GEOM: Introduce partial confxml API

Traditionally the GEOM's primary channel of information from kernel to
user-space was confxml, fetched by libgeom through kern.geom.confxml
sysctl.  It is convenient and informative, representing full state of
GEOM in a single XML document.  But problems start to arise on systems
with hundreds of disks, where the full confxml size reaches many
megabytes, taking significant time to first write it and then parse.

This patch introduces alternative solution, allowing to fetch much
smaller XML document, subset of the full confxml, limited to 64KB and
representing only one specified geom and optionally its parents.  It
uses existing GEOM control interface, extended with new "getxml" verb.
In case of any error, such as the buffer overflow, it just transparently
falls back to traditional full confxml.  This patch uses the new API in
user-space GEOM tools where it is possible.

Reviewed by:	imp
MFC after:	2 month
Sponsored by:	iXsystems, Inc.
Differential Revision:	https://reviews.freebsd.org/D34529

1 files changed, 36 insertions, 14 deletions

diff --git a/lib/libgeom/geom_xml2tree.c b/lib/libgeom/geom_xml2tree.c
index 824800070933..a6da0e6d163f 100644
--- a/lib/libgeom/geom_xml2tree.c
+++ b/lib/libgeom/geom_xml2tree.c
@@ -358,6 +358,17 @@ geom_lookupid(struct gmesh *gmp, const void *id)
 	return (NULL);
 }
 
+static void *
+geom_lookupidptr(struct gmesh *gmp, const void *id)
+{
+	struct gident *gip;
+
+	gip = geom_lookupid(gmp, id);
+	if (gip)
+		return (gip->lg_ptr);
+	return (NULL);
+}
+
 int
 geom_xml2tree(struct gmesh *gmp, char *p)
 {
@@ -430,22 +441,19 @@ geom_xml2tree(struct gmesh *gmp, char *p)
 	/* Substitute all identifiers */
 	LIST_FOREACH(cl, &gmp->lg_class, lg_class) {
 		LIST_FOREACH(ge, &cl->lg_geom, lg_geom) {
-			ge->lg_class =
-			    geom_lookupid(gmp, ge->lg_class)->lg_ptr;
-			LIST_FOREACH(pr, &ge->lg_provider, lg_provider) {
-				pr->lg_geom =
-				    geom_lookupid(gmp, pr->lg_geom)->lg_ptr;
-			}
+			ge->lg_class = geom_lookupidptr(gmp, ge->lg_class);
+			LIST_FOREACH(pr, &ge->lg_provider, lg_provider)
+				pr->lg_geom = geom_lookupidptr(gmp, pr->lg_geom);
 			LIST_FOREACH(co, &ge->lg_consumer, lg_consumer) {
-				co->lg_geom =
-				    geom_lookupid(gmp, co->lg_geom)->lg_ptr;
+				co->lg_geom = geom_lookupidptr(gmp, co->lg_geom);
 				if (co->lg_provider != NULL) {
-					co->lg_provider = 
-					    geom_lookupid(gmp,
-						co->lg_provider)->lg_ptr;
-					LIST_INSERT_HEAD(
-					    &co->lg_provider->lg_consumers,
-					    co, lg_consumers);
+					co->lg_provider = geom_lookupidptr(gmp,
+						co->lg_provider);
+					if (co->lg_provider != NULL) {
+						LIST_INSERT_HEAD(
+						    &co->lg_provider->lg_consumers,
+						    co, lg_consumers);
+					}
 				}
 			}
 		}
@@ -467,6 +475,20 @@ geom_gettree(struct gmesh *gmp)
 	return (error);
 }
 
+int
+geom_gettree_geom(struct gmesh *gmp, const char *c, const char *g, int parents)
+{
+	char *p;
+	int error;
+
+	p = geom_getxml_geom(c, g, parents);
+	if (p == NULL)
+		return (errno);
+	error = geom_xml2tree(gmp, p);
+	free(p);
+	return (error);
+}
+
 static void 
 delete_config(struct gconf *gp)
 {