From 6273966d0b50541a37a652ccf6113f184eff5300 Mon Sep 17 00:00:00 2001 From: Kevin Boulain Date: Thu, 2 Mar 2023 18:59:15 +0100 Subject: [PATCH] lib: replace some uses of Query::MatchAll with a thread-safe alternative This replaces two instances of Xapian::Query::MatchAll with the equivalent but thread-safe alternative Xapian::Query(std::string()). Xapian::Query::MatchAll maintains an internal pointer to a refcounted Xapian::Internal::QueryTerm. None of this is thread-safe but that wouldn't be an issue if Xapian::Query::MatchAll wasn't static. Because it's static, the refcounting goes awry when Notmuch is called from multiple threads. This is actually documented by Xapian: https://github.com/xapian/xapian/blob/4715de3a9fcee741587439dc3cc1d2ff01ffeaf2/xapian-core/include/xapian/query.h#L65 While static, Xapian::Query::MatchNothing is safe because it doesn't maintain an internal object and as such, doesn't use references. Two best-effort tests making use of TSan were added to showcase the issue (I couldn't figure out a way to deterministically reproduce it without making an unmaintainable mess). First, when two databases are created in parallel, a query that uses Xapian::Query::MatchAll is made (lib/query.cc), resulting in the following backtrace on a segfault: #0 0x00007ffff76822af in Xapian::Query::get_terms_begin (this=0x7fffe80137f0) at api/query.cc:141 #1 0x00007ffff7f933f5 in _notmuch_query_cache_terms (query=0x7fffe80137c0) at lib/query.cc:176 #2 0x00007ffff7f93784 in _notmuch_query_ensure_parsed_xapian (query=0x7fffe80137c0) at lib/query.cc:225 #3 0x00007ffff7f9381a in _notmuch_query_ensure_parsed (query=0x7fffe80137c0) at lib/query.cc:260 #4 0x00007ffff7f93bfe in _notmuch_query_search_documents (query=0x7fffe80137c0, type=0x7ffff7fa9b1e "mail", out=0x7ffff666da18) at lib/query.cc:361 #5 0x00007ffff7f93ba4 in notmuch_query_search_messages (query=0x7fffe80137c0, out=0x7ffff666da18) at lib/query.cc:349 #6 0x00007ffff7f83d98 in notmuch_database_upgrade (notmuch=0x7fffe8000bd0, progress_notify=0x0, closure=0x0) at lib/database.cc:934 #7 0x00007ffff7fa110f in notmuch_database_create_with_config (database_path=0x7ffff666dcb0 "/tmp/notmuch.MZ2AGr", config_path=0x7ffff7faab3c "", profile=0x0, database=0x0, status_string=0x7ffff666dc90) at lib/open.cc:754 #8 0x00007ffff7fa0d6f in notmuch_database_create_verbose (path=0x7ffff666dcb0 "/tmp/notmuch.MZ2AGr", database=0x0, status_string=0x7ffff666dc90) at lib/open.cc:653 #9 0x00007ffff7fa0ceb in notmuch_database_create (path=0x7ffff666dcb0 "/tmp/notmuch.MZ2AGr", database=0x0) at lib/open.cc:637 ... Second, some queries would make use of Xapian::Query::MatchAll (lib/regexp-fields.cc), resulting in the following backtrace on a segfault: #0 0x00007f629828b690 in Xapian::Internal::QueryBranch::gather_terms (this=0x7f628800def0, void_terms=0x7f629726d5a0) at api/queryinternal.cc:1245 #1 0x00007f629828c260 in Xapian::Internal::QueryScaleWeight::gather_terms (this=0x7f628800df70, void_terms=0x7f629726d5a0) at api/queryinternal.cc:1434 #2 0x00007f629828b69f in Xapian::Internal::QueryBranch::gather_terms (this=0x7f628800dd90, void_terms=0x7f629726d5a0) at api/queryinternal.cc:1245 #3 0x00007f6298282571 in Xapian::Query::get_unique_terms_begin (this=0x7f628800dcd8) at api/query.cc:166 #4 0x00007f629841a59b in Xapian::Weight::Internal::accumulate_stats (this=0x7f628800dca0, subdb=..., rset=...) at weight/weightinternal.cc:86 #5 0x00007f62983c15ba in LocalSubMatch::prepare_match (this=0x7f628800df20, nowait=true, total_stats=...) at matcher/localsubmatch.cc:172 #6 0x00007f62983c8fcc in prepare_sub_matches (leaves=std::vector of length 1, capacity 1 = {...