cmd/ld: remove Plan 9 symbol table

Update #6853

Nothing reads the Plan 9 symbol table anymore.
The last holdout was 'go tool nm', but since being rewritten in Go
it uses the standard symbol table for the binary format
(ELF, Mach-O, PE) instead.

Removing the Plan 9 symbol table saves ~15% disk space
on most binaries.

Two supporting changes included in this CL:

debug/gosym: use Go 1.2 pclntab to synthesize func-only
symbol table when there is no Plan 9 symbol table

debug/elf, debug/macho, debug/pe: ignore final EOF from ReadAt

LGTM=r
R=r, bradfitz
CC=golang-codereviews
https://golang.org/cl/65740045

5 files changed, 46 insertions, 0 deletions

diff --git a/src/pkg/debug/elf/file.go b/src/pkg/debug/elf/file.go
index a406170996..2840f07674 100644
--- a/src/pkg/debug/elf/file.go
+++ b/src/pkg/debug/elf/file.go
@@ -76,6 +76,9 @@ type Section struct {
 func (s *Section) Data() ([]byte, error) {
 	dat := make([]byte, s.sr.Size())
 	n, err := s.sr.ReadAt(dat, 0)
+	if n == len(dat) {
+		err = nil
+	}
 	return dat[0:n], err
 }
 
diff --git a/src/pkg/debug/gosym/pclntab.go b/src/pkg/debug/gosym/pclntab.go
index 3e6a8046b3..6620aefb05 100644
--- a/src/pkg/debug/gosym/pclntab.go
+++ b/src/pkg/debug/gosym/pclntab.go
@@ -196,6 +196,33 @@ func (t *LineTable) go12Init() {
 	t.go12 = 1 // so far so good
 }
 
+// go12Funcs returns a slice of Funcs derived from the Go 1.2 pcln table.
+func (t *LineTable) go12Funcs() []Func {
+	// Assume it is malformed and return nil on error.
+	defer func() {
+		recover()
+	}()
+
+	n := len(t.functab) / int(t.ptrsize) / 2
+	funcs := make([]Func, n)
+	for i := range funcs {
+		f := &funcs[i]
+		f.Entry = uint64(t.uintptr(t.functab[2*i*int(t.ptrsize):]))
+		f.End = uint64(t.uintptr(t.functab[(2*i+2)*int(t.ptrsize):]))
+		info := t.Data[t.uintptr(t.functab[(2*i+1)*int(t.ptrsize):]):]
+		f.LineTable = t
+		f.FrameSize = int(t.binary.Uint32(info[t.ptrsize+2*4:]))
+		f.Sym = &Sym{
+			Value:  f.Entry,
+			Type:   'T',
+			Name:   t.string(t.binary.Uint32(info[t.ptrsize:])),
+			GoType: 0,
+			Func:   f,
+		}
+	}
+	return funcs
+}
+
 // findFunc returns the func corresponding to the given program counter.
 func (t *LineTable) findFunc(pc uint64) []byte {
 	if pc < t.uintptr(t.functab) || pc >= t.uintptr(t.functab[len(t.functab)-int(t.ptrsize):]) {
diff --git a/src/pkg/debug/gosym/symtab.go b/src/pkg/debug/gosym/symtab.go
index 9ab05bac2f..3864e3cb4f 100644
--- a/src/pkg/debug/gosym/symtab.go
+++ b/src/pkg/debug/gosym/symtab.go
@@ -129,6 +129,9 @@ var (
 )
 
 func walksymtab(data []byte, fn func(sym) error) error {
+	if len(data) == 0 { // missing symtab is okay
+		return nil
+	}
 	var order binary.ByteOrder = binary.BigEndian
 	newTable := false
 	switch {
@@ -455,6 +458,10 @@ func NewTable(symtab []byte, pcln *LineTable) (*Table, error) {
 			i = end - 1 // loop will i++
 		}
 	}
+
+	if t.go12line != nil && nf == 0 {
+		t.Funcs = t.go12line.go12Funcs()
+	}
 	if obj != nil {
 		obj.Funcs = t.Funcs[lastf:]
 	}
diff --git a/src/pkg/debug/macho/file.go b/src/pkg/debug/macho/file.go
index c799fa49df..2b19f7f658 100644
--- a/src/pkg/debug/macho/file.go
+++ b/src/pkg/debug/macho/file.go
@@ -74,6 +74,9 @@ type Segment struct {
 func (s *Segment) Data() ([]byte, error) {
 	dat := make([]byte, s.sr.Size())
 	n, err := s.sr.ReadAt(dat, 0)
+	if n == len(dat) {
+		err = nil
+	}
 	return dat[0:n], err
 }
 
@@ -109,6 +112,9 @@ type Section struct {
 func (s *Section) Data() ([]byte, error) {
 	dat := make([]byte, s.sr.Size())
 	n, err := s.sr.ReadAt(dat, 0)
+	if n == len(dat) {
+		err = nil
+	}
 	return dat[0:n], err
 }
 
diff --git a/src/pkg/debug/pe/file.go b/src/pkg/debug/pe/file.go
index a2859bf370..d0005bacf3 100644
--- a/src/pkg/debug/pe/file.go
+++ b/src/pkg/debug/pe/file.go
@@ -72,6 +72,9 @@ type ImportDirectory struct {
 func (s *Section) Data() ([]byte, error) {
 	dat := make([]byte, s.sr.Size())
 	n, err := s.sr.ReadAt(dat, 0)
+	if n == len(dat) {
+		err = nil
+	}
 	return dat[0:n], err
 }