Merge bitcoin/bitcoin#28981: Replace Boost.Process with cpp-subprocess

d5a715536e497c160a2520f81334aab6c7490213 build: remove boost::process dependency for building external signer support (Sebastian Falbesoner)
70434b1c443d9251a880d0193af771f574c40617 external_signer: replace boost::process with cpp-subprocess (Sebastian Falbesoner)
cc8b9875b104c31f0a5b5e4195a8278ec55f35f7 Add `cpp-subprocess` header-only library (Hennadii Stepanov)

Pull request description:

  Closes https://github.com/bitcoin/bitcoin/issues/24907.

  This PR is based on **theStack**'s [work](https://github.com/bitcoin/bitcoin/issues/24907#issuecomment-1466087049).

  The `subprocess.hpp` header has been sourced from the [upstream repo](https://github.com/arun11299/cpp-subprocess) with the only modification being the removal of convenience functions, which are not utilized in our codebase.

  Windows-related changes will be addressed in subsequent follow-ups.

ACKs for top commit:
  achow101:
    reACK d5a715536e497c160a2520f81334aab6c7490213
  Sjors:
    re-tACK d5a715536e497c160a2520f81334aab6c7490213
  theStack:
    Light re-ACK d5a715536e497c160a2520f81334aab6c7490213
  fanquake:
    ACK d5a715536e497c160a2520f81334aab6c7490213 - with the expectation that this code is going to be maintained as our own. Next PRs should:

Tree-SHA512: d7fb6fecc3f5792496204190afb7d85b3e207b858fb1a75efe483c05260843b81b27d14b299323bb667c990e87a07197059afea3796cf218ed8b614086bd3611

1 files changed, 10 insertions, 52 deletions

diff --git a/configure.ac b/configure.ac
index d8b7159e27..63a012b35d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -307,9 +307,9 @@ AC_ARG_ENABLE([werror],
     [enable_werror=no])
 
 AC_ARG_ENABLE([external-signer],
-    [AS_HELP_STRING([--enable-external-signer],[compile external signer support (default is auto, requires Boost::Process)])],
+    [AS_HELP_STRING([--enable-external-signer],[compile external signer support (default is yes)])],
     [use_external_signer=$enableval],
-    [use_external_signer=auto])
+    [use_external_signer=yes])
 
 AC_LANG_PUSH([C++])
 
@@ -1414,56 +1414,14 @@ if test "$use_boost" = "yes"; then
   fi
 fi
 
-if test "$use_external_signer" != "no"; then
-  AC_MSG_CHECKING([whether Boost.Process can be used])
-  TEMP_CXXFLAGS="$CXXFLAGS"
-  dnl Boost 1.78 requires the following workaround.
-  dnl See: https://github.com/boostorg/process/issues/235
-  CXXFLAGS="$CXXFLAGS -Wno-error=narrowing"
-  TEMP_CPPFLAGS="$CPPFLAGS"
-  CPPFLAGS="$CPPFLAGS $BOOST_CPPFLAGS"
-  TEMP_LDFLAGS="$LDFLAGS"
-  dnl Boost 1.73 and older require the following workaround.
-  LDFLAGS="$LDFLAGS $PTHREAD_CFLAGS"
-  AC_LINK_IFELSE([AC_LANG_PROGRAM([[
-    #define BOOST_PROCESS_USE_STD_FS
-    #include <boost/process.hpp>
-  ]],[[
-    namespace bp = boost::process;
-    bp::opstream stdin_stream;
-    bp::ipstream stdout_stream;
-    bp::child c("dummy", bp::std_out > stdout_stream, bp::std_err > stdout_stream, bp::std_in < stdin_stream);
-    stdin_stream << std::string{"test"} << std::endl;
-    if (c.running()) c.terminate();
-    c.wait();
-    c.exit_code();
-  ]])],
-    [have_boost_process="yes"],
-    [have_boost_process="no"])
-  LDFLAGS="$TEMP_LDFLAGS"
-  CPPFLAGS="$TEMP_CPPFLAGS"
-  CXXFLAGS="$TEMP_CXXFLAGS"
-  AC_MSG_RESULT([$have_boost_process])
-  if test "$have_boost_process" = "yes"; then
-    case $host in
-      dnl Boost Process for Windows uses Boost ASIO. Boost ASIO performs
-      dnl pre-main init of Windows networking libraries, which we do not
-      dnl want.
-      *mingw*)
-        use_external_signer="no"
-      ;;
-      *)
-        use_external_signer="yes"
-        AC_DEFINE([ENABLE_EXTERNAL_SIGNER], [1], [Define if external signer support is enabled])
-        AC_DEFINE([BOOST_PROCESS_USE_STD_FS], [1], [Defined to avoid Boost::Process trying to use Boost Filesystem])
-      ;;
-    esac
-  else
-    if test "$use_external_signer" = "yes"; then
-      AC_MSG_ERROR([External signing is not supported for this Boost version])
-    fi
-    use_external_signer="no";
-  fi
+case $host in
+  dnl Re-enable it after enabling Windows support in cpp-subprocess.
+  *mingw*)
+    use_external_signer="no"
+  ;;
+esac
+if test "$use_external_signer" = "yes"; then
+  AC_DEFINE([ENABLE_EXTERNAL_SIGNER], [1], [Define if external signer support is enabled])
 fi
 AM_CONDITIONAL([ENABLE_EXTERNAL_SIGNER], [test "$use_external_signer" = "yes"])