Make configure error out on missing ghc-tarballs on Windows

Currently checking out the source on windows requires two git
checkouts. One for the GHC sources and one for the GHC-tarballs.

This patch will make configure issue an error if compiling under
windows and the GHC-tarballs folder is missing.

On failure the user is told which command they need to run to get the
tarballs or if they want configure to handle it for them configure
provide the `--enable-tarballs-autodownload` flag.

Test Plan:
1. make sure ghc-tarballs folder is not present
2. run ./configure which should fail giving an error that tarballs is
missing and how to get it
3. run ./configure --enable-tarballs-autodownload and the tarballs
should be downloaded and configure finishes
4. rerun the command in 3, no new download should be done.
5. run configure without --enable-tarballs-autodownload, configure
should finish correctly.

Reviewers: bgamari, austin, thomie

Reviewed By: thomie

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1108

GHC Trac Issues: #10705

4 files changed, 56 insertions, 8 deletions

diff --git a/HACKING.md b/HACKING.md
index b59e747700..6ed39eaf1c 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -33,11 +33,11 @@ Next, clone the repository and all the associated libraries:
 $ git clone --recursive git://git.haskell.org/ghc.git
 ```
 
-On Windows, you need an extra repository containing some build tools:
+On Windows, you need an extra repository containing some build tools.
+These can be downloaded for you by configure. This only needs to be done once by running:
 
 ```
-$ cd ghc/
-$ git clone git://git.haskell.org/ghc-tarballs.git
+$ ./configure --enable-tarballs-autodownload
 ```
 
 First copy `mk/build.mk.sample` to `mk/build.mk` and ensure it has
diff --git a/INSTALL.md b/INSTALL.md
index 1db2595736..58930afeb8 100644
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -23,6 +23,11 @@ Quick start:  the following gives you a default build:
     $ make
     $ make install
 
+  On Windows, you need an extra repository containing some build tools.
+  These can be downloaded for you by configure. This only needs to be done once by running:
+
+    $ ./configure --enable-tarballs-autodownload
+
 The "perl boot" step is only necessary if this is a tree checked out
 from git.  For source distributions downloaded from GHC's web site,
 this step has already been performed.
diff --git a/README.md b/README.md
index 5ad1adb6d0..025140c772 100644
--- a/README.md
+++ b/README.md
@@ -28,11 +28,6 @@ There are two ways to get a source tree:
 
         $ git clone --recursive git://git.haskell.org/ghc.git
 
-  On Windows, you need an extra repository containing some build tools:
-
-        $ cd ghc/
-        $ git clone git://git.haskell.org/ghc-tarballs.git
-
   Note: cloning GHC from Github requires a special setup. See [Getting a GHC
   repository from Github] [7].
 
@@ -66,6 +61,11 @@ dblatex.
     $ make         # can also say 'make -jX' for X number of jobs
     $ make install
 
+  On Windows, you need an extra repository containing some build tools.
+  These can be downloaded for you by configure. This only needs to be done once by running:
+
+    $ ./configure --enable-tarballs-autodownload
+
 (NB: **Do you have multiple cores? Be sure to tell that to `make`!** This can
 save you hours of build time depending on your system configuration, and is
 almost always a win regardless of how many cores you have. As a simple rule,
diff --git a/configure.ac b/configure.ac
index d896c8bf48..d60acb7833 100644
--- a/configure.ac
+++ b/configure.ac
@@ -101,6 +101,12 @@ AC_ARG_ENABLE(bootstrap-with-devel-snapshot,
         EnableBootstrapWithDevelSnaphost=NO
 )
 
+AC_ARG_ENABLE(tarballs-autodownload,
+[AC_HELP_STRING([--enable-tarballs-autodownload],
+                [Automatically download Windows distribution binaries if needed.])],
+        TarballsAutodownload=YES,
+        TarballsAutodownload=NO
+)
 if test "$WithGhc" != ""; then
   FPTOOLS_GHC_VERSION([GhcVersion], [GhcMajVersion], [GhcMinVersion], [GhcPatchLevel])dnl
 
@@ -274,12 +280,49 @@ AC_SUBST(WithHc)
 FP_INTREE_GHC_PWD
 FP_FIND_ROOT
 
+function fail() {
+    echo >&2
+    echo "$1" >&2
+    exit 1
+}
+
+function set_up_tarballs() {
+    local tarball_repo_url="$1"
+    local tarball_dir="$2"
+
+    if ! test -d "${tarball_dir}"
+    then
+        local git_cmd="git clone ${tarball_repo_url} ${tarball_dir}"
+        if test "$TarballsAutodownload" = "NO"
+        then
+            echo >&2
+            echo "ERROR: Windows tarball binary distributions not found." >&2
+            echo "Please rerun configure with --enable-tarballs-autodownload, or clone the repository manually:" >&2
+            echo "  $git_cmd" >&2
+            exit 1
+        fi
+        AC_MSG_NOTICE([Downloading Windows tarball distributions to ${tarball_dir}...])
+        $git_cmd || {
+            rm -f "${tarball_dir}"
+            fail "ERROR: Git clone failed."
+        }
+    else
+        AC_MSG_NOTICE([Using Windows tarball distributions found in ${tarball_dir}...])
+    fi
+}
+
 if test "$HostOS" = "mingw32"
 then
     test -d inplace || mkdir inplace
 
+    # NB. For now just run git clone on the tarball repo
+    ghc_tarball_repo='git://git.haskell.org/ghc-tarballs.git'
+    ghc_tarball_dir='ghc-tarballs'
+    set_up_tarballs "${ghc_tarball_repo}" "${ghc_tarball_dir}"
+
     if test "$HostArch" = "i386"
     then
+
         # NB. If you update the tarballs to a new version of gcc, don't
         # forget to tweak the paths in driver/gcc/gcc.c.
         if ! test -d inplace/mingw ||