debug/elf: add (*File).DynamicSymbols, ErrNoSymbols, and tests for…

debug/elf: add (*File).DynamicSymbols, ErrNoSymbols, and tests for (*File).Symbols and (*File).DynamicSymbols, and formalize symbol order.

Added a complement to (*File).Symbols for the dynamic symbol table.
Would be useful, for instance, if seraching for certain shared objects
compatible with certain libraries (for instance, LADSPA requires an
exported symbol "ladspa_descriptor").

Added a variable ErrNoSymbols that canonicalizes a return from
(*File).Symbols and (*File).DyanmicSymbols if the file has no symbols.

Added tests for both (*File).Symbols and (*File).DynamicSymbols;
there was never a test for (*File).Symbols at all. A small C program using
libelf, included in the test data, was used to produce the golden
symbols to compare against.

As part of the requirements for testing, (*File).Symbols and (*File).DynamicSymbols now document the order in which the symbol tables are returned (in the order the symbols appear in the file).

All tests currently pass.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/107530043

src/pkg/debug/elf/file.go

@@ -405,10 +405,14 @@ func (f *File) getSymbols(typ SectionType) ([]Symbol, []byte, error) {
 	return nil, nil, errors.New("not implemented")
 }
 
+// ErrNoSymbols is returned by File.Symbols and File.DynamicSymbols
+// if there is no such section in the File.
+var ErrNoSymbols = errors.New("no symbol section")
+
 func (f *File) getSymbols32(typ SectionType) ([]Symbol, []byte, error) {
 	symtabSection := f.SectionByType(typ)
 	if symtabSection == nil {
-		return nil, nil, errors.New("no symbol section")
+		return nil, nil, ErrNoSymbols
 	}
 
 	data, err := symtabSection.Data()
@@ -451,7 +455,7 @@ func (f *File) getSymbols32(typ SectionType) ([]Symbol, []byte, error) {
 func (f *File) getSymbols64(typ SectionType) ([]Symbol, []byte, error) {
 	symtabSection := f.SectionByType(typ)
 	if symtabSection == nil {
-		return nil, nil, errors.New("no symbol section")
+		return nil, nil, ErrNoSymbols
 	}
 
 	data, err := symtabSection.Data()
@@ -698,7 +702,8 @@ func (f *File) DWARF() (*dwarf.Data, error) {
 	return d, nil
 }
 
-// Symbols returns the symbol table for f.
+// Symbols returns the symbol table for f. The symbols will be listed in the order
+// they appear in f.
 //
 // For compatibility with Go 1.0, Symbols omits the null symbol at index 0.
 // After retrieving the symbols as symtab, an externally supplied index x
@@ -708,6 +713,17 @@ func (f *File) Symbols() ([]Symbol, error) {
 	return sym, err
 }
 
+// DynamicSymbols returns the dynamic symbol table for f. The symbols
+// will be listed in the order they appear in f.
+//
+// For compatibility with Symbols, DynamicSymbols omits the null symbol at index 0.
+// After retrieving the symbols as symtab, an externally supplied index x
+// corresponds to symtab[x-1], not symtab[x].
+func (f *File) DynamicSymbols() ([]Symbol, error) {
+	sym, _, err := f.getSymbols(SHT_DYNSYM)
+	return sym, err
+}
+
 type ImportedSymbol struct {
 	Name    string
 	Version string