Add Go standard library patch to use VFORK for exec.Command().

For more details: golang/go#5838.

Dockerfile

@@ -18,6 +18,14 @@ RUN apk add glibc-2.23-r3.apk
 # Disable cgo so that binaries we build will be fully static.
 ENV CGO_ENABLED=0
 
+# Apply patches to Go runtime and recompile.
+# See https://github.com/golang/go/issues/5838 for defails of vfork patch.
+COPY patches/use-clone-vfork-b7edfba429d982e3e065d637334bcc63ad49f8f9.patch \
+     /tmp/use-clone-vfork-b7edfba429d982e3e065d637334bcc63ad49f8f9.patch
+RUN cd /usr/local/go && \
+    patch -p 1 < /tmp/use-clone-vfork-b7edfba429d982e3e065d637334bcc63ad49f8f9.patch
+RUN go install -v -a syscall
+
 # Recompile the standard library with cgo disabled.  This prevents the standard library from being
 # marked stale, causing full rebuilds every time.
 RUN go install -v std

patches/use-clone-vfork-b7edfba429d982e3e065d637334bcc63ad49f8f9.patch

@@ -0,0 +1,76 @@
+From b7edfba429d982e3e065d637334bcc63ad49f8f9 Mon Sep 17 00:00:00 2001
+From: Richard Musiol <mail@richard-musiol.de>
+Date: Fri, 29 Apr 2016 19:22:36 +0200
+Subject: [PATCH] syscall: use CLONE_VFORK (WIP)
+
+---
+ src/syscall/asm_linux_amd64.s      | 25 +++++++++++++++++++++++++
+ src/syscall/exec_linux.go          |  7 +++++--
+ src/syscall/syscall_linux_amd64.go |  2 ++
+ 3 files changed, 32 insertions(+), 2 deletions(-)
+
+diff --git a/src/syscall/asm_linux_amd64.s b/src/syscall/asm_linux_amd64.s
+index 6634875..a5255c1 100644
+--- a/src/syscall/asm_linux_amd64.s
++++ b/src/syscall/asm_linux_amd64.s
+@@ -111,6 +111,31 @@ ok2:
+ 	MOVQ	$0, err+72(FP)
+ 	RET
+
++// func rawSyscall6PreserveRet(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
++TEXT ·rawSyscall6PreserveRet(SB),NOSPLIT,$0-80
++	MOVQ	a1+8(FP), DI
++	MOVQ	a2+16(FP), SI
++	MOVQ	a3+24(FP), DX
++	MOVQ	a4+32(FP), R10
++	MOVQ	a5+40(FP), R8
++	MOVQ	a6+48(FP), R9
++	MOVQ	trap+0(FP), AX	// syscall entry
++	POPQ	R12 // preserve return address
++	SYSCALL
++	PUSHQ	R12
++	CMPQ	AX, $0xfffffffffffff001
++	JLS	ok2
++	MOVQ	$-1, r1+56(FP)
++	MOVQ	$0, r2+64(FP)
++	NEGQ	AX
++	MOVQ	AX, err+72(FP)
++	RET
++ok2:
++	MOVQ	AX, r1+56(FP)
++	MOVQ	DX, r2+64(FP)
++	MOVQ	$0, err+72(FP)
++	RET
++
+ // func gettimeofday(tv *Timeval) (err uintptr)
+ TEXT ·gettimeofday(SB),NOSPLIT,$0-16
+ 	MOVQ	tv+0(FP), DI
+diff --git a/src/syscall/exec_linux.go b/src/syscall/exec_linux.go
+index e49bad7..61a72ad 100644
+--- a/src/syscall/exec_linux.go
++++ b/src/syscall/exec_linux.go
+@@ -94,9 +94,12 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
+ 	// About to call fork.
+ 	// No more allocation or calls of non-assembly functions.
+ 	runtime_BeforeFork()
+-	if runtime.GOARCH == "s390x" {
++	switch {
++	case runtime.GOARCH == "amd64" && sys.Cloneflags&CLONE_NEWUSER == 0:
++		r1, _, err1 = rawSyscall6PreserveRet(SYS_CLONE, uintptr(SIGCHLD|CLONE_VFORK|CLONE_VM)|sys.Cloneflags, 0, 0, 0, 0, 0)
++	case runtime.GOARCH == "s390x":
+ 		r1, _, err1 = RawSyscall6(SYS_CLONE, 0, uintptr(SIGCHLD)|sys.Cloneflags, 0, 0, 0, 0)
+-	} else {
++	default:
+ 		r1, _, err1 = RawSyscall6(SYS_CLONE, uintptr(SIGCHLD)|sys.Cloneflags, 0, 0, 0, 0, 0)
+ 	}
+ 	if err1 != 0 {
+diff --git a/src/syscall/syscall_linux_amd64.go b/src/syscall/syscall_linux_amd64.go
+index d1bda29..cf3e5d1 100644
+--- a/src/syscall/syscall_linux_amd64.go
++++ b/src/syscall/syscall_linux_amd64.go
+@@ -144,3 +144,5 @@ func (msghdr *Msghdr) SetControllen(length int) {
+ func (cmsg *Cmsghdr) SetLen(length int) {
+ 	cmsg.Len = uint64(length)
+ }
++
++func rawSyscall6PreserveRet(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)