libart: better defined behavior when a full prefix key is encountered

Summary: a facet of the libart implementation that is undocumented
except for a series of issues (linked below) is this: an inserted key
must not be a full prefix of another key that is already stored in the
tree.  The recommendation is to store keys that have a terminator
character.

armon/libart#4
armon/libart#12
armon/libart#14
armon/libart#17

Our usage in watchman can't guarantee to provide a NUL-terminated input,
so this diff adjusts the key comparison routines to generate an implicit
or synthetic NUL terminator character when comparing one character
beyond the end of a key, and by asserting (or allowing ASAN to complain)
if we try to look more than 1 character beyond the end.

I also tidied up the function signatures of a couple of matching
functions so that the semantics are clearer (return boolean true for
success rather than integer 0) and removed some redundant casts when
invoking callbacks.

Test Plan: added an explicit test for the full-prefix issue.

Built with -fsanitize=address and ran the integration tests as well as
the sparse/unsparse checks I've been using for the pending list changes.

Makefile.am

@@ -201,7 +201,8 @@ integration: all py-integration
 tests_art_t_CPPFLAGS = $(THIRDPARTY_CPPFLAGS)
 tests_art_t_LDADD = $(ART_LIB) $(TAP_LIB)
 tests_art_t_SOURCES = \
-	tests/art_test.c
+	tests/art_test.c \
+	log.c
 
 tests_argv_t_CPPFLAGS = $(THIRDPARTY_CPPFLAGS)
 tests_argv_t_LDADD = $(JSON_LIB) $(TAP_LIB)

tests/art_test.c

@@ -13,11 +13,26 @@
 
 #include "thirdparty/tap.h"
 #include "thirdparty/libart/src/art.h"
+#include "watchman.h"
 
 #define stringy2(line)  #line
 #define stringy(line)   stringy2(line)
 #define fail_unless(__x)  ok(__x, __FILE__ ":" stringy(__LINE__) " " # __x)
 
+void w_request_shutdown(void) {}
+
+bool w_should_log_to_clients(int level)
+{
+  unused_parameter(level);
+  return false;
+}
+
+void w_log_to_clients(int level, const char *buf)
+{
+  unused_parameter(level);
+  unused_parameter(buf);
+}
+
 void test_art_init_and_destroy(void) {
   art_tree t;
   int res = art_tree_init(&t);
@@ -430,6 +445,34 @@ void test_art_long_prefix(void) {
   fail_unless(res == 0);
 }
 
+static int dump_iter(void *data, const unsigned char *key, unsigned int key_len,
+                     void *value) {
+  diag("iter leaf: data=%p key_len=%d %.*s value=%p", data, (int)key_len,
+       (int)key_len, key, value);
+  return 0;
+}
+
+void test_art_prefix(void) {
+  art_tree t;
+  void *v;
+
+  art_tree_init(&t);
+
+  fail_unless(art_insert(&t, (const unsigned char*)"food", 4, "food") == NULL);
+  fail_unless(art_insert(&t, (const unsigned char*)"foo", 3, "foo") == NULL);
+  diag("size is now %d", art_size(&t));
+  fail_unless(art_size(&t) == 2);
+  fail_unless((v = art_search(&t, (const unsigned char*)"food", 4)) != NULL);
+  diag("food lookup yields %s", v);
+  fail_unless(v && strcmp((char*)v, "food") == 0);
+
+  art_iter(&t, dump_iter, NULL);
+
+  fail_unless((v = art_search(&t, (const unsigned char*)"foo", 3)) != NULL);
+  diag("foo lookup yields %s", v);
+  fail_unless(v && strcmp((char*)v, "foo") == 0);
+}
+
 void test_art_insert_search_uuid(void) {
   art_tree t;
   art_leaf *l;
@@ -486,7 +529,7 @@ int main(int argc, char **argv) {
   (void)argc;
   (void)argv;
 
-  plan_tests(109);
+  plan_tests(116);
   test_art_init_and_destroy();
   test_art_insert();
   test_art_insert_verylong();
@@ -496,6 +539,7 @@ int main(int argc, char **argv) {
   test_art_iter_prefix();
   test_art_long_prefix();
   test_art_insert_search_uuid();
+  test_art_prefix();
 
   return exit_status();
 }

tests/ignore_test.c

@@ -3,7 +3,6 @@
 
 #include "watchman.h"
 #include "thirdparty/tap.h"
-#include "thirdparty/libart/src/art.h"
 
 // A list that looks similar to one used in one of our repos
 const char *ignore_dirs[] = {".buckd",