Updated Windows build doc for WSL/Xenial workarounds.

doc/build-windows.md

@@ -4,11 +4,14 @@ WINDOWS BUILD NOTES
 Below are some notes on how to build Bitcoin Core for Windows.
 
 Most developers use cross-compilation from Ubuntu to build executables for
-Windows. Cross-compilation is also used to build the release binaries.
+Windows. This is also used to build the release binaries.
 
-Currently only building on Ubuntu Trusty 14.04 or Ubuntu Zesty 17.04 or later is supported.
+Building on Ubuntu Trusty 14.04 is recommended.
-Building on Ubuntu Xenial 16.04 is known to be broken, see extensive discussion in issue [8732](https://github.com/bitcoin/bitcoin/issues/8732).
+At the time of writing the Windows Subsystem for Linux installs Ubuntu Xenial 16.04. The default cross
-While it may be possible to do so with work arounds, it's potentially dangerous and not recommended.
+compiler package for Ubuntu Xenial does not produce working executables for some of the bitcoin applications.
+It is possible to build on Ubuntu Xenial by installing the cross compiler packages from
+Ubuntu Zesty, see the steps below.
+Building on Ubuntu Zesty 17.04 up to 17.10 has been verified to work.
 
 While there are potentially a number of ways to build on Windows (for example using msys / mingw-w64),
 using the Windows Subsystem For Linux is the most straightforward. If you are building with
@@ -19,7 +22,7 @@ Compiling with Windows Subsystem For Linux
 -------------------------------------------
 
 With Windows 10, Microsoft has released a new feature named the [Windows
-Subsystem for Linux](https://msdn.microsoft.com/commandline/wsl/about). This
+Subsystem for Linux (WSL)](https://msdn.microsoft.com/commandline/wsl/about). This
 feature allows you to run a bash shell directly on Windows in an Ubuntu-based
 environment. Within this environment you can cross compile for Windows without
 the need for a separate Linux VM or server.
@@ -57,7 +60,7 @@ installing the toolchain will be different.
 
 First, install the general dependencies:
 
-    sudo apt-get install build-essential libtool autotools-dev automake pkg-config bsdmainutils curl
+    sudo apt install build-essential libtool autotools-dev automake pkg-config bsdmainutils curl git
 
 A host toolchain (`build-essential`) is necessary because some dependency
 packages (such as `protobuf`) need to build host utilities that are used in the
@@ -65,20 +68,32 @@ build process.
 
 See also: [dependencies.md](dependencies.md).
 
-If you're building on Ubuntu 17.04 or later, run these two commands, selecting the 'posix' variant for both,
-to work around issues with mingw-w64. See issue [8732](https://github.com/bitcoin/bitcoin/issues/8732) for more information.
-```
-sudo update-alternatives --config x86_64-w64-mingw32-g++
-sudo update-alternatives --config x86_64-w64-mingw32-gcc
-```
-
 ## Building for 64-bit Windows
 
-To build executables for Windows 64-bit, install the following dependencies:
+The first step is to install the mingw-w64 cross-compilation tool chain. Due to different Ubuntu
+packages for each distribution and problems with the Xenial packages the steps for each are different.
 
-    sudo apt-get install g++-mingw-w64-x86-64 mingw-w64-x86-64-dev
+Common steps to install mingw32 cross compiler tool chain:
 
-Then build using:
+    sudo apt install g++-mingw-w64-x86-64
+
+Ubuntu Trusty 14.04:
+
+    No further steps required
+
+Ubuntu Xenial 16.04 and Windows Subsystem for Linux <sup>[1](#footnote1),[2](#footnote2)</sup>:
+
+    sudo apt install software-properties-common
+    sudo add-apt-repository "deb http://archive.ubuntu.com/ubuntu zesty universe"
+    sudo apt update
+    sudo apt upgrade
+    sudo update-alternatives --config x86_64-w64-mingw32-g++ # Set the default mingw32 g++ compiler option to posix.
+
+Ubuntu Zesty 17.04 <sup>[2](#footnote2)</sup>:
+
+    sudo update-alternatives --config x86_64-w64-mingw32-g++ # Set the default mingw32 g++ compiler option to posix.
+
+Once the tool chain is installed the build steps are common:
 
     PATH=$(echo "$PATH" | sed -e 's/:\/mnt.*//g') # strip out problematic Windows %PATH% imported var
     cd depends
@@ -92,7 +107,11 @@ Then build using:
 
 To build executables for Windows 32-bit, install the following dependencies:
 
-    sudo apt-get install g++-mingw-w64-i686 mingw-w64-i686-dev
+    sudo apt install g++-mingw-w64-i686 mingw-w64-i686-dev
+
+For Ubuntu Xenial 16.04, Ubuntu Zesty 17.04 and Windows Subsystem for Linux <sup>[2](#footnote2)</sup>:
+
+    sudo update-alternatives --config i686-w64-mingw32-g++  # Set the default mingw32 g++ compiler option to posix.
 
 Then build using:
 
@@ -117,3 +136,20 @@ as they appear in the release `.zip` archive. This can be done in the following
 way. This will install to `c:\workspace\bitcoin`, for example:
 
     make install DESTDIR=/mnt/c/workspace/bitcoin
+
+Footnotes
+---------
+
+<a name="footnote1">1</a>: There is currently a bug in the 64 bit mingw-w64 cross compiler packaged for WSL/Ubuntu Xenial 16.04 that
+causes two of the bitcoin executables to crash shortly after start up. The bug is related to the
+-fstack-protector-all g++ compiler flag which is used to mitigate buffer overflows.
+Installing the mingw-w64 packages from the Ubuntu 17 distribution solves the issue, however, this is not
+an officially supported approach and it's only recommended if you are prepared to reinstall WSL/Ubutntu should
+something break.
+
+<a name="footnote2">2</a>: Starting from Ubuntu Xenial 16.04 both the 32 and 64 bit mingw-w64 packages install two different
+compiler options to allow a choice between either posix or win32 threads. The default option is win32 threads which is the more
+efficient since it will result in binary code that links directly with the Windows kernel32.lib. Unfortunately, the headers
+required to support win32 threads conflict with some of the classes in the C++11 standard library in particular std::mutex.
+It's not possible to build the bitcoin code using the win32 version of the mingw-w64 cross compilers (at least not without
+modifying headers in the bitcoin source code).