Revert "[dev.link] cmd/link: remove buffered file I/O from OutBuf"

This reverts commit b2def42d9e.

Reason for revert: trybots failing

Change-Id: I920be6d8de158b1e513154ac0eb0c8fa0cffa9f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/228657
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/amd64/asm.go

@@ -756,6 +756,7 @@ func asmb2(ctxt *ld.Link) {
 			if ctxt.IsELF {
 				ctxt.Out.SeekSet(symo)
 				ld.Asmelfsym(ctxt)
+				ctxt.Out.Flush()
 				ctxt.Out.Write(ld.Elfstrdat)
 
 				if ctxt.LinkMode == ld.LinkExternal {
@@ -765,11 +766,13 @@ func asmb2(ctxt *ld.Link) {
 
 		case objabi.Hplan9:
 			ld.Asmplan9sym(ctxt)
+			ctxt.Out.Flush()
 
 			sym := ctxt.Syms.Lookup("pclntab", 0)
 			if sym != nil {
 				ld.Lcsize = int32(len(sym.P))
 				ctxt.Out.Write(sym.P)
+				ctxt.Out.Flush()
 			}
 
 		case objabi.Hwindows:
@@ -814,6 +817,8 @@ func asmb2(ctxt *ld.Link) {
 	case objabi.Hwindows:
 		ld.Asmbpe(ctxt)
 	}
+
+	ctxt.Out.Flush()
 }
 
 func tlsIEtoLE(s *sym.Symbol, off, size int) {

src/cmd/link/internal/arm/asm.go

@@ -816,6 +816,7 @@ func asmb2(ctxt *ld.Link) {
 		default:
 			if ctxt.IsELF {
 				ld.Asmelfsym(ctxt)
+				ctxt.Out.Flush()
 				ctxt.Out.Write(ld.Elfstrdat)
 
 				if ctxt.LinkMode == ld.LinkExternal {
@@ -825,11 +826,13 @@ func asmb2(ctxt *ld.Link) {
 
 		case objabi.Hplan9:
 			ld.Asmplan9sym(ctxt)
+			ctxt.Out.Flush()
 
 			sym := ctxt.Syms.Lookup("pclntab", 0)
 			if sym != nil {
 				ld.Lcsize = int32(len(sym.P))
 				ctxt.Out.Write(sym.P)
+				ctxt.Out.Flush()
 			}
 
 		case objabi.Hwindows:
@@ -860,6 +863,7 @@ func asmb2(ctxt *ld.Link) {
 		ld.Asmbpe(ctxt)
 	}
 
+	ctxt.Out.Flush()
 	if *ld.FlagC {
 		fmt.Printf("textsize=%d\n", ld.Segtext.Filelen)
 		fmt.Printf("datsize=%d\n", ld.Segdata.Filelen)

src/cmd/link/internal/arm64/asm.go

@@ -866,6 +866,7 @@ func asmb2(ctxt *ld.Link) {
 		default:
 			if ctxt.IsELF {
 				ld.Asmelfsym(ctxt)
+				ctxt.Out.Flush()
 				ctxt.Out.Write(ld.Elfstrdat)
 
 				if ctxt.LinkMode == ld.LinkExternal {
@@ -875,11 +876,13 @@ func asmb2(ctxt *ld.Link) {
 
 		case objabi.Hplan9:
 			ld.Asmplan9sym(ctxt)
+			ctxt.Out.Flush()
 
 			sym := ctxt.Syms.Lookup("pclntab", 0)
 			if sym != nil {
 				ld.Lcsize = int32(len(sym.P))
 				ctxt.Out.Write(sym.P)
+				ctxt.Out.Flush()
 			}
 
 		case objabi.Hdarwin:
@@ -912,6 +915,7 @@ func asmb2(ctxt *ld.Link) {
 		ld.Asmbmacho(ctxt)
 	}
 
