mirror of https://go.googlesource.com/go
99 lines
2.6 KiB
Go
99 lines
2.6 KiB
Go
// Copyright 2017 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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//go:build dragonfly || freebsd || linux || netbsd || openbsd || solaris
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package syscall
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import "sync"
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// forkExecPipe atomically opens a pipe with O_CLOEXEC set on both file
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// descriptors.
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func forkExecPipe(p []int) error {
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return Pipe2(p, O_CLOEXEC)
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}
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var (
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// Guard the forking variable.
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forkingLock sync.Mutex
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// Number of goroutines currently forking, and thus the
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// number of goroutines holding a conceptual write lock
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// on ForkLock.
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forking int
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)
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// hasWaitingReaders reports whether any goroutine is waiting
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// to acquire a read lock on rw. It is defined in the sync package.
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func hasWaitingReaders(rw *sync.RWMutex) bool
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// acquireForkLock acquires a write lock on ForkLock.
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// ForkLock is exported and we've promised that during a fork
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// we will call ForkLock.Lock, so that no other threads create
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// new fds that are not yet close-on-exec before we fork.
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// But that forces all fork calls to be serialized, which is bad.
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// But we haven't promised that serialization, and it is essentially
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// undetectable by other users of ForkLock, which is good.
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// Avoid the serialization by ensuring that ForkLock is locked
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// at the first fork and unlocked when there are no more forks.
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func acquireForkLock() {
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forkingLock.Lock()
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defer forkingLock.Unlock()
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if forking == 0 {
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// There is no current write lock on ForkLock.
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ForkLock.Lock()
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forking++
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return
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}
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// ForkLock is currently locked for writing.
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if hasWaitingReaders(&ForkLock) {
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// ForkLock is locked for writing, and at least one
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// goroutine is waiting to read from it.
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// To avoid lock starvation, allow readers to proceed.
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// The simple way to do this is for us to acquire a
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// read lock. That will block us until all current
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// conceptual write locks are released.
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//
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// Note that this case is unusual on modern systems
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// with O_CLOEXEC and SOCK_CLOEXEC. On those systems
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// the standard library should never take a read
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// lock on ForkLock.
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forkingLock.Unlock()
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ForkLock.RLock()
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ForkLock.RUnlock()
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forkingLock.Lock()
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// Readers got a chance, so now take the write lock.
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if forking == 0 {
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ForkLock.Lock()
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}
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}
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forking++
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}
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// releaseForkLock releases the conceptual write lock on ForkLock
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// acquired by acquireForkLock.
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func releaseForkLock() {
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forkingLock.Lock()
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defer forkingLock.Unlock()
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if forking <= 0 {
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panic("syscall.releaseForkLock: negative count")
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}
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forking--
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if forking == 0 {
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// No more conceptual write locks.
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ForkLock.Unlock()
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}
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}
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