copy: check if the source file still exists

If the source file is removed during copying it, then the result file may
be broken. Let's check if the file still exists during and after copying.

This is important when archiving a journal file on btrfs. Archiving
journal files is done asynchronously. So, a journal file may be removed
by the main thread of journald by journal_directory_vacuum(), while
another thread is copying the file. If that happens, a broken journal
file is created.

Fixes systemd#24150 and systemd#31222.

src/shared/copy.c

@@ -247,6 +247,10 @@ int copy_bytes_full(
         for (;;) {
                 ssize_t n;
 
+                r = fd_verify_regular_full(fdf, /* check_removed = */ true);
+                if (r < 0)
+                        return r;
+
                 if (max_bytes <= 0)
                         break;
 
@@ -484,6 +488,10 @@ int copy_bytes_full(
                 copied_something = true;
         }
 
+        r = fd_verify_regular_full(fdf, /* check_removed = */ true);
+        if (r < 0)
+                return r;
+
         if (copy_flags & COPY_TRUNCATE) {
                 off_t off = lseek(fdt, 0, SEEK_CUR);
                 if (off < 0)
@@ -798,6 +806,10 @@ static int fd_copy_regular(
         (void) futimens(fdt, (struct timespec[]) { st->st_atim, st->st_mtim });
         (void) copy_xattr(fdf, NULL, fdt, NULL, copy_flags);
 
+        r = fd_verify_regular_full(fdf, /* check_removed = */ true);
+        if (r < 0)
+                return r;
+
         if (copy_flags & COPY_FSYNC) {
                 if (fsync(fdt) < 0) {
                         r = -errno;
@@ -1314,7 +1326,7 @@ int copy_file_fd_at_full(
         if (fdf < 0)
                 return -errno;
 
-        r = fd_verify_regular(fdf);
+        r = fd_verify_regular_full(fdf, /* check_removed = */ true);
         if (r < 0)
                 return r;
 
@@ -1333,6 +1345,10 @@ int copy_file_fd_at_full(
                 (void) copy_xattr(fdf, NULL, fdt, NULL, copy_flags);
         }
 
+        r = fd_verify_regular_full(fdf, /* check_removed = */ true);
+        if (r < 0)
+                return r;
+
         if (copy_flags & COPY_FSYNC_FULL) {
                 r = fsync_full(fdt);
                 if (r < 0)
@@ -1374,7 +1390,7 @@ int copy_file_at_full(
         if (fstat(fdf, &st) < 0)
                 return -errno;
 
-        r = stat_verify_regular(&st);
+        r = stat_verify_regular_full(&st, /* check_removed = */ true);
         if (r < 0)
                 return r;
 
@@ -1404,6 +1420,10 @@ int copy_file_at_full(
         (void) copy_times(fdf, fdt, copy_flags);
         (void) copy_xattr(fdf, NULL, fdt, NULL, copy_flags);
 
+        r = fd_verify_regular_full(fdf, /* check_removed = */ true);
+        if (r < 0)
+                return r;
+
         if (chattr_mask != 0)
                 (void) chattr_fd(fdt, chattr_flags, chattr_mask & ~CHATTR_EARLY_FL, NULL);