+	ctxt.Out.Flush()
 	if *ld.FlagC {
 		fmt.Printf("textsize=%d\n", ld.Segtext.Filelen)
 		fmt.Printf("datsize=%d\n", ld.Segdata.Filelen)

src/cmd/link/internal/ld/data.go

@@ -32,6 +32,7 @@
 package ld
 
 import (
+	"bufio"
 	"bytes"
 	"cmd/internal/gcprog"
 	"cmd/internal/objabi"
@@ -957,10 +958,11 @@ func Datblk(ctxt *Link, out *OutBuf, addr, size int64) {
 
 // Used only on Wasm for now.
 func DatblkBytes(ctxt *Link, addr int64, size int64) []byte {
-	buf := make([]byte, size)
+	buf := bytes.NewBuffer(make([]byte, 0, size))
-	out := &OutBuf{heap: buf}
+	out := &OutBuf{w: bufio.NewWriter(buf)}
 	writeDatblkToOutBuf(ctxt, out, addr, size)
-	return buf
+	out.Flush()
+	return buf.Bytes()
 }
 
 func writeDatblkToOutBuf(ctxt *Link, out *OutBuf, addr int64, size int64) {

src/cmd/link/internal/ld/main.go

@@ -319,25 +319,35 @@ func Main(arch *sys.Arch, theArch Arch) {
 	// for which we have computed the size and offset, in a
 	// mmap'd region. The second part writes more content, for
 	// which we don't know the size.
+	var outputMmapped bool
 	if ctxt.Arch.Family != sys.Wasm {
 		// Don't mmap if we're building for Wasm. Wasm file
 		// layout is very different so filesize is meaningless.
-		if err := ctxt.Out.Mmap(filesize); err != nil {
+		err := ctxt.Out.Mmap(filesize)
-			ctxt.Errorf(0, "error mapping file: %v", err)
+		outputMmapped = err == nil
-		}
+	}
+	if outputMmapped {
+		// Asmb will redirect symbols to the output file mmap, and relocations
+		// will be applied directly there.
+		bench.Start("Asmb")
+		thearch.Asmb(ctxt)
+		bench.Start("reloc")
+		ctxt.reloc()
+	} else {
+		// If we don't mmap, we need to apply relocations before
+		// writing out.
+		bench.Start("reloc")
+		ctxt.reloc()
+		bench.Start("Asmb")
+		thearch.Asmb(ctxt)
 	}
-
-	// Asmb will redirect symbols to the output file mmap, and relocations
-	// will be applied directly there.
-	bench.Start("Asmb")
-	thearch.Asmb(ctxt)
-	bench.Start("reloc")
-	ctxt.reloc()
 	bench.Start("Asmb2")
 	thearch.Asmb2(ctxt)
 
-	bench.Start("Munmap")
+	if outputMmapped {
-	ctxt.Out.Close() // Close handles Munmapping if necessary.
+		bench.Start("Munmap")
+		ctxt.Out.Munmap()
+	}
 
 	bench.Start("undef")
 	ctxt.undef()

src/cmd/link/internal/ld/outbuf.go

@@ -5,6 +5,7 @@
 package ld
 
 import (
+	"bufio"
 	"cmd/internal/sys"
 	"cmd/link/internal/sym"
 	"encoding/binary"
@@ -60,6 +61,7 @@ type OutBuf struct {
 	buf  []byte // backing store of mmap'd output file
 	heap []byte // backing store for non-mmapped data
 
+	w      *bufio.Writer
 	name   string
 	f      *os.File
 	encbuf [8]byte // temp buffer used by WriteN methods
@@ -76,6 +78,7 @@ func (out *OutBuf) Open(name string) error {
 	}
 	out.off = 0
 	out.name = name
+	out.w = bufio.NewWriter(f)
 	out.f = f
 	return nil
 }
@@ -107,17 +110,10 @@ func (out *OutBuf) Close() error {
 	if out.isView {
 		return viewCloseError
 	}
-	if out.isMmapped() {
+	out.Flush()
-		return out.Munmap()
-	}
 	if out.f == nil {
 		return nil
 	}
-	if len(out.heap) != 0 {
-		if _, err := out.f.Write(out.heap); err != nil {
-			return err
-		}
-	}
 	if err := out.f.Close(); err != nil {
 		return err
 	}
@@ -155,6 +151,10 @@ func (out *OutBuf) Munmap() error {
 // writing. When the mmapped section is full, we switch over the heap memory
 // for writing.
 func (out *OutBuf) writeLoc(lenToWrite int64) (int64, []byte) {
+	if !out.isMmapped() {
+		panic("shouldn't happen")
+	}
+
 	// See if we have enough space in the mmaped area.
 	bufLen := int64(len(out.buf))
 	if out.off+lenToWrite <= bufLen {
@@ -178,6 +178,15 @@ func (out *OutBuf) writeLoc(lenToWrite int64) (int64, []byte) {
 }
 
