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Under a burst of concurrent uploads, bazel-remote could be OOM-killed by a transient Go heap overshoot even though idle/steady-state memory is small. Heap profiling (alloc_space) pointed at zstd.(*Encoder).encodeAll: the write path compresses each blob in 1 MiB chunks and called EncodeAll(in, nil) per chunk, so a fresh output slice was allocated for every chunk of every upload. The chunk size is exactly 1<<20, and klauspost's EncodeAll only pre-allocates an output buffer when len(src) < 1<<20 (strictly less-than). A full chunk therefore starts from a nil dst and grows it by repeated append-doubling, so each 1 MiB chunk churned several MiB of transient garbage. With no memory backpressure on concurrent Puts, the allocation rate outran the GC and the process was killed. Thread a reusable dst through zstdimpl.EncodeAll (both the pure-Go and cgo backends already accept one) and have casblob.WriteAndClose allocate a single output buffer, sized to compressBound(chunkSize), reused across all chunks of the blob. Sizing to the ZSTD_compressBound worst case ensures EncodeAll never has to grow (and reallocate) the buffer, even for incompressible chunks whose output is slightly larger than the input. A microbenchmark of the write path (16 MiB blob, incompressible data) shows per-upload allocations drop from ~79 MB/op to ~1.1 MB/op.
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This is a squashed and rebased version of #907, so I can trigger CI on it in that state before landing.