Merge #21064: refactor: use std::shared_mutex & remove Boost Thread

060a2a64d40d75fecb60b7d2b9946a67e46aa6fc ci: remove boost thread installation (fanquake)
06e1d7d81d5a56d136c6fc88f09a2b0654a164f9 build: don't build or use Boost Thread (fanquake)
7097add83c8596f81be9edd66971ffd2486357eb refactor: replace Boost shared_mutex with std shared_mutex in sigcache (fanquake)
8e55981ef834490c438436719f95cbaf888c4914 refactor: replace Boost shared_mutex with std shared_mutex in cuckoocache tests (fanquake)

Pull request description:

  This replaces `boost::shared_mutex` and `boost::unique_lock` with [`std::shared_mutex`](https://en.cppreference.com/w/cpp/thread/shared_mutex) & [`std::unique_lock`](https://en.cppreference.com/w/cpp/thread/unique_lock).

  Even though [some concerns were raised](https://github.com/bitcoin/bitcoin/issues/16684#issuecomment-726214696) in #16684 with regard to `std::shared_mutex` being unsafe to use across some glibc versions, I still think this change is an improvement. As I mentioned in #21022, I also think trying to restrict standard library feature usage based on bugs in glibc is not only hard to do, but it's not currently clear exactly how we do that in practice (does it also extend to patching out use in our dependencies, should we be implementing more runtime checks for features we are using, when do we consider an affected glibc "old enough" not to worry about? etc). If you take a look through the [glibc bug tracker](https://sourceware.org/bugzilla/describecomponents.cgi?product=glibc) you'll no doubt find plenty of (active) bug reports for standard library code we already using. Obviously not to say we shouldn't try and avoid buggy code where possible.

  Two other points:

  [Cory mentioned in #21022](https://github.com/bitcoin/bitcoin/pull/21022#issuecomment-769274179):
  > It also seems reasonable to me to worry that boost hits the same underlying glibc bug, and we've just not happened to trigger the right conditions yet.

  Moving away from Boost to the standard library also removes the potential for differences related to Boosts configuration. Boost has multiple versions of `shared_mutex`, and what you end up using, and what it's backed by depends on:
  * The version of Boost.
  * The platform you're building for.
  * Which version of `BOOST_THREAD_VERSION` is defined: (2,3,4 or 5) default=2. (see [here](https://www.boost.org/doc/libs/1_70_0/doc/html/thread/build.html#thread.build.configuration) for some of the differences).
  * Is `BOOST_THREAD_V2_SHARED_MUTEX` defined? (not by default). If so, you might get the ["less performant, but more robust"](https://github.com/boostorg/thread/issues/230#issuecomment-475937761) version of `shared_mutex`.

  A lot of these factors are eliminated by our use of depends, but users will have varying configurations. It's also not inconceivable to think that a distro, or some package manager might start defining something like `BOOST_THREAD_VERSION=3`. Boost tried to change the default from 2 to 3 at one point.

  With this change, we no longer use Boost Thread, so this PR also removes it from depends, the build system, CI etc.

  Previous similar PRs were #19183 & #20922. The authors are included in the commits here.
  Also related to #21022 - pthread sanity checking.

ACKs for top commit:
  laanwj:
    Code review ACK 060a2a64d40d75fecb60b7d2b9946a67e46aa6fc
  vasild:
    ACK 060a2a64d40d75fecb60b7d2b9946a67e46aa6fc

Tree-SHA512: 572d14d8c9de20bc434511f20d3f431836393ff915b2fe9de5a47a02dca76805ad5c3fc4cceecb4cd43f3ba939a0508178c4e60e62abdbaaa6b3e8db20b75b03

1 files changed, 2 insertions, 3 deletions

diff --git a/configure.ac b/configure.ac
index e759ff4f87..62f1c308b7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -657,7 +657,7 @@ case $host in
        AC_MSG_ERROR("windres not found")
      fi
 
-     CPPFLAGS="$CPPFLAGS -D_MT -DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB -D_WIN32_WINNT=0x0601 -D_WIN32_IE=0x0501 -DWIN32_LEAN_AND_MEAN"
+     CPPFLAGS="$CPPFLAGS -D_MT -DWIN32 -D_WINDOWS -D_WIN32_WINNT=0x0601 -D_WIN32_IE=0x0501 -DWIN32_LEAN_AND_MEAN"
 
      dnl libtool insists upon adding -nostdlib and a list of objects/libs to link against.
      dnl That breaks our ability to build dll's with static libgcc/libstdc++/libssp. Override
@@ -1387,7 +1387,6 @@ if test x$want_boost = xno; then
 fi
 AX_BOOST_SYSTEM
 AX_BOOST_FILESYSTEM
-AX_BOOST_THREAD
 
 dnl Opt-in to boost-process
 AS_IF([ test x$with_boost_process != x ], [ AX_BOOST_PROCESS ], [ ax_cv_boost_process=no ] )
@@ -1401,7 +1400,7 @@ dnl counter implementations. In 1.63 and later the std::atomic approach is defau
 m4_pattern_allow(DBOOST_AC_USE_STD_ATOMIC) dnl otherwise it's treated like a macro
 BOOST_CPPFLAGS="-DBOOST_SP_USE_STD_ATOMIC -DBOOST_AC_USE_STD_ATOMIC $BOOST_CPPFLAGS"
 
-BOOST_LIBS="$BOOST_LDFLAGS $BOOST_SYSTEM_LIB $BOOST_FILESYSTEM_LIB $BOOST_THREAD_LIB"
+BOOST_LIBS="$BOOST_LDFLAGS $BOOST_SYSTEM_LIB $BOOST_FILESYSTEM_LIB"
 fi
 
 dnl Check for reduced exports