# Destroying futures after 'fdb\_transaction\_on\_error()' **URL:** \ **Category:** Using FoundationDB\ **Created:** [April 16, 2019, 1:33pm UTC](https://forums.foundationdb.org/t/destroying-futures-after-fdb-transaction-on-error/1307 "2019-04-16T13:33:52Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![doublemax](https://avatars.discourse-cdn.com/v4/letter/d/8e7dd6/32.png) [@doublemax](https://forums.foundationdb.org/u/doublemax)\ **Post date:** [April 16, 2019, 1:33pm UTC](https://forums.foundationdb.org/t/destroying-futures-after-fdb-transaction-on-error/1307/1 "2019-04-16T13:33:52Z")
I experience a strange crash after “1031: Operation aborted because the transaction timed out” errors (reading from multiple futures in parallel): ```auto ERROR: AddressSanitizer: SEGV on unknown address 0x7f9e30800016 The signal is caused by a READ memory access. 0x7f9ec55fe402 in fdb_future_destroy (/lib64/libfdb_c.so+0xd5402) ``` The documentation of `fdb_transaction_on_error()` is unclear whether to also destroy the original future. Pseudo-code: ```auto FDBFuture *future_read = fdb_transaction_get( transaction, "test", 4, true ); fdb_error_t error_read = fdb_future_block_until_ready( future_read ); if( error_read == 0 ) error_read = fdb_future_get_error( future_read ); if( error_read != 0 ) { FDBFuture *future_error = fdb_transaction_on_error( transaction, error_read ); fdb_error_t error_retry = fdb_future_block_until_ready( future_error ); if( error_retry == 0 ) error_retry = fdb_future_get_error( error_retry ); fdb_future_destroy( future_error ); // Here be dragons: Is the next 'fdb_future_destroy' required? fdb_future_destroy( future_read ); } ``` ---
**Author:** ![ajbeamon](https://sea1.discourse-cdn.com/foundationdb/user_avatar/forums.foundationdb.org/ajbeamon/32/13_2.png) [@ajbeamon](https://forums.foundationdb.org/u/ajbeamon)\ **Post date:** [April 16, 2019, 3:21pm UTC](https://forums.foundationdb.org/t/destroying-futures-after-fdb-transaction-on-error/1307/2 "2019-04-16T15:21:04Z")
You always need to destroy your futures, and at first glance that aspect of your code looks correct. I think the issue you may be running into comes from the following line: > [@doublemax](#): > > `error_retry = fdb_future_get_error( error_retry );` Your intent was probably to pass `future_error` into `fdb_future_get_error`, and passing `error_retry` is going to cause an invalid pointer dereference. ---
**Author:** ![doublemax](https://avatars.discourse-cdn.com/v4/letter/d/8e7dd6/32.png) [@doublemax](https://forums.foundationdb.org/u/doublemax)\ **Post date:** [April 16, 2019, 3:42pm UTC](https://forums.foundationdb.org/t/destroying-futures-after-fdb-transaction-on-error/1307/3 "2019-04-16T15:42:02Z")
This happens only if this two conditions met: a)The FDB **server** times out 5 sec (overloaded due swapping in my case) b) Multiple reads in parallel, pseudo-code: ```auto FDBFuture futures[10]; for( int i=0; i < 10; ++i ) futures[i] = fdb_transaction_get( transaction, i, 4, true ); for( int i=0; i < 10; ++i ) fdb_future_block_until_ready( futures[i] ); ``` I have core-dumps - but they are not very helpful. I will try to compile FDB using ASAN. ---
**Author:** ![ajbeamon](https://sea1.discourse-cdn.com/foundationdb/user_avatar/forums.foundationdb.org/ajbeamon/32/13_2.png) [@ajbeamon](https://forums.foundationdb.org/u/ajbeamon)\ **Post date:** [April 16, 2019, 4:21pm UTC](https://forums.foundationdb.org/t/destroying-futures-after-fdb-transaction-on-error/1307/4 "2019-04-16T16:21:18Z")
Does that mean that making the fix I suggested didn’t solve all of your crashes? The example you originally pasted crashed for me as written but works if I make that change. Do you have some code for a reproduction that I could play with? ---
