Allow by-host worktree-sharable configure caches

The Git configuration value ocaml.configure-cache can be used to specify
a directory to keep autoconf cache files in, relative to the worktree
root (so `git config ocaml.configure-cache .` enables the feature, and
`git config --global ocaml.configure-cache ..` enables it for all
worktrees, assuming they're at the same level).

autoconf's --cache-file option speeds up future runs of configure by
caching the results of previous tests. The cache is invalidated if any
environment variables differ (e.g. LDFLAGS) or if the build-host-target
triplet differs. This is a nuisance on Windows, where configure is both
very slow and it's also common to build with multiple different --host
values.

This PR allows a tree to be quickly reconfigured from one Windows port
to another.

.gitignore

@@ -45,6 +45,7 @@ _build
 /autom4te.cache
 /ocamlc
 /config.cache
+/ocaml-*.cache
 /config.log
 /config.status
 /libtool

Changes

@@ -266,7 +266,9 @@ Working version
   (David Allsopp, review by Nicolás Ojeda Bär, report by Sebastian Rasmussen)
 
 - #8995: allow developers to specify frequently-used configure options in
-  Git (ocaml.configure option) See HACKING.adoc for further details.
+  Git (ocaml.configure option) and a directory for host-specific, shareable
+  config.cache files (ocaml.configure-cache option). See HACKING.adoc for
+  further details.
   (David Allsopp, review by Gabriel Scherer)
 
 ### Compiler user-interface and warnings:

HACKING.adoc

@@ -249,6 +249,23 @@ debug runtime, since the enable flag appears after the disable flag. You can
 also use the full power of Git's `config` command and have options specific to
 particular clone or worktree.
 
+=== Speeding up configure
+
+`configure` includes the standard `-C` option which caches various test results
+in the file `config.cache` and can use those results to avoid running tests in
+subsequent invocations. This mechanism works fine, except that it is easy to
+clean the cache by mistake (e.g. with `git clean -dfX`). The cache is also
+host-specific which means the file has to be deleted if you run `configure` with
+a new `--host` value (this is quite common on Windows, where `configure` is
+also quite slow to run).
+
+You can elect to have host-specific cache files by issuing
+`git config --global ocaml.configure-cache .`. The `configure` script will now
+automatically create `ocaml-host.cache` (e.g. `ocaml-x86_64-pc-windows.cache`,
+or `ocaml-default.cache`). If you work with multiple worktrees, you can share
+these cache files by issuing `git config --global ocaml.configure-cache ..`. The
+directory is interpreted _relative_ to the `configure` script.
+
 === Bootstrapping
 
 The OCaml compiler is bootstrapped. This means that

configure

@@ -1,16 +1,57 @@
 #! /bin/sh
 
 if test -e '.git' ; then :
-  extra_args=$(git config ocaml.configure 2>/dev/null)
-  if test -n "$extra_args" ; then :
-    if test -z "$ac_read_git_config" ; then :
+  if test -z "$ac_read_git_config" ; then :
+    extra_args=$(git config ocaml.configure 2>/dev/null)
+    extended_cache=$(git config ocaml.configure-cache 2>/dev/null)
+    cache_file=
+
+    # If ocaml.configure-cache is set, parse the command-line for the --host
+    # option, in order to determine the name of the cache file.
+    if test -n "$extended_cache" ; then :
+      echo "Detected Git configuration option ocaml.configure-cache set to \
+\"$extended_cache\""
+      dashdash=
+      prev=
+      host=default
+      # The logic here is pretty borrowed from autoconf's
+      for option in $extra_args "$@"
+      do
+        if test -n "$prev" ; then :
+          host=$option
+          continue
+        fi
+
+        case $dashdash$option in
+          --)
+            dashdash=yes ;;
+          -host | --host | --hos | --ho)
+            prev=host ;;
+          -host=* | --host=* | --hos=* | --ho=*)
+            case $option in
+              *=?*) host=$(expr "X$option" : '[^=]*=\(.*\)') ;;
+              *=) host= ;;
+            esac ;;
+        esac
+      done
+      cache_file="`dirname "$0"`/$extended_cache/ocaml-$host.cache"
+    fi
+
+    # If either option has a value, re-invoke configure
+    if test -n "$extra_args$cache_file" ; then :
       echo "Detected Git configuration option ocaml.configure set to \
 \"$extra_args\""
       # Too much effort to get the echo to show appropriate quoting - the
       # invocation itself intentionally quotes $0 and passes $@ exactly as given
       # but allows a single expansion of ocaml.configure
-      echo "Re-running $0 $extra_args $@"
-      ac_read_git_config=true exec "$0" $extra_args "$@"
+      if test -n "$cache_file" ; then :
+        echo "Re-running $0 $extra_args --cache-file \"$cache_file\" $@"
+        ac_read_git_config=true exec "$0" $extra_args \
+                                          --cache-file "$cache_file" "$@"
+      else
+        echo "Re-running $0 $extra_args $@"
+        ac_read_git_config=true exec "$0" $extra_args "$@"
+      fi
     fi
   fi
 fi

tools/git-dev-options.sh

@@ -15,16 +15,57 @@
 
 # This script should have the same shebang as configure
 if test -e '.git' ; then :
-  extra_args=$(git config ocaml.configure 2>/dev/null)
-  if test -n "$extra_args" ; then :
-    if test -z "$ac_read_git_config" ; then :
+  if test -z "$ac_read_git_config" ; then :
+    extra_args=$(git config ocaml.configure 2>/dev/null)
+    extended_cache=$(git config ocaml.configure-cache 2>/dev/null)
+    cache_file=
+
+    # If ocaml.configure-cache is set, parse the command-line for the --host
+    # option, in order to determine the name of the cache file.
+    if test -n "$extended_cache" ; then :
+      echo "Detected Git configuration option ocaml.configure-cache set to \
+\"$extended_cache\""
+      dashdash=
+      prev=
+      host=default
+      # The logic here is pretty borrowed from autoconf's
+      for option in $extra_args "$@"
+      do
+        if test -n "$prev" ; then :
+          host=$option
+          continue
+        fi
+
+        case $dashdash$option in
+          --)
+            dashdash=yes ;;
+          -host | --host | --hos | --ho)
+            prev=host ;;
+          -host=* | --host=* | --hos=* | --ho=*)
+            case $option in
+              *=?*) host=$(expr "X$option" : '[^=]*=\(.*\)') ;;
+              *=) host= ;;
+            esac ;;
+        esac
+      done
+      cache_file="`dirname "$0"`/$extended_cache/ocaml-$host.cache"
+    fi
+
+    # If either option has a value, re-invoke configure
+    if test -n "$extra_args$cache_file" ; then :
       echo "Detected Git configuration option ocaml.configure set to \
 \"$extra_args\""
       # Too much effort to get the echo to show appropriate quoting - the
       # invocation itself intentionally quotes $0 and passes $@ exactly as given
       # but allows a single expansion of ocaml.configure
-      echo "Re-running $0 $extra_args $@"
-      ac_read_git_config=true exec "$0" $extra_args "$@"
+      if test -n "$cache_file" ; then :
+        echo "Re-running $0 $extra_args --cache-file \"$cache_file\" $@"
+        ac_read_git_config=true exec "$0" $extra_args \
+                                          --cache-file "$cache_file" "$@"
+      else
+        echo "Re-running $0 $extra_args $@"
+        ac_read_git_config=true exec "$0" $extra_args "$@"
+      fi
     fi
   fi
 fi