coredump: get rid of _META_MANDATORY_MAX

No functional change. This change is done in preparation for future changes.
Currently, the list of fields which are received on the command line is a
strict subset of the fields which are always expected to be received on a
socket. But when we add new kernel args in the future, we'll have two
non-overlapping sets and this approach will not work. Get rid of the variable
and enumerate the required fields. This set will never change, so this is
actually more maintainable.

The message with the hint where to add new fields is switched with
_META_ARGV_MAX. The new order is more correct.

(cherry-picked from 49f1f2d)
(cherry-picked from aea6a631bca93e8b04a11aaced694f25f4da155e)
(cherry picked from cf16b6b6b2e0a656531bfd73ad66be3817b155cd)

(cherry picked from commit b46a4f0)

Author: keszybz

src/coredump/coredump.c

@@ -80,7 +80,7 @@
  * size. See DATA_SIZE_MAX in journal-importer.h. */
 assert_cc(JOURNAL_SIZE_MAX <= DATA_SIZE_MAX);
 
-enum {
+typedef enum {
         /* We use these as array indexes for our process metadata cache.
          *
          * The first indices of the cache stores the same metadata as the ones passed by
@@ -96,26 +96,25 @@ enum {
         _META_ARGV_REQUIRED,
         /* The fields below were added to kernel/core_pattern at later points, so they might be missing. */
         META_ARGV_HOSTNAME = _META_ARGV_REQUIRED,  /* %h: hostname */
-        _META_ARGV_MAX,
         /* If new fields are added, they should be added here, to maintain compatibility
          * with callers which don't know about the new fields. */
+        _META_ARGV_MAX,
 
         /* The following indexes are cached for a couple of special fields we use (and
          * thereby need to be retrieved quickly) for naming coredump files, and attaching
          * xattrs. Unlike the previous ones they are retrieved from the runtime
          * environment. */
 
         META_COMM = _META_ARGV_MAX,
-        _META_MANDATORY_MAX,
 
         /* The rest are similar to the previous ones except that we won't fail if one of
          * them is missing in a message sent over the socket. */
 
-        META_EXE = _META_MANDATORY_MAX,
+        META_EXE,
         META_UNIT,
         META_PROC_AUXV,
         _META_MAX
-};
+} meta_argv_t;
 
 static const char * const meta_field_names[_META_MAX] = {
         [META_ARGV_PID]       = "COREDUMP_PID=",
@@ -1138,12 +1137,24 @@ static int process_socket(int fd) {
         if (r < 0)
                 goto finish;
 
-        /* Make sure we received at least all fields we need. */
-        for (int i = 0; i < _META_MANDATORY_MAX; i++)
+        /* Make sure we received all the expected fields. We support being called by an *older*
+         * systemd-coredump from the outside, so we require only the basic set of fields that
+         * was being sent when the support for sending to containers over a socket was added
+         * in a108c43e36d3ceb6e34efe37c014fc2cda856000. */
+        meta_argv_t i;
+        VA_ARGS_FOREACH(i,
+                        META_ARGV_PID,
+                        META_ARGV_UID,
+                        META_ARGV_GID,
+                        META_ARGV_SIGNAL,
+                        META_ARGV_TIMESTAMP,
+                        META_ARGV_RLIMIT,
+                        META_ARGV_HOSTNAME,
+                        META_COMM)
                 if (!context.meta[i]) {
                         r = log_error_errno(SYNTHETIC_ERRNO(EINVAL),
-                                            "A mandatory argument (%i) has not been sent, aborting.",
-                                            i);
+                                            "Mandatory argument %s not received on socket, aborting.",
+                                            meta_field_names[i]);
                         goto finish;
                 }