test: regression test for n_t_get_thread_id

This is just cached data, so as long as we don't prematurely free
anything, it should be fine.

test/T568-lib-thread.sh

@@ -0,0 +1,63 @@
+#!/usr/bin/env bash
+test_description="API tests for notmuch_thread_*"
+
+. $(dirname "$0")/test-lib.sh || exit 1
+
+add_email_corpus
+
+test_begin_subtest "building database"
+test_expect_success "NOTMUCH_NEW"
+
+cat <<'EOF' > c_tail
+   if (stat) {
+       const char *stat_str = notmuch_database_status_string (db);
+       if (stat_str)
+           fputs (stat_str, stderr);
+    }
+
+}
+EOF
+
+cat <<EOF > c_head
+#include <stdio.h>
+#include <notmuch.h>
+#include <notmuch-test.h>
+int main (int argc, char** argv)
+{
+   notmuch_database_t *db;
+   notmuch_status_t stat;
+   char *msg = NULL;
+   notmuch_thread_t *thread = NULL;
+   notmuch_threads_t *threads = NULL;
+   notmuch_query_t *query = NULL;
+   const char *id = "thread:0000000000000009";
+
+   stat = notmuch_database_open_verbose (argv[1], NOTMUCH_DATABASE_MODE_READ_WRITE, &db, &msg);
+   if (stat != NOTMUCH_STATUS_SUCCESS) {
+     fprintf (stderr, "error opening database: %d %s\n", stat, msg ? msg : "");
+     exit (1);
+   }
+
+   query = notmuch_query_create (db, id);
+   EXPECT0(notmuch_query_search_threads (query, &threads));
+   thread = notmuch_threads_get (threads);
+   EXPECT0(notmuch_database_close (db));
+EOF
+
+test_begin_subtest "get thread-id from closed database"
+cat c_head - c_tail <<'EOF' | test_C ${MAIL_DIR}
+    {
+        const char *id2;
+        id2 = notmuch_thread_get_thread_id (thread);
+        printf("%d\n%s\n", thread != NULL, id2);
+    }
+EOF
+cat <<EOF > EXPECTED
+== stdout ==
+1
+0000000000000009
+== stderr ==
+EOF
+test_expect_equal_file EXPECTED OUTPUT
+
+test_done