chattr: optionally, return the old flags when updating them

src/basic/btrfs-util.c

@@ -1715,7 +1715,7 @@ int btrfs_subvol_snapshot_fd(int old_fd, const char *new_path, BtrfsSnapshotFlag
                                  * it: the IMMUTABLE bit. Let's use this here, if this is requested. */
 
                                 if (flags & BTRFS_SNAPSHOT_FALLBACK_IMMUTABLE)
-                                        (void) chattr_path(new_path, FS_IMMUTABLE_FL, FS_IMMUTABLE_FL);
+                                        (void) chattr_path(new_path, FS_IMMUTABLE_FL, FS_IMMUTABLE_FL, NULL);
                         } else {
                                 r = btrfs_subvol_set_read_only(new_path, true);
                                 if (r < 0)

src/basic/chattr-util.c

@@ -10,7 +10,7 @@
 #include "fd-util.h"
 #include "macro.h"
 
-int chattr_fd(int fd, unsigned value, unsigned mask) {
+int chattr_fd(int fd, unsigned value, unsigned mask, unsigned *previous) {
         unsigned old_attr, new_attr;
         struct stat st;
 
@@ -28,23 +28,29 @@ int chattr_fd(int fd, unsigned value, unsigned mask) {
         if (!S_ISDIR(st.st_mode) && !S_ISREG(st.st_mode))
                 return -ENOTTY;
 
-        if (mask == 0)
+        if (mask == 0 && !previous)
                 return 0;
 
         if (ioctl(fd, FS_IOC_GETFLAGS, &old_attr) < 0)
                 return -errno;
 
         new_attr = (old_attr & ~mask) | (value & mask);
-        if (new_attr == old_attr)
+        if (new_attr == old_attr) {
+                if (previous)
+                        *previous = old_attr;
                 return 0;
+        }
 
         if (ioctl(fd, FS_IOC_SETFLAGS, &new_attr) < 0)
                 return -errno;
 
+        if (previous)
+                *previous = old_attr;
+
         return 1;
 }
 
-int chattr_path(const char *p, unsigned value, unsigned mask) {
+int chattr_path(const char *p, unsigned value, unsigned mask, unsigned *previous) {
         _cleanup_close_ int fd = -1;
 
         assert(p);
@@ -56,7 +62,7 @@ int chattr_path(const char *p, unsigned value, unsigned mask) {
         if (fd < 0)
                 return -errno;
 
-        return chattr_fd(fd, value, mask);
+        return chattr_fd(fd, value, mask, previous);
 }
 
 int read_attr_fd(int fd, unsigned *ret) {

src/basic/chattr-util.h

@@ -1,8 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
-int chattr_fd(int fd, unsigned value, unsigned mask);
-int chattr_path(const char *p, unsigned value, unsigned mask);
+int chattr_fd(int fd, unsigned value, unsigned mask, unsigned *previous);
+int chattr_path(const char *p, unsigned value, unsigned mask, unsigned *previous);
 
 int read_attr_fd(int fd, unsigned *ret);
 int read_attr_path(const char *p, unsigned *ret);

src/basic/copy.c

@@ -695,7 +695,7 @@ int copy_file(const char *from, const char *to, int flags, mode_t mode, unsigned
         }
 
         if (chattr_flags != 0)
-                (void) chattr_fd(fdt, chattr_flags, (unsigned) -1);
+                (void) chattr_fd(fdt, chattr_flags, (unsigned) -1, NULL);
 
         r = copy_file_fd(from, fdt, copy_flags);
         if (r < 0) {
@@ -743,7 +743,7 @@ int copy_file_atomic(const char *from, const char *to, mode_t mode, unsigned cha
         }
 
         if (chattr_flags != 0)
-                (void) chattr_fd(fdt, chattr_flags, (unsigned) -1);
+                (void) chattr_fd(fdt, chattr_flags, (unsigned) -1, NULL);
 
         r = copy_file_fd(from, fdt, copy_flags);
         if (r < 0)