}, stats=...) at matcher/multimatch.cc:237 #7 0x00007f62983c98a3 in MultiMatch::MultiMatch (this=0x7f629726d9a0, db_=..., query_=..., qlen=3, omrset=0x0, collapse_max_=0, collapse_key_=4294967295, percent_cutoff_=0, weight_cutoff_=0, order_=Xapian::Enquire::ASCENDING, sort_key_=0, sort_by_=Xapian::Enquire::Internal::VAL, sort_value_forward_=true, time_limit_=0, stats=..., weight_=0x7f6288008d50, matchspies_=std::vector of length 0, capacity 0, have_sorter=false, have_mdecider=false) at matcher/multimatch.cc:353 #8 0x00007f629826fcba in Xapian::Enquire::Internal::get_mset (this=0x7f628800e0b0, first=0, maxitems=0, check_at_least=0, rset=0x0, mdecider=0x0) at api/omenquire.cc:569 #9 0x00007f629827181c in Xapian::Enquire::get_mset (this=0x7f629726db80, first=0, maxitems=0, check_at_least=0, rset=0x0, mdecider=0x0) at api/omenquire.cc:937 #10 0x00007f6298be529a in _notmuch_query_search_documents (query=0x7f6288009750, type=0x7f6298bfaafe "mail", out=0x7f629726dcc0) at lib/query.cc:447 #11 0x00007f6298be4ae8 in notmuch_query_search_messages (query=0x7f6288009750, out=0x7f629726dcc0) at lib/query.cc:349 ... Printing Xapian::Query::MatchAll->internal.px->_refs in these circumstances can help quickly identifying this scenario. This is motivated by some test frameworks (like Rust's Cargo) that runs unit tests in parallel and would easily encounter this issue, unless client code gates every call to Notmuch behind a lock. This is what can be expected from the tests when they fail: == stderr == +================== +WARNING: ThreadSanitizer: data race (pid=207931) + Read of size 1 at 0x7b10000001a0 by thread T2: + #0 memcpy (libtsan.so.2+0x62506) + #1 void std::__cxx11::basic_string, std::allocator >::_M_construct(char*, char*, std::forward_iterator_tag) [clone .isra.0] (libxapian.so.30+0x872b3) + + Previous write of size 8 at 0x7b10000001a0 by thread T1: + #0 operator new(unsigned long) (libtsan.so.2+0x8ba83) + #1 Xapian::Query::Query(std::__cxx11::basic_string, std::allocator > const&, unsigned int, unsigned int) (libxapian.so.30+0x855cd) ... --- configure | 15 +++++- lib/query.cc | 3 +- lib/regexp-fields.cc | 3 +- test/T800-asan.sh | 2 +- test/T810-tsan.sh | 92 +++++++++++++++++++++++++++++++++++++ test/T810-tsan.suppressions | 5 ++ util/xapian-extra.h | 15 ++++++ 7 files changed, 131 insertions(+), 4 deletions(-) create mode 100755 test/T810-tsan.sh create mode 100644 test/T810-tsan.suppressions create mode 100644 util/xapian-extra.h diff --git a/configure b/configure index c3629a73..7afd08c7 100755 --- a/configure +++ b/configure @@ -422,6 +422,18 @@ else fi unset test_cmdline +printf "C compiler supports thread sanitizer... " +test_cmdline="${CC} ${CFLAGS} ${CPPFLAGS} -fsanitize=thread minimal.c ${LDFLAGS} -o minimal" +if ${test_cmdline} >/dev/null 2>&1 && ./minimal +then + printf "Yes.\n" + have_tsan=1 +else + printf "Nope, skipping those tests.\n" + have_tsan=0 +fi +unset test_cmdline + printf "Reading libnotmuch version from source... " cat > _libversion.c < @@ -1590,8 +1602,9 @@ NOTMUCH_GMIME_VERIFY_WITH_SESSION_KEY=${gmime_verify_with_session_key} NOTMUCH_ZLIB_CFLAGS="${zlib_cflags}" NOTMUCH_ZLIB_LDFLAGS="${zlib_ldflags}" -# Does the C compiler support the address sanitizer +# Does the C compiler support the sanitizers NOTMUCH_HAVE_ASAN=${have_asan} +NOTMUCH_HAVE_TSAN=${have_tsan} # do we have man pages? NOTMUCH_HAVE_MAN=$((have_sphinx)) diff --git a/lib/query.cc b/lib/query.cc index 707f6222..1c60c122 100644 --- a/lib/query.cc +++ b/lib/query.cc @@ -20,6 +20,7 @@ #include "notmuch-private.h" #include "database-private.h" +#include "xapian-extra.h" #include /* GHashTable, GPtrArray */ @@ -186,7 +187,7 @@ _notmuch_query_string_to_xapian_query (notmuch_database_t *notmuch, { try { if (query_string == "" || query_string == "*") { - output = Xapian::Query::MatchAll; + output = xapian_query_match_all (); } else { output = notmuch->query_parser-> diff --git a/lib/regexp-fields.cc b/lib/regexp-fields.cc index 539915d8..3a775261 100644 --- a/lib/regexp-fields.cc +++ b/lib/regexp-fields.cc @@ -25,6 +25,7 @@ #include "regexp-fields.h" #include "notmuch-private.h" #include "database-private.h" +#include "xapian-extra.h" notmuch_status_t compile_regex (regex_t ®exp, const char *str, std::string &msg) @@ -200,7 +201,7 @@ RegexpFieldProcessor::operator() (const std::string & str) if (str.empty ()) { if (options & NOTMUCH_FIELD_PROBABILISTIC) { return Xapian::Query (Xapian::Query::OP_AND_NOT, - Xapian::Query::MatchAll, + xapian_query_match_all (), Xapian::Query (Xapian::Query::OP_WILDCARD, term_prefix)); } else { return Xapian::Query (term_prefix); diff --git a/test/T800-asan.sh b/test/T800-asan.sh index 8607732e..5055c93e 100755 --- a/test/T800-asan.sh +++ b/test/T800-asan.sh @@ -9,7 +9,7 @@ fi add_email_corpus -TEST_CFLAGS="-fsanitize=address" +TEST_CFLAGS="${TEST_CFLAGS:-} -fsanitize=address" test_begin_subtest "open and destroy" test_C ${MAIL_DIR} ${NOTMUCH_CONFIG} < /dev/null && pwd) + +test_description='run code with TSan enabled against the library' +# Note it is hard to ensure race conditions are deterministic so this +# only provides best effort detection. + +. "$test_directory"/test-lib.sh || exit 1 + +if [ $NOTMUCH_HAVE_TSAN -ne 1 ]; then + printf "Skipping due to missing TSan support\n" + test_done +fi + +export TSAN_OPTIONS="suppressions=$test_directory/T810-tsan.suppressions" +TEST_CFLAGS="${TEST_CFLAGS:-} -fsanitize=thread" + +cp -r ${MAIL_DIR} ${MAIL_DIR}-2 + +test_begin_subtest "create" +test_C ${MAIL_DIR} ${MAIL_DIR}-2 < +#include + +void *thread (void *arg) { + char *mail_dir = arg; + /* + * Calls into notmuch_query_search_messages which was using the thread-unsafe + * Xapian::Query::MatchAll. + */ + EXPECT0(notmuch_database_create (mail_dir, NULL)); + return NULL; +} + +int main (int argc, char **argv) { + pthread_t t1, t2; + EXPECT0(pthread_create (&t1, NULL, thread, argv[1])); + EXPECT0(pthread_create (&t2, NULL, thread, argv[2])); + EXPECT0(pthread_join (t1, NULL)); + EXPECT0(pthread_join (t2, NULL)); + return 0; +} +EOF +cat < EXPECTED +== stdout == +== stderr == +EOF +test_expect_equal_file EXPECTED OUTPUT + +add_email_corpus +rm -r ${MAIL_DIR}-2 +cp -r ${MAIL_DIR} ${MAIL_DIR}-2 + +test_begin_subtest "query" +test_C ${MAIL_DIR} ${MAIL_DIR}-2 < +#include + +void *thread (void *arg) { + char *mail_dir = arg; + notmuch_database_t *db; + /* + * 'from' is NOTMUCH_FIELD_PROBABILISTIC | NOTMUCH_FIELD_PROCESSOR and an + * empty string gets us to RegexpFieldProcessor::operator which was using + * the tread-unsafe Xapian::Query::MatchAll. + */ + EXPECT0(notmuch_database_open_with_config (mail_dir, + NOTMUCH_DATABASE_MODE_READ_ONLY, + NULL, NULL, &db, NULL)); + notmuch_query_t *query = notmuch_query_create (db, "from:\"\""); + notmuch_messages_t *messages; + EXPECT0(notmuch_query_search_messages (query, &messages)); + return NULL; +} + +int main (int argc, char **argv) { + pthread_t t1, t2; + EXPECT0(pthread_create (&t1, NULL, thread, argv[1])); + EXPECT0(pthread_create (&t2, NULL, thread, argv[2])); + EXPECT0(pthread_join (t1, NULL)); + EXPECT0(pthread_join (t2, NULL)); + return 0; +} +EOF +cat < EXPECTED +== stdout == +== stderr == +EOF +test_expect_equal_file EXPECTED OUTPUT + +test_done diff --git a/test/T810-tsan.suppressions b/test/T810-tsan.suppressions new file mode 100644 index 00000000..dbd16a94 --- /dev/null +++ b/test/T810-tsan.suppressions @@ -0,0 +1,5 @@ +# It's unclear how TSan-friendly GLib is: +# https://gitlab.gnome.org/GNOME/glib/-/issues/1672 +race:g_rw_lock_reader_lock +# https://gitlab.gnome.org/GNOME/glib/-/issues/1952 +race:g_slice_alloc0 diff --git a/util/xapian-extra.h b/util/xapian-extra.h new file mode 100644 index 00000000..39c7f48f --- /dev/null +++ b/util/xapian-extra.h @@ -0,0 +1,15 @@ +#ifndef _XAPIAN_EXTRA_H +#define _XAPIAN_EXTRA_H + +#include +#include + +inline Xapian::Query +xapian_query_match_all (void) +{ + // Xapian::Query::MatchAll isn't thread safe (a static object with reference + // counting) so instead reconstruct the equivalent on demand. + return Xapian::Query (std::string ()); +} + +#endif