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>

diff --git a/src/archive/tar/stat_unix.go b/src/archive/tar/stat_unix.go
index cb843db4cfd65815830128cc0aa285dee36f7123..aa6d82b1c6e3e5bb655e1cde49d3a0e67db50153 100644 (file)
--- a/src/archive/tar/stat_unix.go
+++ b/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
 }

diff --git a/src/go/build/deps_test.go b/src/go/build/deps_test.go
index 87abfba9219db3c3f79162dd8b3dcecdddd98a55..041c52310a73ccecc4f9d12cc758bd4b7a6a8408 100644 (file)
--- a/src/go/build/deps_test.go
+++ b/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"},