podman

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// Copyright 2019 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build darwin dragonfly freebsd linux netbsd openbsd
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package bio
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import (
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	"runtime"
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	"sync/atomic"
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	"syscall"
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)
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// mmapLimit is the maximum number of mmaped regions to create before
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// falling back to reading into a heap-allocated slice. This exists
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// because some operating systems place a limit on the number of
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// distinct mapped regions per process. As of this writing:
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//
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//  Darwin    unlimited
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//  DragonFly   1000000 (vm.max_proc_mmap)
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//  FreeBSD   unlimited
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//  Linux         65530 (vm.max_map_count) // TODO: query /proc/sys/vm/max_map_count?
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//  NetBSD    unlimited
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//  OpenBSD   unlimited
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var mmapLimit int32 = 1<<31 - 1
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func init() {
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	// Linux is the only practically concerning OS.
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	if runtime.GOOS == "linux" {
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		mmapLimit = 30000
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	}
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}
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func (r *Reader) sliceOS(length uint64) ([]byte, bool) {
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	// For small slices, don't bother with the overhead of a
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	// mapping, especially since we have no way to unmap it.
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	const threshold = 16 << 10
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	if length < threshold {
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		return nil, false
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	}
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	// Have we reached the mmap limit?
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	if atomic.AddInt32(&mmapLimit, -1) < 0 {
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		atomic.AddInt32(&mmapLimit, 1)
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		return nil, false
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	}
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	// Page-align the offset.
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	off := r.Offset()
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	align := syscall.Getpagesize()
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	aoff := off &^ int64(align-1)
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	data, err := syscall.Mmap(int(r.f.Fd()), aoff, int(length+uint64(off-aoff)), syscall.PROT_READ, syscall.MAP_SHARED|syscall.MAP_FILE)
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	if err != nil {
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		return nil, false
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	}
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	data = data[off-aoff:]
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	r.MustSeek(int64(length), 1)
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	return data, true
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}
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