Separate darwin syscalls for mmap (hypermodeinc#1149)

Using the unix package by default for other systems instead of syscall.

This way when systems get added to unix, they will work without change.
And the special case is restricted only to darwin builds.

I encountered problems building on Solaris prior to this patch:
github.com/dgraph-io/badger/y/mmap_unix.go:57:30: undefined: syscall.SYS_MADVISE
But unix.Madvise works fine on that platform (and others).

y/mmap.go

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2019 Dgraph Labs, Inc. and Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package y
+
+import (
+	"os"
+)
+
+// Mmap uses the mmap system call to memory-map a file. If writable is true,
+// memory protection of the pages is set so that they may be written to as well.
+func Mmap(fd *os.File, writable bool, size int64) ([]byte, error) {
+	return mmap(fd, writable, size)
+}
+
+// Munmap unmaps a previously mapped slice.
+func Munmap(b []byte) error {
+	return munmap(b)
+}
+
+// Madvise uses the madvise system call to give advise about the use of memory
+// when using a slice that is memory-mapped to a file. Set the readahead flag to
+// false if page references are expected in random order.
+func Madvise(b []byte, readahead bool) error {
+	return madvise(b, readahead)
+}

y/mmap_darwin.go

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2019 Dgraph Labs, Inc. and Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package y
+
+import (
+	"os"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+// Mmap uses the mmap system call to memory-map a file. If writable is true,
+// memory protection of the pages is set so that they may be written to as well.
+func mmap(fd *os.File, writable bool, size int64) ([]byte, error) {
+	mtype := unix.PROT_READ
+	if writable {
+		mtype |= unix.PROT_WRITE
+	}
+	return unix.Mmap(int(fd.Fd()), 0, int(size), mtype, unix.MAP_SHARED)
+}
+
+// Munmap unmaps a previously mapped slice.
+func munmap(b []byte) error {
+	return unix.Munmap(b)
+}
+
+// This is required because the unix package does not support the madvise system call on OS X.
+func madvise(b []byte, readahead bool) error {
+	advice := unix.MADV_NORMAL
+	if !readahead {
+		advice = unix.MADV_RANDOM
+	}
+
+	_, _, e1 := syscall.Syscall(syscall.SYS_MADVISE, uintptr(unsafe.Pointer(&b[0])),
+		uintptr(len(b)), uintptr(advice))
+	if e1 != 0 {
+		return e1
+	}
+	return nil
+}

y/mmap_unix.go

@@ -1,7 +1,7 @@
-// +build !windows
+// +build !windows,!darwin
 
 /*
- * Copyright 2017 Dgraph Labs, Inc. and Contributors
+ * Copyright 2019 Dgraph Labs, Inc. and Contributors
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,15 +20,13 @@ package y
 
 import (
 	"os"
-	"syscall"
-	"unsafe"
 
 	"golang.org/x/sys/unix"
 )
 
 // Mmap uses the mmap system call to memory-map a file. If writable is true,
 // memory protection of the pages is set so that they may be written to as well.
-func Mmap(fd *os.File, writable bool, size int64) ([]byte, error) {
+func mmap(fd *os.File, writable bool, size int64) ([]byte, error) {
 	mtype := unix.PROT_READ
 	if writable {
 		mtype |= unix.PROT_WRITE
@@ -37,27 +35,17 @@ func Mmap(fd *os.File, writable bool, size int64) ([]byte, error) {
 }
 
 // Munmap unmaps a previously mapped slice.
-func Munmap(b []byte) error {
+func munmap(b []byte) error {
 	return unix.Munmap(b)
 }
 
 // Madvise uses the madvise system call to give advise about the use of memory
 // when using a slice that is memory-mapped to a file. Set the readahead flag to
 // false if page references are expected in random order.
-func Madvise(b []byte, readahead bool) error {
+func madvise(b []byte, readahead bool) error {
 	flags := unix.MADV_NORMAL
 	if !readahead {
 		flags = unix.MADV_RANDOM
 	}
-	return madvise(b, flags)
-}
-
-// This is required because the unix package does not support the madvise system call on OS X.
-func madvise(b []byte, advice int) (err error) {
-	_, _, e1 := syscall.Syscall(syscall.SYS_MADVISE, uintptr(unsafe.Pointer(&b[0])),
-		uintptr(len(b)), uintptr(advice))
-	if e1 != 0 {
-		err = e1
-	}
-	return
+	return unix.Madvise(b, flags)
 }

y/mmap_windows.go

@@ -1,7 +1,7 @@
 // +build windows
 
 /*
- * Copyright 2017 Dgraph Labs, Inc. and Contributors
+ * Copyright 2019 Dgraph Labs, Inc. and Contributors
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,7 +25,7 @@ import (
 	"unsafe"
 )
 
-func Mmap(fd *os.File, write bool, size int64) ([]byte, error) {
+func mmap(fd *os.File, write bool, size int64) ([]byte, error) {
 	protect := syscall.PAGE_READONLY
 	access := syscall.FILE_MAP_READ
 
@@ -81,11 +81,11 @@ func Mmap(fd *os.File, write bool, size int64) ([]byte, error) {
 	return data, nil
 }
 
-func Munmap(b []byte) error {
+func munmap(b []byte) error {
 	return syscall.UnmapViewOfFile(uintptr(unsafe.Pointer(&b[0])))
 }
 
-func Madvise(b []byte, readahead bool) error {
+func madvise(b []byte, readahead bool) error {
 	// Do Nothing. We don’t care about this setting on Windows
 	return nil
 }