Merge bitcoin/bitcoin#24065: build: explicitly disable support for external signing on Windows

e2ab9f83f8655bf09ea392beeee36b2bbe29769b build: disable external signer on Windows (fanquake)

Pull request description:

  This change explicitly disables support for external signing when targeting Windows and OpenBSD. The driver for this is that Boost Process uses boost::filesystem internally, when targeting Windows, which gets in the way of removing our usage of it (#20744). While we could adjust #20744 to still link against the Boost libs when building for Windows, that would be disappointing, as we wouldn't have cleanly removed the Boost usage we're trying too (including the build infrastructure), and, we'd be in a position where we would be building releases differently depending on the platform, which is something I want to avoid.

  After discussion with Sjors, Achow and Hebasto, this seemed like a reasonable step to move #20744 forward (as-is). Note that support for external signing ([while already being experimental](https://github.com/bitcoin/bitcoin/blob/master/doc/external-signer.md#example-usage)), could be considered even more experimental on Windows. Also, oddly, we have external-signing [explicitly disabled in our Windows (cross-compile) CI](https://github.com/bitcoin/bitcoin/blob/807169e10b4a18324356ed6ee4d69587b96a7c70/ci/test/00_setup_env_win64.sh#L16), it's not clear why this is the case, as, if it's a feature being built into releases, it should be being built and tested in the CI which is most-like the release process.

  There is an [issue open upstream](https://github.com/boostorg/process/issues/207), in regards to migrating Boost Process to std::filesystem, or having an option to use it. However there hasn't been much discussion since it was opened ~9 months ago. There is another related issue here: https://github.com/klemens-morgenstern/boost-process/issues/164.

  Resolves #24036.

ACKs for top commit:
  Sjors:
    utACK e2ab9f8
  achow101:
    ACK e2ab9f83f8655bf09ea392beeee36b2bbe29769b
  kallewoof:
    utACK e2ab9f83f8655bf09ea392beeee36b2bbe29769b
  hebasto:
    ACK e2ab9f83f8655bf09ea392beeee36b2bbe29769b, tested on Linux Mint 20.2 (x86_64).

Tree-SHA512: 36fcfc0e1a008a8271dc76b8e12e93d3e1d1e528bf668e95a559e9f6fd7d5f031bd7a6a6bc8b9fa9d057b2cd56f9ec8838c7f74e87899bf9a6aeb787afbd112c

3 files changed, 17 insertions, 8 deletions

diff --git a/ci/test/00_setup_env_win64.sh b/ci/test/00_setup_env_win64.sh
index 44b6eb7ae3..6619852423 100755
--- a/ci/test/00_setup_env_win64.sh
+++ b/ci/test/00_setup_env_win64.sh
@@ -13,4 +13,4 @@ export DPKG_ADD_ARCH="i386"
 export PACKAGES="python3 nsis g++-mingw-w64-x86-64 wine-binfmt wine64 wine32 file"
 export RUN_FUNCTIONAL_TESTS=false
 export GOAL="deploy"
-export BITCOIN_CONFIG="--enable-reduce-exports --disable-gui-tests --disable-external-signer"
+export BITCOIN_CONFIG="--enable-reduce-exports --disable-gui-tests"
diff --git a/configure.ac b/configure.ac
index 5a6b54a1ae..bef3973996 100644
--- a/configure.ac
+++ b/configure.ac
@@ -321,7 +321,7 @@ AC_ARG_ENABLE([werror],
 AC_ARG_ENABLE([external-signer],
     [AS_HELP_STRING([--enable-external-signer],[compile external signer support (default is yes, requires Boost::Process)])],
     [use_external_signer=$enableval],
-    [use_external_signer=yes])
+    [use_external_signer=auto])
 
 AC_ARG_ENABLE([lto],
     [AS_HELP_STRING([--enable-lto],[build using LTO (default is no)])],
@@ -1415,7 +1415,21 @@ if test "$use_boost" = "yes"; then
 fi
 
 if test "$use_external_signer" != "no"; then
-  AC_DEFINE([ENABLE_EXTERNAL_SIGNER], [], [Define if external signer support is enabled])
+  case $host in
+    *mingw*)
+      dnl Boost Process uses Boost Filesystem when targeting Windows. Also,
+      dnl since Boost 1.71.0, Process does not work with mingw-w64 without
+      dnl workarounds. See 67669ab425b52a2b6be3d2f3b3b7e3939b676a2c.
+      if test "$use_external_signer" = "yes"; then
+        AC_MSG_ERROR([External signing is not supported on Windows])
+      fi
+      use_external_signer="no";
+    ;;
+    *)
+      use_external_signer="yes"
+      AC_DEFINE([ENABLE_EXTERNAL_SIGNER], [1], [Define if external signer support is enabled])
+    ;;
+  esac
 fi
 AM_CONDITIONAL([ENABLE_EXTERNAL_SIGNER], [test "$use_external_signer" = "yes"])
 
diff --git a/src/util/system.cpp b/src/util/system.cpp
index 8f35b7b6c6..e34cdc7fb9 100644
--- a/src/util/system.cpp
+++ b/src/util/system.cpp
@@ -6,11 +6,6 @@
 #include <util/system.h>
 
 #ifdef ENABLE_EXTERNAL_SIGNER
-#if defined(WIN32) && !defined(__kernel_entry)
-// A workaround for boost 1.71 incompatibility with mingw-w64 compiler.
-// For details see https://github.com/bitcoin/bitcoin/pull/22348.
-#define __kernel_entry
-#endif
 #include <boost/process.hpp>
 #endif // ENABLE_EXTERNAL_SIGNER