test: add known broken test for uncaught DatabaseModifiedError

There are several of these to track down, but one that is in quite a
few code paths is _notmuch_message_ensure_metadata.

test/T640-database-modified.sh

@@ -0,0 +1,67 @@
+#!/usr/bin/env bash
+test_description="DatabaseModifiedError handling"
+. ./test-lib.sh || exit 1
+
+# add enough messages to trigger the exception
+add_email_corpus
+
+test_begin_subtest "catching DatabaseModifiedError in _notmuch_message_ensure_metadata"
+test_subtest_known_broken
+# it seems to need to be an early document to trigger the exception
+first_id=$(notmuch search --output=messages '*'| head -1 | sed s/^id://)
+
+test_C ${MAIL_DIR} <<EOF
+#include <unistd.h>
+#include <stdlib.h>
+#include <notmuch-test.h>
+#include <talloc.h>
+#include <assert.h>
+int
+main (int argc, char **argv)
+{
+    const char *path = argv[1];
+
+    notmuch_database_t *rw_db, *ro_db;
+    notmuch_messages_t *messages;
+    notmuch_message_t *message, *ro_message;
+    notmuch_query_t *query;
+    notmuch_tags_t *tags;
+
+    EXPECT0 (notmuch_database_open (path, NOTMUCH_DATABASE_MODE_READ_ONLY, &ro_db));
+    assert(ro_db);
+
+    EXPECT0 (notmuch_database_find_message (ro_db, "${first_id}", &ro_message));
+    assert(ro_message);
+
+    EXPECT0 (notmuch_database_open (path, NOTMUCH_DATABASE_MODE_READ_WRITE, &rw_db));
+    query = notmuch_query_create(rw_db, "");
+    EXPECT0 (notmuch_query_search_messages_st (query, &messages));
+
+    for (int count=0;
+	 notmuch_messages_valid (messages);
+	 notmuch_messages_move_to_next (messages), count++) {
+	message = notmuch_messages_get (messages);
+	for (int i=0; i<200; i++) {
+	    char *tag_str = talloc_asprintf(rw_db, "%d", i);
+	    EXPECT0 (notmuch_message_add_tag (message, tag_str));
+	    talloc_free (tag_str);
+	}
+    }
+
+    notmuch_database_close (rw_db);
+
+    tags = notmuch_message_get_tags (ro_message);
+    if (tags)
+	printf("SUCCESS\n");
+    return 0;
+}
+EOF
+
+cat <<'EOF' >EXPECTED
+== stdout ==
+SUCCESS
+== stderr ==
+EOF
+test_expect_equal_file EXPECTED OUTPUT
+
+test_done