os/user: make Lookup* functions properly handle ENOENT

The libc docs say that these functions can return ENOENT, and
per issue #67912, this does happen in practice sometimes. Handle
both the ENOENT and !found cases the same way, for Lookup,
LookupId, LookupGroup, LookupGroupId.

Fixes #67912

Change-Id: I993935af44c83ad785b6cd735fc313a3647daa19
Reviewed-on: https://go-review.googlesource.com/c/go/+/591555
Reviewed-by: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: benhoyt

src/os/user/cgo_lookup_unix.go

@@ -31,12 +31,12 @@ func lookupUser(username string) (*User, error) {
 			(*_C_char)(unsafe.Pointer(&buf[0])), _C_size_t(len(buf)))
 		return errno
 	})
+	if err == syscall.ENOENT || (err == nil && !found) {
+		return nil, UnknownUserError(username)
+	}
 	if err != nil {
 		return nil, fmt.Errorf("user: lookup username %s: %v", username, err)
 	}
-	if !found {
-		return nil, UnknownUserError(username)
-	}
 	return buildUser(&pwd), err
 }
 
@@ -58,12 +58,12 @@ func lookupUnixUid(uid int) (*User, error) {
 			(*_C_char)(unsafe.Pointer(&buf[0])), _C_size_t(len(buf)))
 		return errno
 	})
+	if err == syscall.ENOENT || (err == nil && !found) {
+		return nil, UnknownUserIdError(uid)
+	}
 	if err != nil {
 		return nil, fmt.Errorf("user: lookup userid %d: %v", uid, err)
 	}
-	if !found {
-		return nil, UnknownUserIdError(uid)
-	}
 	return buildUser(&pwd), nil
 }
 
@@ -96,12 +96,12 @@ func lookupGroup(groupname string) (*Group, error) {
 			(*_C_char)(unsafe.Pointer(&buf[0])), _C_size_t(len(buf)))
 		return errno
 	})
+	if err == syscall.ENOENT || (err == nil && !found) {
+		return nil, UnknownGroupError(groupname)
+	}
 	if err != nil {
 		return nil, fmt.Errorf("user: lookup groupname %s: %v", groupname, err)
 	}
-	if !found {
-		return nil, UnknownGroupError(groupname)
-	}
 	return buildGroup(&grp), nil
 }
 
@@ -123,12 +123,12 @@ func lookupUnixGid(gid int) (*Group, error) {
 			(*_C_char)(unsafe.Pointer(&buf[0])), _C_size_t(len(buf)))
 		return syscall.Errno(errno)
 	})
+	if err == syscall.ENOENT || (err == nil && !found) {
+		return nil, UnknownGroupIdError(strconv.Itoa(gid))
+	}
 	if err != nil {
 		return nil, fmt.Errorf("user: lookup groupid %d: %v", gid, err)
 	}
-	if !found {
-		return nil, UnknownGroupIdError(strconv.Itoa(gid))
-	}
 	return buildGroup(&grp), nil
 }