Fix crash in contrib/ltree's lca() function for empty input array.

lca_inner() wasn't prepared for the possibility of getting no inputs.
Fix that, and make some cosmetic improvements to the code while at it.

Also, I thought the documentation of this function as returning the
"longest common prefix" of the paths was entirely misleading; it really
returns a path one shorter than the longest common prefix, for the typical
definition of "prefix".  Don't use that term in the docs, and adjust the
examples to clarify what really happens.

This has been broken since its beginning, so back-patch to all supported
branches.

Per report from Hailong Li.  Thanks to Pierre Ducroquet for diagnosing
and for the initial patch, though I whacked it around some and added
test cases.

Discussion: https://postgr.es/m/5b0d8e4f-f2a3-1305-d612-e00e35a7be66@qunar.com

4 files changed, 50 insertions, 13 deletions

diff --git a/contrib/ltree/expected/ltree.out b/contrib/ltree/expected/ltree.out
index 3d5737d41b1..82269309056 100644
--- a/contrib/ltree/expected/ltree.out
+++ b/contrib/ltree/expected/ltree.out
@@ -259,6 +259,24 @@ SELECT lca('{1.2.3,1.2.3.4.5.6}');
  1.2
 (1 row)
 
+SELECT lca('{1.2.3}');
+ lca 
+-----
+ 1.2
+(1 row)
+
+SELECT lca('{1}'), lca('{1}') IS NULL;
+ lca | ?column? 
+-----+----------
+     | f
+(1 row)
+
+SELECT lca('{}') IS NULL;
+ ?column? 
+----------
+ t
+(1 row)
+
 SELECT lca('1.la.2.3','1.2.3.4.5.6');
  lca 
 -----
diff --git a/contrib/ltree/ltree_op.c b/contrib/ltree/ltree_op.c
index d62ca02521b..759f1f8d29b 100644
--- a/contrib/ltree/ltree_op.c
+++ b/contrib/ltree/ltree_op.c
@@ -402,22 +402,34 @@ ltree_textadd(PG_FUNCTION_ARGS)
 	PG_RETURN_POINTER(r);
 }
 
+/*
+ * Common code for variants of lca(), find longest common ancestor of inputs
+ *
+ * Returns NULL if there is no common ancestor, ie, the longest common
+ * prefix is empty.
+ */
 ltree *
 lca_inner(ltree **a, int len)
 {
 	int			tmp,
-				num = ((*a)->numlevel) ? (*a)->numlevel - 1 : 0;
-	ltree	  **ptr = a + 1;
-	int			i,
-				reslen = LTREE_HDRSIZE;
+				num,
+				i,
+				reslen;
+	ltree	  **ptr;
 	ltree_level *l1,
 			   *l2;
 	ltree	   *res;
 
-
+	if (len <= 0)
+		return NULL;			/* no inputs? */
 	if ((*a)->numlevel == 0)
-		return NULL;
+		return NULL;			/* any empty input means NULL result */
+
+	/* num is the length of the longest common ancestor so far */
+	num = (*a)->numlevel - 1;
 
+	/* Compare each additional input to *a */
+	ptr = a + 1;
 	while (ptr - a < len)
 	{
 		if ((*ptr)->numlevel == 0)
@@ -428,11 +440,12 @@ lca_inner(ltree **a, int len)
 		{
 			l1 = LTREE_FIRST(*a);
 			l2 = LTREE_FIRST(*ptr);
-			tmp = num;
+			tmp = Min(num, (*ptr)->numlevel - 1);
 			num = 0;
-			for (i = 0; i < Min(tmp, (*ptr)->numlevel - 1); i++)
+			for (i = 0; i < tmp; i++)
 			{
-				if (l1->len == l2->len && memcmp(l1->name, l2->name, l1->len) == 0)
+				if (l1->len == l2->len &&
+					memcmp(l1->name, l2->name, l1->len) == 0)
 					num = i + 1;
 				else
 					break;
@@ -443,6 +456,8 @@ lca_inner(ltree **a, int len)
 		ptr++;
 	}
 
+	/* Now compute size of result ... */
+	reslen = LTREE_HDRSIZE;
 	l1 = LTREE_FIRST(*a);
 	for (i = 0; i < num; i++)
 	{
@@ -450,6 +465,7 @@ lca_inner(ltree **a, int len)
 		l1 = LEVEL_NEXT(l1);
 	}
 
+	/* ... and construct it by copying from *a */
 	res = (ltree *) palloc0(reslen);
 	SET_VARSIZE(res, reslen);
 	res->numlevel = num;
diff --git a/contrib/ltree/sql/ltree.sql b/contrib/ltree/sql/ltree.sql
index e9f74909a64..846b04e48ee 100644
--- a/contrib/ltree/sql/ltree.sql
+++ b/contrib/ltree/sql/ltree.sql
@@ -54,6 +54,9 @@ SELECT lca('{la.2.3,1.2.3.4.5.6,""}') IS NULL;
 SELECT lca('{la.2.3,1.2.3.4.5.6}') IS NULL;
 SELECT lca('{1.la.2.3,1.2.3.4.5.6}');
 SELECT lca('{1.2.3,1.2.3.4.5.6}');
+SELECT lca('{1.2.3}');
+SELECT lca('{1}'), lca('{1}') IS NULL;
+SELECT lca('{}') IS NULL;
 SELECT lca('1.la.2.3','1.2.3.4.5.6');
 SELECT lca('1.2.3','1.2.3.4.5.6');
 SELECT lca('1.2.2.3','1.2.3.4.5.6');
diff --git a/doc/src/sgml/ltree.sgml b/doc/src/sgml/ltree.sgml
index 98e0ca5c623..3ddd335b8c9 100644
--- a/doc/src/sgml/ltree.sgml
+++ b/doc/src/sgml/ltree.sgml
@@ -457,17 +457,17 @@ Europe &amp; Russia*@ &amp; !Transportation
      <row>
       <entry><function>lca(ltree, ltree, ...)</function><indexterm><primary>lca</primary></indexterm></entry>
       <entry><type>ltree</type></entry>
-      <entry>lowest common ancestor, i.e., longest common prefix of paths
+      <entry>longest common ancestor of paths
        (up to 8 arguments supported)</entry>
-      <entry><literal>lca('1.2.2.3','1.2.3.4.5.6')</literal></entry>
+      <entry><literal>lca('1.2.3','1.2.3.4.5.6')</literal></entry>
       <entry><literal>1.2</literal></entry>
      </row>
 
      <row>
       <entry><function>lca(ltree[])</function></entry>
       <entry><type>ltree</type></entry>
-      <entry>lowest common ancestor, i.e., longest common prefix of paths</entry>
-      <entry><literal>lca(array['1.2.2.3'::ltree,'1.2.3'])</literal></entry>
+      <entry>longest common ancestor of paths in array</entry>
+      <entry><literal>lca(array['1.2.3'::ltree,'1.2.3.4'])</literal></entry>
       <entry><literal>1.2</literal></entry>
      </row>