doc: note why we can't use thread_local with glibc back compat #18709

pull fanquake wants to merge 2 commits into bitcoin:master from fanquake:document_thread_local_glibc changing 1 files +4 −1

fanquake commented at 9:02 AM on April 20, 2020: member
Given that we went through a gitian build to remember why this is the case, we might as well make a note of it in configure.ac.

From [#18681](/bitcoin-bitcoin/18681/):

Looking at the Linux build log, this has failed with:
```
Checking glibc back compat...
bitcoind: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
bitcoind: failed IMPORTED_SYMBOLS
bitcoin-cli: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
bitcoin-cli: failed IMPORTED_SYMBOLS
bitcoin-tx: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
bitcoin-tx: failed IMPORTED_SYMBOLS
bitcoin-wallet: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
bitcoin-wallet: failed IMPORTED_SYMBOLS
test/test_bitcoin: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
test/test_bitcoin: failed IMPORTED_SYMBOLS
bench/bench_bitcoin: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
bench/bench_bitcoin: failed IMPORTED_SYMBOLS
qt/bitcoin-qt: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
```
__cxa_thread_atexit_impl is used for thread_local variable destruction:

To implement this support, glibc defines __cxa_thread_atexit_impl exclusively for use by libstdc++ (which has the __cxa_thread_atexit to wrap around it), that registers destructors for thread_local variables in a list. Upon thread or process exit, the destructors are called in reverse order in which they were added.

As suggested, this only became available in glibc 2.18. From the 2.18 release notes:
Add support for calling C++11 thread_local object destructors on thread and program exit. This needs compiler support for offloading C++11 destructor calls to glibc.
doc: note why we can't use thread_local with glibc back compat 20a30922fb
fanquake added the label Docs on Apr 20, 2020
fanquake added the label Build system on Apr 20, 2020
MarcoFalke commented at 9:49 AM on April 20, 2020: member

ACK
DrahtBot commented at 12:26 PM on April 20, 2020: member



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hebasto approved
hebasto commented at 2:18 PM on April 20, 2020: member

ACK 20a30922fbf6ba14e250ca649239af115dbbe7b0
hebasto commented at 7:51 PM on April 20, 2020: member

It is absolutely unrelated, but would you mind adding a typo fix in https://github.com/bitcoin/bitcoin/blob/c4c3f110eb93243fc8f740070240f50b0008f206/configure.ac#L259 s/liner/linker/ ?
MarcoFalke added the label Needs Guix build on Apr 20, 2020
MarcoFalke removed the label Needs Guix build on Apr 21, 2020
doc: fix typo in configure.ac
s/liner/linker/
b155fcda51
fanquake commented at 2:07 AM on April 21, 2020: member

It is absolutely unrelated, but would you mind adding a typo fix in

Done.
hebasto approved
hebasto commented at 9:22 AM on April 21, 2020: member

ACK b155fcda5186c59fc4fb2a9eaaf791d132e0ab30
fanquake merged this on Apr 22, 2020
fanquake closed this on Apr 22, 2020
fanquake deleted the branch on Apr 22, 2020
sidhujag referenced this in commit e466d0eb3a on Apr 23, 2020
DrahtBot locked this on Feb 15, 2022

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#18681 donotmerge: build: Enable thread-local with glibc compat

doc: note why we can't use thread_local with glibc back compat #18709

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