Generate intermediate image into a temp file

All of the `output*` functions took a `[]byte` and immediately wrapped it in a
`bytes.Buffer` to produce an `io.Reader`. Make them take an `io.Reader` instead
and satisfy this further up the call chain by directing `moby.Build` to output
to a temp file instead of another `bytes.Buffer`.

In my test case (building kube master image) this reduces Maximum RSS (as
measured by time(1)) from 6.7G to 2.8G and overall allocations from 9.7G to
5.3G. When building a tar (output to /dev/null) the Maximum RSS fell slightly
from 2.2G to 2.1G. Overall allocations remained stable at around 5.3G.

Signed-off-by: Ian Campbell <ijc@docker.com>

cmd/moby/build.go

@@ -185,14 +185,18 @@ func build(args []string) {
 		m.Trust = moby.TrustConfig{}
 	}
 
-	var buf *bytes.Buffer
+	var tf *os.File
 	var w io.Writer
 	if outputFile != nil {
 		w = outputFile
 	} else {
-		buf = new(bytes.Buffer)
+		if tf, err = ioutil.TempFile("", ""); err != nil {
-		w = buf
+			log.Fatalf("Error creating tempfile: %v", err)
+		}
+		defer os.Remove(tf.Name())
+		w = tf
 	}
+
 	// this is a weird interface, but currently only streamable types can have additional files
 	// need to split up the base tarball outputs from the secondary stages
 	var tp string
@@ -205,7 +209,11 @@ func build(args []string) {
 	}
 
 	if outputFile == nil {
-		image := buf.Bytes()
+		image := tf.Name()
+		if err := tf.Close(); err != nil {
+			log.Fatalf("Error closing tempfile: %v", err)
+		}
+
 		log.Infof("Create outputs:")
 		err = moby.Formats(filepath.Join(*buildDir, name), image, buildFormats, size)
 		if err != nil {

src/moby/linuxkit.go

@@ -1,7 +1,6 @@
 package moby
 
 import (
-	"bytes"
 	"crypto/sha256"
 	"fmt"
 	"io"
@@ -58,9 +57,21 @@ func ensureLinuxkitImage(name string) error {
 		return err
 	}
 	// TODO pass through --pull to here
-	buf := new(bytes.Buffer)
+	tf, err := ioutil.TempFile("", "")
-	Build(m, buf, false, "")
+	if err != nil {
-	image := buf.Bytes()
+		return err
+	}
+	defer os.Remove(tf.Name())
+	Build(m, tf, false, "")
+	if err := tf.Close(); err != nil {
+		return err
+	}
+
+	image, err := os.Open(tf.Name())
+	if err != nil {
+		return err
+	}
+	defer image.Close()
 	kernel, initrd, cmdline, err := tarToInitrd(image)
 	if err != nil {
 		return fmt.Errorf("Error converting to initrd: %v", err)

src/moby/output.go

@@ -4,6 +4,7 @@ import (
 	"archive/tar"
 	"bytes"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
 	"runtime"
@@ -24,8 +25,8 @@ const (
 	rpi3       = "linuxkit/mkimage-rpi3:0735656fff247ca978135e3aeb62864adc612180"
 )
 
