± 0% -18.58% (p=0.008 n=5+5) name old allocs/op new allocs/op delta EncoderReader/buffer-sized_lines-16 718 ± 0% 519 ± 0% -27.72% (p=0.008 n=5+5) EncoderReader/short_lines-16 522 ± 0% 421 ± 0% -19.35% (p=0.008 n=5+5) EncoderReader/long_lines-16 2.65k ± 0% 1.58k ± 0% -40.54% (p=0.008 n=5+5) EncoderReader/skip_lines-16 420 ± 0% 419 ± 0% -0.24% (p=0.008 n=5+5) ``` (cherry picked from commit 0e3788b)
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