1 files changed, 98 insertions, 0 deletions
diff --git a/src/syscall/forkpipe2.go b/src/syscall/forkpipe2.go
new file mode 100644
index 0000000..bbecfda
--- /dev/null
+++ b/src/syscall/forkpipe2.go
@@ -0,0 +1,98 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build dragonfly || freebsd || linux || netbsd || openbsd || solaris
+
+package syscall
+
+import "sync"
+
+// forkExecPipe atomically opens a pipe with O_CLOEXEC set on both file
+// descriptors.
+func forkExecPipe(p []int) error {
+	return Pipe2(p, O_CLOEXEC)
+}
+
+var (
+	// Guard the forking variable.
+	forkingLock sync.Mutex
+	// Number of goroutines currently forking, and thus the
+	// number of goroutines holding a conceptual write lock
+	// on ForkLock.
+	forking int
+)
+
+// hasWaitingReaders reports whether any goroutine is waiting
+// to acquire a read lock on rw. It is defined in the sync package.
+func hasWaitingReaders(rw *sync.RWMutex) bool
+
+// acquireForkLock acquires a write lock on ForkLock.
+// ForkLock is exported and we've promised that during a fork
+// we will call ForkLock.Lock, so that no other threads create
+// new fds that are not yet close-on-exec before we fork.
+// But that forces all fork calls to be serialized, which is bad.
+// But we haven't promised that serialization, and it is essentially
+// undetectable by other users of ForkLock, which is good.
+// Avoid the serialization by ensuring that ForkLock is locked
+// at the first fork and unlocked when there are no more forks.
+func acquireForkLock() {
+	forkingLock.Lock()
+	defer forkingLock.Unlock()
+
+	if forking == 0 {
+		// There is no current write lock on ForkLock.
+		ForkLock.Lock()
+		forking++
+		return
+	}
+
+	// ForkLock is currently locked for writing.
+
+	if hasWaitingReaders(&ForkLock) {
+		// ForkLock is locked for writing, and at least one
+		// goroutine is waiting to read from it.
+		// To avoid lock starvation, allow readers to proceed.
+		// The simple way to do this is for us to acquire a
+		// read lock. That will block us until all current
+		// conceptual write locks are released.
+		//
+		// Note that this case is unusual on modern systems
+		// with O_CLOEXEC and SOCK_CLOEXEC. On those systems
+		// the standard library should never take a read
+		// lock on ForkLock.
+
+		forkingLock.Unlock()
+
+		ForkLock.RLock()
+		ForkLock.RUnlock()
+
+		forkingLock.Lock()
+
+		// Readers got a chance, so now take the write lock.
+
+		if forking == 0 {
+			ForkLock.Lock()
+		}
+	}
+
+	forking++
+}
+
+// releaseForkLock releases the conceptual write lock on ForkLock
+// acquired by acquireForkLock.
+func releaseForkLock() {
+	forkingLock.Lock()
+	defer forkingLock.Unlock()
+
+	if forking <= 0 {
+		panic("syscall.releaseForkLock: negative count")
+	}
+
+	forking--
+
+	if forking == 0 {
+		// No more conceptual write locks.
+		ForkLock.Unlock()
+	}
+}