-var outFuns = map[string]func(string, []byte, int) error{
+var outFuns = map[string]func(string, io.Reader, int) error{
-	"kernel+initrd": func(base string, image []byte, size int) error {
+	"kernel+initrd": func(base string, image io.Reader, size int) error {
 		kernel, initrd, cmdline, err := tarToInitrd(image)
 		if err != nil {
 			return fmt.Errorf("Error converting to initrd: %v", err)
@@ -36,7 +37,7 @@ var outFuns = map[string]func(string, []byte, int) error{
 		}
 		return nil
 	},
-	"tar-kernel-initrd": func(base string, image []byte, size int) error {
+	"tar-kernel-initrd": func(base string, image io.Reader, size int) error {
 		kernel, initrd, cmdline, err := tarToInitrd(image)
 		if err != nil {
 			return fmt.Errorf("Error converting to initrd: %v", err)
@@ -46,21 +47,21 @@ var outFuns = map[string]func(string, []byte, int) error{
 		}
 		return nil
 	},
-	"iso-bios": func(base string, image []byte, size int) error {
+	"iso-bios": func(base string, image io.Reader, size int) error {
 		err := outputIso(isoBios, base+".iso", image)
 		if err != nil {
 			return fmt.Errorf("Error writing iso-bios output: %v", err)
 		}
 		return nil
 	},
-	"iso-efi": func(base string, image []byte, size int) error {
+	"iso-efi": func(base string, image io.Reader, size int) error {
 		err := outputIso(isoEfi, base+"-efi.iso", image)
 		if err != nil {
 			return fmt.Errorf("Error writing iso-efi output: %v", err)
 		}
 		return nil
 	},
-	"raw-bios": func(base string, image []byte, size int) error {
+	"raw-bios": func(base string, image io.Reader, size int) error {
 		kernel, initrd, cmdline, err := tarToInitrd(image)
 		if err != nil {
 			return fmt.Errorf("Error converting to initrd: %v", err)
@@ -71,7 +72,7 @@ var outFuns = map[string]func(string, []byte, int) error{
 		}
 		return nil
 	},
-	"raw-efi": func(base string, image []byte, size int) error {
+	"raw-efi": func(base string, image io.Reader, size int) error {
 		kernel, initrd, cmdline, err := tarToInitrd(image)
 		if err != nil {
 			return fmt.Errorf("Error converting to initrd: %v", err)
@@ -82,7 +83,7 @@ var outFuns = map[string]func(string, []byte, int) error{
 		}
 		return nil
 	},
-	"aws": func(base string, image []byte, size int) error {
+	"aws": func(base string, image io.Reader, size int) error {
 		filename := base + ".raw"
 		log.Infof("  %s", filename)
 		kernel, initrd, cmdline, err := tarToInitrd(image)
@@ -95,7 +96,7 @@ var outFuns = map[string]func(string, []byte, int) error{
 		}
 		return nil
 	},
-	"gcp": func(base string, image []byte, size int) error {
+	"gcp": func(base string, image io.Reader, size int) error {
 		kernel, initrd, cmdline, err := tarToInitrd(image)
 		if err != nil {
 			return fmt.Errorf("Error converting to initrd: %v", err)
@@ -106,7 +107,7 @@ var outFuns = map[string]func(string, []byte, int) error{
 		}
 		return nil
 	},
-	"qcow2-bios": func(base string, image []byte, size int) error {
+	"qcow2-bios": func(base string, image io.Reader, size int) error {
 		filename := base + ".qcow2"
 		log.Infof("  %s", filename)
 		kernel, initrd, cmdline, err := tarToInitrd(image)
@@ -119,7 +120,7 @@ var outFuns = map[string]func(string, []byte, int) error{
 		}
 		return nil
 	},
-	"vhd": func(base string, image []byte, size int) error {
+	"vhd": func(base string, image io.Reader, size int) error {
 		kernel, initrd, cmdline, err := tarToInitrd(image)
 		if err != nil {
 			return fmt.Errorf("Error converting to initrd: %v", err)
@@ -130,7 +131,7 @@ var outFuns = map[string]func(string, []byte, int) error{
 		}
 		return nil
 	},
-	"dynamic-vhd": func(base string, image []byte, size int) error {
+	"dynamic-vhd": func(base string, image io.Reader, size int) error {
 		kernel, initrd, cmdline, err := tarToInitrd(image)
 		if err != nil {
 			return fmt.Errorf("Error converting to initrd: %v", err)
@@ -141,7 +142,7 @@ var outFuns = map[string]func(string, []byte, int) error{
 		}
 		return nil
 	},
-	"vmdk": func(base string, image []byte, size int) error {
+	"vmdk": func(base string, image io.Reader, size int) error {
 		kernel, initrd, cmdline, err := tarToInitrd(image)
 		if err != nil {
 			return fmt.Errorf("Error converting to initrd: %v", err)
@@ -152,7 +153,7 @@ var outFuns = map[string]func(string, []byte, int) error{
 		}
 		return nil
 	},
-	"rpi3": func(base string, image []byte, size int) error {
+	"rpi3": func(base string, image io.Reader, size int) error {
 		if runtime.GOARCH != "arm64" {
 			return fmt.Errorf("Raspberry Pi output currently only supported on arm64")
 		}
@@ -197,7 +198,7 @@ func ValidateFormats(formats []string) error {
 }
 
 // Formats generates all the specified output formats
-func Formats(base string, image []byte, formats []string, size int) error {
+func Formats(base string, image string, formats []string, size int) error {
 	log.Debugf("format: %v %s", formats, base)
 
 	err := ValidateFormats(formats)
@@ -205,20 +206,23 @@ func Formats(base string, image []byte, formats []string, size int) error {
 		return err
 	}
 	for _, o := range formats {
-		f := outFuns[o]
+		ir, err := os.Open(image)
-		err := f(base, image, size)
 		if err != nil {
 			return err
 		}
+		defer ir.Close()
+		f := outFuns[o]
+		if err := f(base, ir, size); err != nil {
+			return err
+		}
 	}
 
 	return nil
 }
 
-func tarToInitrd(image []byte) ([]byte, []byte, string, error) {
+func tarToInitrd(r io.Reader) ([]byte, []byte, string, error) {
 	w := new(bytes.Buffer)
 	iw := initrd.NewWriter(w)
-	r := bytes.NewReader(image)
 	tr := tar.NewReader(r)
 	kernel, cmdline, err := initrd.CopySplitTar(iw, tr)
 	if err != nil {
@@ -288,7 +292,7 @@ func outputImg(image, filename string, kernel []byte, initrd []byte, cmdline str
 	return dockerRun(buf, output, true, image, cmdline)
 }
 
-func outputIso(image, filename string, filesystem []byte) error {
+func outputIso(image, filename string, filesystem io.Reader) error {
 	log.Debugf("output ISO: %s %s", image, filename)
 	log.Infof("  %s", filename)
 	output, err := os.Create(filename)
@@ -296,10 +300,10 @@ func outputIso(image, filename string, filesystem []byte) error {
 		return err
 	}
 	defer output.Close()
-	return dockerRun(bytes.NewBuffer(filesystem), output, true, image)
+	return dockerRun(filesystem, output, true, image)
 }
 
-func outputRPi3(image, filename string, filesystem []byte) error {
+func outputRPi3(image, filename string, filesystem io.Reader) error {
 	log.Debugf("output RPi3: %s %s", image, filename)
 	log.Infof("  %s", filename)
 	output, err := os.Create(filename)
@@ -307,7 +311,7 @@ func outputRPi3(image, filename string, filesystem []byte) error {
 		return err
 	}
 	defer output.Close()
-	return dockerRun(bytes.NewBuffer(filesystem), output, true, image)
+	return dockerRun(filesystem, output, true, image)
 }
 
 func outputKernelInitrd(base string, kernel []byte, initrd []byte, cmdline string) error {