* New "general-purpose" datastore upgrade callback added which i called once on startup, intended for lo

w-level general upgrades and as a complement to module-specific upgrade.
  * Called on startup after initial XML parsing, but before module-specific upgrades
  * Enabled by definign the `.ca_datastore_upgrade`
  * [General-purpose upgrade documentation](https://clixon-docs.readthedocs.io/en/latest/backend.html#ge
neral-purpose)
* JSON parse error messages change from ` on line x: syntax error,..` to `json_parse: line x: syntax err
or`
* Unknown-element error message is more descriptive, eg from `namespace is: urn:example:clixon` to: `Fai
led to find YANG spec of XML node: x with parent: xp in namespace urn:example:clixon`.
* C-API parse and validation API more capable
  * `xml_spec_populate` family of functions extended with three-value return values
    * -1: error, 0: parse OK, 1: parse and YANG binding OK.
  * `xml_parse` and `json_parse` API changes
    * Three value returns: -1: error, 0: parse OK, 1: parse and YANG binding OK.
    * Extended `xml_parse_file2` and `xml_parse_string2` extended API functions with all options available.
      * New concept called `yang_bind` that defines how XML symbols are bound to YANG after parsing
    * Existing API same except `xml_parse_file` `endtag` argument moved to `xml_parse_file2`
* C-API: Added instrumentation: `xml_size` and `xml_stats_get`.
* Fixed: Enabling modstate (CLICON_XMLDB_MODSTATE), changing a revision on a yang, and restarting made the backend daemon exit at start (thanks Matt)
  * Also: ensure to load `ietf-yang-library.yang ` if CLICON_XMLDB_MODSTATE is set

test/test_upgrade_simple.sh

@@ -0,0 +1,127 @@
+#!/usr/bin/env bash
+# A very simple case - Error in this detected by msmith@netgate
+# Enable modstate and save running on a simple system without upgrade callback
+# Upgrade yang revision, but no other (upgrade) changes
+# Then start from running with modstate enabled and the new revision
+
+# Magic line must be first in script (see README.md)
+s="$_" ; . ./lib.sh || if [ "$s" = $0 ]; then exit 0; else return 0; fi
+
+APPNAME=simple
+
+cfg=$dir/conf_yang.xml
+
+# Create configuration
+cat <<EOF > $cfg
+<clixon-config xmlns="http://clicon.org/config">
+  <CLICON_CONFIGFILE>/usr/local/etc/clixon.xml</CLICON_CONFIGFILE>
+  <CLICON_FEATURE>*:*</CLICON_FEATURE>
+  <CLICON_YANG_DIR>/usr/local/share/clixon</CLICON_YANG_DIR>
+  <CLICON_YANG_DIR>$dir</CLICON_YANG_DIR>
+  <CLICON_YANG_MODULE_MAIN>$APPNAME</CLICON_YANG_MODULE_MAIN>
+  <CLICON_CLI_MODE>hello</CLICON_CLI_MODE>
+  <CLICON_CLISPEC_DIR>/usr/local/lib/hello/clispec</CLICON_CLISPEC_DIR>
+  <CLICON_SOCK>/usr/local/var/hello.sock</CLICON_SOCK>
+  <CLICON_BACKEND_PIDFILE>/usr/local/var/hello.pidfile</CLICON_BACKEND_PIDFILE>
+  <CLICON_XMLDB_DIR>$dir</CLICON_XMLDB_DIR>
+  <CLICON_XMLDB_MODSTATE>true</CLICON_XMLDB_MODSTATE>
+  <CLICON_STARTUP_MODE>init</CLICON_STARTUP_MODE>
+  <CLICON_MODULE_LIBRARY_RFC7895>false</CLICON_MODULE_LIBRARY_RFC7895>
+</clixon-config>
+EOF
+
+cat <<EOF > $dir/$APPNAME.yang
+module $APPNAME {
+    yang-version 1.1;
+    namespace "urn:example:simple";
+    prefix he;
+    revision 2019-04-17 {
+	description
+	    "Clixon hello world example";
+    }
+    container hello{
+	container world{
+	    presence true;
+	}
+    }
+}
+EOF
+
+new "test params: -f $cfg"
+# Bring your own backend
+if [ $BE -ne 0 ]; then
+    # kill old backend (if any)
+    new "kill old backend"
+    sudo clixon_backend -zf $cfg
+    if [ $? -ne 0 ]; then
+	err
+    fi
+    new "start backend -s init -f $cfg"
+    start_backend -s init -f $cfg
+
+    new "waiting"
+    wait_backend
+fi
+
+new "add hello world (with modstate)"
+expecteof "$clixon_netconf -qf $cfg" 0 '<rpc><edit-config><target><candidate/></target><config><hello xmlns="urn:example:simple"><world/></hello></config></edit-config></rpc>]]>]]>' "^<rpc-reply><ok/></rpc-reply>]]>]]>$"
+
+new "netconf commit"
+expecteof "$clixon_netconf -qf $cfg" 0 "<rpc><commit/></rpc>]]>]]>" "^<rpc-reply><ok/></rpc-reply>]]>]]>$"
+
+new "Kill backend"
+# Check if premature kill
+pid=$(pgrep -u root -f clixon_backend)
+if [ -z "$pid" ]; then
+    err "backend already dead"
+fi
+# kill backend
+stop_backend -f $cfg
+
+# Now add a new yang for hello
+cat <<EOF > $dir/$APPNAME.yang
+module $APPNAME {
+    yang-version 1.1;
+    namespace "urn:example:simple";
+    prefix he;
+    revision 2020-01-01 {
+	description
+	    "Test new revision";
+    }
+    revision 2019-04-17 {
+	description
+	    "Clixon hello world example";
+    }
+    container hello{
+	container world{
+	    presence true;
+	}
+    }
+}
+EOF
+
+# Now start again from running with modstate enabled and new revision
+
+if [ $BE -ne 0 ]; then
+
+    new "start backend -s running -f $cfg"
+    start_backend -s running -f $cfg
+
+    new "waiting"
+    wait_backend
+fi
+
+new "netconf get config"
+expecteof "$clixon_netconf -qf $cfg" 0 "<rpc><get-config><source><candidate/></source></get-config></rpc>]]>]]>" '^<rpc-reply><data><hello xmlns="urn:example:simple"><world/></hello></data></rpc-reply>]]>]]>$'
+
+new "Kill backend"
+# Check if premature kill
+pid=$(pgrep -u root -f clixon_backend)
+if [ -z "$pid" ]; then
+    err "backend already dead"
+fi
+# kill backend
+stop_backend -f $cfg
+
+rm -rf $dir
+