 func (out *OutBuf) SeekSet(p int64) {
+	if p == out.off {
+		return
+	}
+	if !out.isMmapped() {
+		out.Flush()
+		if _, err := out.f.Seek(p, 0); err != nil {
+			Exitf("seeking to %d in %s: %v", p, out.name, err)
+		}
+	}
 	out.off = p
 }
 
@@ -186,18 +195,33 @@ func (out *OutBuf) Offset() int64 {
 }
 
 // Write writes the contents of v to the buffer.
+//
+// As Write is backed by a bufio.Writer, callers do not have
+// to explicitly handle the returned error as long as Flush is
+// eventually called.
 func (out *OutBuf) Write(v []byte) (int, error) {
-	n := len(v)
+	if out.isMmapped() {
-	pos, buf := out.writeLoc(int64(n))
+		n := len(v)
-	copy(buf[pos:], v)
+		pos, buf := out.writeLoc(int64(n))
+		copy(buf[pos:], v)
+		out.off += int64(n)
+		return n, nil
+	}
+	n, err := out.w.Write(v)
 	out.off += int64(n)
-	return n, nil
+	return n, err
 }
 
 func (out *OutBuf) Write8(v uint8) {
-	pos, buf := out.writeLoc(1)
+	if out.isMmapped() {
-	buf[pos] = v
+		pos, buf := out.writeLoc(1)
-	out.off++
+		buf[pos] = v
+		out.off++
+		return
+	}
+	if err := out.w.WriteByte(v); err == nil {
+		out.off++
+	}
 }
 
 // WriteByte is an alias for Write8 to fulfill the io.ByteWriter interface.
@@ -232,11 +256,16 @@ func (out *OutBuf) Write64b(v uint64) {
 }
 
 func (out *OutBuf) WriteString(s string) {
-	pos, buf := out.writeLoc(int64(len(s)))
+	if out.isMmapped() {
-	n := copy(buf[pos:], s)
+		pos, buf := out.writeLoc(int64(len(s)))
-	if n != len(s) {
+		n := copy(buf[pos:], s)
-		log.Fatalf("WriteString truncated. buffer size: %d, offset: %d, len(s)=%d", len(out.buf), out.off, len(s))
+		if n != len(s) {
+			log.Fatalf("WriteString truncated. buffer size: %d, offset: %d, len(s)=%d", len(out.buf), out.off, len(s))
+		}
+		out.off += int64(n)
+		return
 	}
+	n, _ := out.w.WriteString(s)
 	out.off += int64(n)
 }
 
@@ -268,10 +297,26 @@ func (out *OutBuf) WriteStringPad(s string, n int, pad []byte) {
 // edit to the symbol content.
 // If the output file is not Mmap'd, just writes the content.
 func (out *OutBuf) WriteSym(s *sym.Symbol) {
-	n := int64(len(s.P))
+	// NB: We inline the Write call for speediness.
-	pos, buf := out.writeLoc(n)
+	if out.isMmapped() {
-	copy(buf[pos:], s.P)
+		n := int64(len(s.P))
-	out.off += n
+		pos, buf := out.writeLoc(n)
-	s.P = buf[pos : pos+n]
+		copy(buf[pos:], s.P)
-	s.Attr.Set(sym.AttrReadOnly, false)
+		out.off += n
+		s.P = buf[pos : pos+n]
+		s.Attr.Set(sym.AttrReadOnly, false)
+	} else {
+		n, _ := out.w.Write(s.P)
+		out.off += int64(n)
+	}
+}
+
+func (out *OutBuf) Flush() {
+	var err error
+	if out.w != nil {
+		err = out.w.Flush()
+	}
+	if err != nil {
+		Exitf("flushing %s: %v", out.name, err)
+	}
 }

