internal/gcimporter: copy GOROOT/internal/exportdata functions

Copies functions from GOROOT/internal/exportdata as they are.
Only local modification is to avoid saferio.

Updates golang/go#70651

Change-Id: If2f4e503f69897c51b708f995dad2720b9f59e8d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/633659
Reviewed-by: Robert Findley <rfindley@google.com>
Commit-Queue: Tim King <taking@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: timothy-king

internal/gcimporter/exportdata.go

@@ -13,6 +13,7 @@ import (
 	"errors"
 	"fmt"
 	"go/build"
+	"io"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -106,6 +107,216 @@ func FindExportData(r *bufio.Reader) (size int64, err error) {
 	return
 }
 
+// ReadUnified reads the contents of the unified export data from a reader r
+// that contains the contents of a GC-created archive file.
+//
+// On success, the reader will be positioned after the end-of-section marker "\n$$\n".
+//
+// Supported GC-created archive files have 4 layers of nesting:
+//   - An archive file containing a package definition file.
+//   - The package definition file contains headers followed by a data section.
+//     Headers are lines (≤ 4kb) that do not start with "$$".
+//   - The data section starts with "$$B\n" followed by export data followed
+//     by an end of section marker "\n$$\n". (The section start "$$\n" is no
+//     longer supported.)
+//   - The export data starts with a format byte ('u') followed by the <data> in
+//     the given format. (See ReadExportDataHeader for older formats.)
+//
+// Putting this together, the bytes in a GC-created archive files are expected
+// to look like the following.
+// See cmd/internal/archive for more details on ar file headers.
+//
+// | <!arch>\n             | ar file signature
+// | __.PKGDEF...size...\n | ar header for __.PKGDEF including size.
+// | go object <...>\n     | objabi header
+// | <optional headers>\n  | other headers such as build id
+// | $$B\n                 | binary format marker
+// | u<data>\n             | unified export <data>
+// | $$\n                  | end-of-section marker
+// | [optional padding]    | padding byte (0x0A) if size is odd
+// | [ar file header]      | other ar files
+// | [ar file data]        |
+func ReadUnified(r *bufio.Reader) (data []byte, err error) {
+	// We historically guaranteed headers at the default buffer size (4096) work.
+	// This ensures we can use ReadSlice throughout.
+	const minBufferSize = 4096
+	r = bufio.NewReaderSize(r, minBufferSize)
+
+	size, err := FindPackageDefinition(r)
+	if err != nil {
+		return
+	}
+	n := size
+
+	objapi, headers, err := ReadObjectHeaders(r)
+	if err != nil {
+		return
+	}
+	n -= len(objapi)
+	for _, h := range headers {
+		n -= len(h)
+	}
+
+	hdrlen, err := ReadExportDataHeader(r)
+	if err != nil {
+		return
+	}
+	n -= hdrlen
+
+	// size also includes the end of section marker. Remove that many bytes from the end.
+	const marker = "\n$$\n"
+	n -= len(marker)
+
+	if n < 0 {
+		err = fmt.Errorf("invalid size (%d) in the archive file: %d bytes remain without section headers (recompile package)", size, n)
+		return
+	}
+
+	// Read n bytes from buf.
+	data = make([]byte, n)
+	_, err = io.ReadFull(r, data)
+	if err != nil {
+		return
+	}
+
+	// Check for marker at the end.
+	var suffix [len(marker)]byte
+	_, err = io.ReadFull(r, suffix[:])
+	if err != nil {
+		return
+	}
+	if s := string(suffix[:]); s != marker {
+		err = fmt.Errorf("read %q instead of end-of-section marker (%q)", s, marker)
+		return
+	}
+
+	return
+}
+
+// FindPackageDefinition positions the reader r at the beginning of a package
+// definition file ("__.PKGDEF") within a GC-created archive by reading
+// from it, and returns the size of the package definition file in the archive.
+//
+// The reader must be positioned at the start of the archive file before calling
+// this function, and "__.PKGDEF" is assumed to be the first file in the archive.
+//
+// See cmd/internal/archive for details on the archive format.
+func FindPackageDefinition(r *bufio.Reader) (size int, err error) {
+	// Uses ReadSlice to limit risk of malformed inputs.
+
+	// Read first line to make sure this is an object file.
+	line, err := r.ReadSlice('\n')
+	if err != nil {
+		err = fmt.Errorf("can't find export data (%v)", err)
+		return
+	}
+
+	// Is the first line an archive file signature?
+	if string(line) != "!<arch>\n" {
+		err = fmt.Errorf("not the start of an archive file (%q)", line)
+		return
+	}
+
+	// package export block should be first
+	size = readArchiveHeader(r, "__.PKGDEF")
+	if size <= 0 {
+		err = fmt.Errorf("not a package file")
+		return
+	}
+
+	return
+}
+
+// ReadObjectHeaders reads object headers from the reader. Object headers are
+// lines that do not start with an end-of-section marker "$$". The first header
+// is the objabi header. On success, the reader will be positioned at the beginning
+// of the end-of-section marker.
+//
+// It returns an error if any header does not fit in r.Size() bytes.
+func ReadObjectHeaders(r *bufio.Reader) (objapi string, headers []string, err error) {
+	// line is a temporary buffer for headers.
+	// Use bounded reads (ReadSlice, Peek) to limit risk of malformed inputs.
+	var line []byte
+
+	// objapi header should be the first line
+	if line, err = r.ReadSlice('\n'); err != nil {
+		err = fmt.Errorf("can't find export data (%v)", err)
+		return
+	}
+	objapi = string(line)
+
+	// objapi header begins with "go object ".
+	if !strings.HasPrefix(objapi, "go object ") {
+		err = fmt.Errorf("not a go object file: %s", objapi)
+		return
+	}
+
+	// process remaining object header lines
+	for {
+		// check for an end of section marker "$$"
+		line, err = r.Peek(2)
+		if err != nil {
+			return
+		}
+		if string(line) == "$$" {
+			return // stop
+		}
+
+		// read next header
+		line, err = r.ReadSlice('\n')
+		if err != nil {
+			return
+		}
+		headers = append(headers, string(line))
+	}
+}
+
+// ReadExportDataHeader reads the export data header and format from r.
+// It returns the number of bytes read, or an error if the format is no longer
+// supported or it failed to read.
+//
+// The only currently supported format is binary export data in the
+// unified export format.
+func ReadExportDataHeader(r *bufio.Reader) (n int, err error) {
+	// Read export data header.
+	line, err := r.ReadSlice('\n')
+	if err != nil {
+		return
+	}
+
+	hdr := string(line)
+	switch hdr {
+	case "$$\n":
+		err = fmt.Errorf("old textual export format no longer supported (recompile package)")
+		return
+
+	case "$$B\n":
+		var format byte
+		format, err = r.ReadByte()
+		if err != nil {
+			return
+		}
+		// The unified export format starts with a 'u'.
+		switch format {
+		case 'u':
+		default:
+			// Older no longer supported export formats include:
+			// indexed export format which started with an 'i'; and
+			// the older binary export format which started with a 'c',
+			// 'd', or 'v' (from "version").
+			err = fmt.Errorf("binary export format %q is no longer supported (recompile package)", format)
+			return
+		}
+
+	default:
+		err = fmt.Errorf("unknown export data header: %q", hdr)
+		return
+	}
+
+	n = len(hdr) + 1 // + 1 is for 'u'
+	return
+}
+
 // FindPkg returns the filename and unique package id for an import
 // path based on package information provided by build.Import (using
 // the build.Default build.Context). A relative srcDir is interpreted