MDEV-16676 Using malloc-lib=jemalloc in MariaDB 10.2 causes non-critical error...

MDEV-16676 Using malloc-lib=jemalloc in MariaDB 10.2 causes non-critical error about missing mysql_config on startup

- This commit is based on patch 84fe9720a4d2483ff67b6a and suggestion of
Jean Weisbuch to use `ldconfig -p`
- Format of ldconfig output:
"libjemalloc.so.1 (libc6,x86-64) => /usr/lib/x86_64-linux-gnu/libjemalloc.so.1"

```
$ ./scripts/mysqld_safe
201013 13:36:50 mysqld_safe Adding '/usr/lib/x86_64-linux-gnu/libjemalloc.so.1' to LD_PRELOAD for mysqld
201013 13:36:50 mysqld_safe Logging to '/home/anel/builds/data-10.5/mysqld_safe_anel.err'.
201013 13:36:50 mysqld_safe Starting mysqld daemon with databases from /home/anel/builds/data-10.5
```
Reviewed by: Daniel Black, Faustin Lammler

scripts/mysqld_safe.sh

@@ -456,25 +456,10 @@ mysqld_ld_preload_text() {
   echo "$text"
 }
 
-
-mysql_config=
-get_mysql_config() {
-  if [ -z "$mysql_config" ]; then
-    mysql_config=`echo "$0" | sed 's,/[^/][^/]*$,/mysql_config,'`
-    if [ ! -x "$mysql_config" ]; then
-      log_error "Can not run mysql_config $@ from '$mysql_config'"
-      exit 1
-    fi
-  fi
-
-  "$mysql_config" "$@"
-}
-
-
 # set_malloc_lib LIB
 # - If LIB is empty, do nothing and return
-# - If LIB starts with 'tcmalloc' or 'jemalloc', look for the shared library in
-#   /usr/lib, /usr/lib64 and then pkglibdir.
+# - If LIB starts with 'tcmalloc' or 'jemalloc', look for the shared library
+#   using `ldconfig`.
 #   tcmalloc is part of the Google perftools project.
 # - If LIB is an absolute path, assume it is a malloc shared library
 #
@@ -482,28 +467,28 @@ get_mysql_config() {
 # running mysqld.  See ld.so for details.
 set_malloc_lib() {
   malloc_lib="$1"
-
   if expr "$malloc_lib" : "\(tcmalloc\|jemalloc\)" > /dev/null ; then
-    pkglibdir=`get_mysql_config --variable=pkglibdir`
-    where=''
-    # This list is kept intentionally simple.  Simply set --malloc-lib
-    # to a full path if another location is desired.
-    for libdir in /usr/lib /usr/lib64 "$pkglibdir" "$pkglibdir/mysql"; do
-       tmp=`echo "$libdir/lib$malloc_lib.so".[0-9]`
-       where="$where $libdir"
-       # log_notice "DEBUG: Checking for malloc lib '$tmp'"
-       [ -r "$tmp" ] || continue
-       malloc_lib="$tmp"
-       where=''
-       break
-    done
+    if ! my_which ldconfig > /dev/null 2>&1
+    then
+      log_error "ldconfig command not found, required for ldconfig -p"
+      exit 1
+    fi
+    # format from ldconfig:
+    # "libjemalloc.so.1 (libc6,x86-64) => /usr/lib/x86_64-linux-gnu/libjemalloc.so.1"
+    libmalloc_path="$(ldconfig -p | sed -n "/lib${malloc_lib}/p" | cut -d '>' -f2)"
 
-    if [ -n "$where" ]; then
-      log_error "no shared library for lib$malloc_lib.so.[0-9] found in$where"
+    if [ -z "$libmalloc_path" ]; then
+      log_error "no shared library for lib$malloc_lib.so.[0-9] found."
       exit 1
     fi
-  fi
 
+    for f in $libmalloc_path; do
+      if [ -f "$f" ]; then
+        malloc_lib=$f # get the first path if many
+        break
+      fi
+    done
+  fi
   # Allow --malloc-lib='' to override other settings
   [ -z  "$malloc_lib" ] && return
 
@@ -520,7 +505,6 @@ set_malloc_lib() {
       exit 1
       ;;
   esac
-
   add_mysqld_ld_preload "$malloc_lib"
 }