Two small bugs in guc.c

I investigated the report at [1] about pg_file_settings not reporting
invalid values of "log_connections".  It turns out it's broken for
PGC_BACKEND and PGC_SU_BACKEND parameters, but not other ones.
The cause is a bit of premature optimization in this logic:

                 * If a PGC_BACKEND or PGC_SU_BACKEND parameter is changed in
                 * the config file, we want to accept the new value in the
                 * postmaster (whence it will propagate to
                 * subsequently-started backends), but ignore it in existing
                 * backends. ...

Upon detecting that case, set_config_option just returns -1 immediately
without bothering to validate the value.  It should check for invalid
input before returning -1, which we can mechanize with a one-line fix:

-                    return -1;
+                    changeVal = false;

While studying this, I also noted that the bit to prevent changes in
parallel workers seems seriously broken:

    if (IsInParallelMode() && changeVal && action != GUC_ACTION_SAVE)
        ereport(elevel,
                (errcode(ERRCODE_INVALID_TRANSACTION_STATE),
                 errmsg("cannot set parameters during a parallel operation")));

This is evidently assuming that ereport() won't return, which seems
like a very dubious assumption given the various values that elevel
can have.  Maybe it's accidentally true -- I don't recall any
reports of trouble here -- but it sure looks fragile.

Hence, proposed patch attached.

            regards, tom lane

[1] https://www.postgresql.org/message-id/flat/CAK30z9T9gaF_isNquccZxi7agXCSjPjMsFXiifmkfu4VpZguxw%40mail.gmail.com

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 959a1c76bf..4126b90ad7 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -3412,9 +3412,12 @@ set_config_with_handle(const char *name, config_handle *handle,
      * Other changes might need to affect other workers, so forbid them.
      */
     if (IsInParallelMode() && changeVal && action != GUC_ACTION_SAVE)
+    {
         ereport(elevel,
                 (errcode(ERRCODE_INVALID_TRANSACTION_STATE),
                  errmsg("cannot set parameters during a parallel operation")));
+        return -1;
+    }

     /* if handle is specified, no need to look up option */
     if (!handle)
@@ -3513,7 +3516,9 @@ set_config_with_handle(const char *name, config_handle *handle,
                  * postmaster (whence it will propagate to
                  * subsequently-started backends), but ignore it in existing
                  * backends.  This is a tad klugy, but necessary because we
-                 * don't re-read the config file during backend start.
+                 * don't re-read the config file during backend start.  Handle
+                 * this by clearing changeVal but continuing, since we do want
+                 * to report incorrect values.
                  *
                  * In EXEC_BACKEND builds, this works differently: we load all
                  * non-default settings from the CONFIG_EXEC_PARAMS file
@@ -3526,7 +3531,7 @@ set_config_with_handle(const char *name, config_handle *handle,
                  * applies.
                  */
                 if (IsUnderPostmaster && !is_reload)
-                    return -1;
+                    changeVal = false;
             }
             else if (context != PGC_POSTMASTER &&
                      context != PGC_BACKEND &&