**Author:** ![doublemax](https://avatars.discourse-cdn.com/v4/letter/d/8e7dd6/32.png) [@doublemax](https://forums.foundationdb.org/u/doublemax)\ **Post date:** [April 16, 2019, 6:59pm UTC](https://forums.foundationdb.org/t/destroying-futures-after-fdb-transaction-on-error/1307/5 "2019-04-16T18:59:27Z")
Yes, the above pseudo-code (“error\_retry” -\> “future\_error”) is wrong. But this is not the problem. So far I could not create a reproducible testcase. It happens when I read multiple records within a transaction and (one?) read times out because the server is overloaded (a cheap Intel NUC in my case). I will compile FDB with address-sanitizer - maybe it will report some useful information. ---
**Author:** ![doublemax](https://avatars.discourse-cdn.com/v4/letter/d/8e7dd6/32.png) [@doublemax](https://forums.foundationdb.org/u/doublemax)\ **Post date:** [April 18, 2019, 4:47pm UTC](https://forums.foundationdb.org/t/destroying-futures-after-fdb-transaction-on-error/1307/6 "2019-04-18T16:47:24Z")
I forgot to mention, that the client crashes (not the server). The segfault is a rare case. The `future` passed to `fdb_future_destroy()` is correct. FDB 6.1.2 - compiled `USE_ASAN=on` ```auto ================================================================= ==26316==ERROR: AddressSanitizer: SEGV on unknown address 0x7f1c5280001a (pc 0x7f1c42679efb bp 0x7fff98c214e0 sp 0x7fff98c1db80 T0) ==26316==The signal is caused by a READ memory access. #0 0x7f1c42679efa in fdb_future_destroy /home/login/foundationdb-6.1.2/bindings/c/fdb_c.cpp:154 #1 0x16e3aea in load_records foundationdb.cpp:982 [...snip...] AddressSanitizer can not provide additional info. SUMMARY: AddressSanitizer: SEGV /home/login/foundationdb-6.1.2/bindings/c/fdb_c.cpp:154 in fdb_future_destroy ==26316==ABORTING ``` As far as I can tell this happens only: a) Reading multiple records in parallel b) A read returns the error: `1031: Operation aborted because the transaction timed out` ASAN is not very helpful. Is there a compile option to get a better backtrace? ---
**Author:** ![doublemax](https://avatars.discourse-cdn.com/v4/letter/d/8e7dd6/32.png) [@doublemax](https://forums.foundationdb.org/u/doublemax)\ **Post date:** [April 20, 2019, 1:57pm UTC](https://forums.foundationdb.org/t/destroying-futures-after-fdb-transaction-on-error/1307/7 "2019-04-20T13:57:09Z")
Wrong. It did not - and does not - use address sanitizer. This is what I did: ```auto tar xvfz 6.1.2.tar.gz cd foundationdb-6.1.2/build cmake USE_ASAN=ON CMAKE_VERBOSE_MAKEFILE=ON -DBOOST_ROOT=/tmp/backup/boost_1_67_0 .. make ``` But it compiles without “-fno-omit-frame-pointer -fsanitize=address”. How do you enable address sanitizer? ---
**Author:** ![markus.pilman](https://sea1.discourse-cdn.com/foundationdb/user_avatar/forums.foundationdb.org/markus.pilman/32/379_2.png) [@markus.pilman](https://forums.foundationdb.org/u/markus.pilman)\ **Post date:** [April 21, 2019, 3:36am UTC](https://forums.foundationdb.org/t/destroying-futures-after-fdb-transaction-on-error/1307/8 "2019-04-21T03:36:30Z")
The proper syntax for cmake variables is `-DVARIABLE=VALUE`. So in your command should look like this: ```auto cmake -DUSE_ASAN=ON CMAKE_VERBOSE_MAKEFILE=ON -DBOOST_ROOT=/tmp/backup/boost_1_67_0 .. ``` ---
**Author:** ![doublemax](https://avatars.discourse-cdn.com/v4/letter/d/8e7dd6/32.png) [@doublemax](https://forums.foundationdb.org/u/doublemax)\ **Post date:** [April 24, 2019, 6:38pm UTC](https://forums.foundationdb.org/t/destroying-futures-after-fdb-transaction-on-error/1307/9 "2019-04-24T18:38:45Z")
@ajbeamon @markus.pilman This issue was my fault - I called `fdb_future_destroy()` accidentally twice - which lead to use-after-free. Sorry for the inconvenience.