src/cmd/link/internal/ld/outbuf_test.go

@@ -50,7 +50,6 @@ func TestWriteLoc(t *testing.T) {
 		{100, 100, 0, 100, 100, 0, true},
 		{10, 10, 0, 100, 100, 0, true},
 		{10, 20, 10, 100, 110, 10, true},
-		{0, 0, 0, 100, 100, 0, true},
 	}
 
 	for i, test := range tests {

src/cmd/link/internal/ld/xcoff.go

@@ -1389,6 +1389,7 @@ func (f *xcoffFile) writeLdrScn(ctxt *Link, globalOff uint64) {
 	}
 
 	f.loaderSize = off + uint64(stlen)
+	ctxt.Out.Flush()
 
 	/* again for printing */
 	if !*flagA {
@@ -1558,6 +1559,8 @@ func Asmbxcoff(ctxt *Link, fileoff int64) {
 	// write string table
 	xfile.stringTable.write(ctxt.Out)
 
+	ctxt.Out.Flush()
+
 	// write headers
 	xcoffwrite(ctxt)
 }

src/cmd/link/internal/mips/asm.go

@@ -204,6 +204,7 @@ func asmb2(ctxt *ld.Link) {
 
 		ctxt.Out.SeekSet(int64(symo))
 		ld.Asmelfsym(ctxt)
+		ctxt.Out.Flush()
 		ctxt.Out.Write(ld.Elfstrdat)
 
 		if ctxt.LinkMode == ld.LinkExternal {
@@ -219,6 +220,7 @@ func asmb2(ctxt *ld.Link) {
 		ld.Asmbelf(ctxt, int64(symo))
 	}
 
+	ctxt.Out.Flush()
 	if *ld.FlagC {
 		fmt.Printf("textsize=%d\n", ld.Segtext.Filelen)
 		fmt.Printf("datsize=%d\n", ld.Segdata.Filelen)

src/cmd/link/internal/mips64/asm.go

@@ -223,6 +223,7 @@ func asmb2(ctxt *ld.Link) {
 		default:
 			if ctxt.IsELF {
 				ld.Asmelfsym(ctxt)
+				ctxt.Out.Flush()
 				ctxt.Out.Write(ld.Elfstrdat)
 
 				if ctxt.LinkMode == ld.LinkExternal {
@@ -232,11 +233,13 @@ func asmb2(ctxt *ld.Link) {
 
 		case objabi.Hplan9:
 			ld.Asmplan9sym(ctxt)
+			ctxt.Out.Flush()
 
 			sym := ctxt.Syms.Lookup("pclntab", 0)
 			if sym != nil {
 				ld.Lcsize = int32(len(sym.P))
 				ctxt.Out.Write(sym.P)
+				ctxt.Out.Flush()
 			}
 		}
 	}
@@ -265,6 +268,7 @@ func asmb2(ctxt *ld.Link) {
 		ld.Asmbelf(ctxt, int64(symo))
 	}
 
+	ctxt.Out.Flush()
 	if *ld.FlagC {
 		fmt.Printf("textsize=%d\n", ld.Segtext.Filelen)
 		fmt.Printf("datsize=%d\n", ld.Segdata.Filelen)

src/cmd/link/internal/ppc64/asm.go

@@ -1130,6 +1130,7 @@ func asmb2(ctxt *ld.Link) {
 		default:
 			if ctxt.IsELF {
 				ld.Asmelfsym(ctxt)
+				ctxt.Out.Flush()
 				ctxt.Out.Write(ld.Elfstrdat)
 
 				if ctxt.LinkMode == ld.LinkExternal {
@@ -1139,15 +1140,18 @@ func asmb2(ctxt *ld.Link) {
 
 		case objabi.Hplan9:
 			ld.Asmplan9sym(ctxt)
+			ctxt.Out.Flush()
 
 			sym := ctxt.Syms.Lookup("pclntab", 0)
 			if sym != nil {
 				ld.Lcsize = int32(len(sym.P))
 				ctxt.Out.Write(sym.P)
+				ctxt.Out.Flush()
 			}
 
 		case objabi.Haix:
 			// symtab must be added once sections have been created in ld.Asmbxcoff
+			ctxt.Out.Flush()
 		}
 	}
 
