archive/tar: populate uname/gname/devmajor/devminor in FileInfoHeader

We take a best-effort approach since information for these fields
are not well supported on all platforms.

user.LookupId+user.LookupGroupId is currently 15x slower than os.Stat.
For performance reasons, we perpetually cache username and groupname
with a sync.Map. As a result, this function will not be updated whenever
the user or group names are renamed in the OS. However, this is a better
situation than before, where those fields were not populated at all.

Change-Id: I3cec8291aed7675dea89ee1cbda92bd493c8831f
Reviewed-on: https://go-review.googlesource.com/59531
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/archive/tar/stat_unix.go

@@ -8,6 +8,10 @@ package tar
 
 import (
 	"os"
+	"os/user"
+	"runtime"
+	"strconv"
+	"sync"
 	"syscall"
 )
 
@@ -15,6 +19,10 @@ func init() {
 	sysStat = statUnix
 }
 
+// userMap and groupMap caches UID and GID lookups for performance reasons.
+// The downside is that renaming uname or gname by the OS never takes effect.
+var userMap, groupMap sync.Map // map[int]string
+
 func statUnix(fi os.FileInfo, h *Header) error {
 	sys, ok := fi.Sys().(*syscall.Stat_t)
 	if !ok {
@@ -22,11 +30,40 @@ func statUnix(fi os.FileInfo, h *Header) error {
 	}
 	h.Uid = int(sys.Uid)
 	h.Gid = int(sys.Gid)
-	// TODO(bradfitz): populate username & group.  os/user
-	// doesn't cache LookupId lookups, and lacks group
-	// lookup functions.
+
+	// Best effort at populating Uname and Gname.
+	// The os/user functions may fail for any number of reasons
+	// (not implemented on that platform, cgo not enabled, etc).
+	if u, ok := userMap.Load(h.Uid); ok {
+		h.Uname = u.(string)
+	} else if u, err := user.LookupId(strconv.Itoa(h.Uid)); err == nil {
+		h.Uname = u.Username
+		userMap.Store(h.Uid, h.Uname)
+	}
+	if g, ok := groupMap.Load(h.Gid); ok {
+		h.Gname = g.(string)
+	} else if g, err := user.LookupGroupId(strconv.Itoa(h.Gid)); err == nil {
+		h.Gname = g.Name
+		groupMap.Store(h.Gid, h.Gname)
+	}
+
 	h.AccessTime = statAtime(sys)
 	h.ChangeTime = statCtime(sys)
-	// TODO(bradfitz): major/minor device numbers?
+
+	// Best effort at populating Devmajor and Devminor.
+	if h.Typeflag == TypeChar || h.Typeflag == TypeBlock {
+		dev := uint64(sys.Rdev) // May be int32 or uint32
+		switch runtime.GOOS {
+		case "linux":
+			// Copied from golang.org/x/sys/unix/dev_linux.go.
+			major := uint32((dev & 0x00000000000fff00) >> 8)
+			major |= uint32((dev & 0xfffff00000000000) >> 32)
+			minor := uint32((dev & 0x00000000000000ff) >> 0)
+			minor |= uint32((dev & 0x00000ffffff00000) >> 12)
+			h.Devmajor, h.Devminor = int64(major), int64(minor)
+		default:
+			// TODO: Implement others (see https://golang.org/issue/8106)
+		}
+	}
 	return nil
 }

src/go/build/deps_test.go

@@ -226,7 +226,7 @@ var pkgDeps = map[string][]string{
 	"go/types":                  {"L4", "GOPARSER", "container/heap", "go/constant"},
 
 	// One of a kind.
-	"archive/tar":              {"L4", "OS", "syscall"},
+	"archive/tar":              {"L4", "OS", "syscall", "os/user"},
 	"archive/zip":              {"L4", "OS", "compress/flate"},
 	"container/heap":           {"sort"},
 	"compress/bzip2":           {"L4"},