os/user: fix buffer retry loop on macOS

getpwnam_r and friends return the errno as the result,
not in the global errno. The code changes in CL 449316
inadvertently started using the global errno.
So if a lookup didn't fit in the first buffer size,
it was treated as not found instead of growing the buffer.

Fixes #56942.

Change-Id: Ic5904fbeb31161bccd858e5adb987e919fb3e9d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/455815
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>

5 files changed, 32 insertions, 1 deletions

diff --git a/src/os/user/cgo_lookup_unix.go b/src/os/user/cgo_lookup_unix.go
index 81787fee2b..b745ffd9cf 100644
--- a/src/os/user/cgo_lookup_unix.go
+++ b/src/os/user/cgo_lookup_unix.go
@@ -141,7 +141,7 @@ func buildGroup(grp *_C_struct_group) *Group {
 
 type bufferKind _C_int
 
-const (
+var (
 	userBuffer  = bufferKind(_C__SC_GETPW_R_SIZE_MAX)
 	groupBuffer = bufferKind(_C__SC_GETGR_R_SIZE_MAX)
 )
diff --git a/src/os/user/lookup_plan9.go b/src/os/user/lookup_plan9.go
index dcc9319268..c2aabd504c 100644
--- a/src/os/user/lookup_plan9.go
+++ b/src/os/user/lookup_plan9.go
@@ -20,6 +20,13 @@ func init() {
 	groupListImplemented = false
 }
 
+var (
+	// unused variables (in this implementation)
+	// modified during test to exercise code paths in the cgo implementation.
+	userBuffer  = 0
+	groupBuffer = 0
+)
+
 func current() (*User, error) {
 	ubytes, err := os.ReadFile("/dev/user")
 	if err != nil {
diff --git a/src/os/user/lookup_stubs.go b/src/os/user/lookup_stubs.go
index b02c1ffa28..89dfe455b5 100644
--- a/src/os/user/lookup_stubs.go
+++ b/src/os/user/lookup_stubs.go
@@ -13,6 +13,13 @@ import (
 	"strconv"
 )
 
+var (
+	// unused variables (in this implementation)
+	// modified during test to exercise code paths in the cgo implementation.
+	userBuffer  = 0
+	groupBuffer = 0
+)
+
 func current() (*User, error) {
 	uid := currentUID()
 	// $USER and /etc/passwd may disagree; prefer the latter if we can get it.
diff --git a/src/os/user/lookup_windows.go b/src/os/user/lookup_windows.go
index f65773ced3..e64b8ae028 100644
--- a/src/os/user/lookup_windows.go
+++ b/src/os/user/lookup_windows.go
@@ -192,6 +192,13 @@ func newUser(uid, gid, dir, username, domain string) (*User, error) {
 	return u, nil
 }
 
+var (
+	// unused variables (in this implementation)
+	// modified during test to exercise code paths in the cgo implementation.
+	userBuffer  = 0
+	groupBuffer = 0
+)
+
 func current() (*User, error) {
 	t, e := syscall.OpenCurrentProcessToken()
 	if e != nil {
diff --git a/src/os/user/user_test.go b/src/os/user/user_test.go
index 80251749a7..0fa963dae0 100644
--- a/src/os/user/user_test.go
+++ b/src/os/user/user_test.go
@@ -16,6 +16,11 @@ func checkUser(t *testing.T) {
 }
 
 func TestCurrent(t *testing.T) {
+	old := userBuffer
+	defer func() {
+		userBuffer = old
+	}()
+	userBuffer = 1 // force use of retry code
 	u, err := Current()
 	if err != nil {
 		t.Fatalf("Current: %v (got %#v)", err, u)
@@ -91,6 +96,11 @@ func checkGroup(t *testing.T) {
 }
 
 func TestLookupGroup(t *testing.T) {
+	old := groupBuffer
+	defer func() {
+		groupBuffer = old
+	}()
+	groupBuffer = 1 // force use of retry code
 	checkGroup(t)
 	user, err := Current()
 	if err != nil {