@@ -1176,6 +1180,7 @@ func asmb2(ctxt *ld.Link) {
 		ld.Asmbxcoff(ctxt, int64(fileoff))
 	}
 
+	ctxt.Out.Flush()
 	if *ld.FlagC {
 		fmt.Printf("textsize=%d\n", ld.Segtext.Filelen)
 		fmt.Printf("datsize=%d\n", ld.Segdata.Filelen)

src/cmd/link/internal/riscv64/asm.go

@@ -142,6 +142,7 @@ func asmb2(ctxt *ld.Link) {
 		ctxt.Out.SeekSet(int64(symo))
 
 		ld.Asmelfsym(ctxt)
+		ctxt.Out.Flush()
 		ctxt.Out.Write(ld.Elfstrdat)
 
 		if ctxt.LinkMode == ld.LinkExternal {
@@ -156,6 +157,7 @@ func asmb2(ctxt *ld.Link) {
 	default:
 		ld.Errorf(nil, "unsupported operating system")
 	}
+	ctxt.Out.Flush()
 
 	if *ld.FlagC {
 		fmt.Printf("textsize=%d\n", ld.Segtext.Filelen)

src/cmd/link/internal/s390x/asm.go

@@ -515,6 +515,7 @@ func asmb2(ctxt *ld.Link) {
 
 		ctxt.Out.SeekSet(int64(symo))
 		ld.Asmelfsym(ctxt)
+		ctxt.Out.Flush()
 		ctxt.Out.Write(ld.Elfstrdat)
 
 		if ctxt.LinkMode == ld.LinkExternal {
@@ -530,6 +531,7 @@ func asmb2(ctxt *ld.Link) {
 		ld.Asmbelf(ctxt, int64(symo))
 	}
 
+	ctxt.Out.Flush()
 	if *ld.FlagC {
 		fmt.Printf("textsize=%d\n", ld.Segtext.Filelen)
 		fmt.Printf("datsize=%d\n", ld.Segdata.Filelen)

src/cmd/link/internal/wasm/asm.go

@@ -187,6 +187,8 @@ func asmb2(ctxt *ld.Link) {
 	if !*ld.FlagS {
 		writeNameSec(ctxt, len(hostImports), fns)
 	}
+
+	ctxt.Out.Flush()
 }
 
 func lookupType(sig *wasmFuncType, types *[]*wasmFuncType) uint32 {

src/cmd/link/internal/x86/asm.go

@@ -646,6 +646,7 @@ func asmb2(ctxt *ld.Link) {
 		default:
 			if ctxt.IsELF {
 				ld.Asmelfsym(ctxt)
+				ctxt.Out.Flush()
 				ctxt.Out.Write(ld.Elfstrdat)
 
 				if ctxt.LinkMode == ld.LinkExternal {
@@ -655,11 +656,13 @@ func asmb2(ctxt *ld.Link) {
 
 		case objabi.Hplan9:
 			ld.Asmplan9sym(ctxt)
+			ctxt.Out.Flush()
 
 			sym := ctxt.Syms.Lookup("pclntab", 0)
 			if sym != nil {
 				ld.Lcsize = int32(len(sym.P))
 				ctxt.Out.Write(sym.P)
+				ctxt.Out.Flush()
 			}
 
 		case objabi.Hwindows:
@@ -699,4 +702,6 @@ func asmb2(ctxt *ld.Link) {
 	case objabi.Hwindows:
 		ld.Asmbpe(ctxt)
 	}
+
+	ctxt.Out.Flush()
 }