[PATCH] add a script to diff rendered documentation

[Jeff King <peff@peff.net>]

After making a change to the documentation, it's easy to
forget to check the rendered version to make sure it was
formatted as you intended. And simply doing a diff between
the two built versions is less trivial than you might hope:

  - diffing the roff or html output isn't particularly
    readable; what we really care about is what the end user
    will see

  - you have to tweak a few build variables to avoid
    spurious differences (e.g., version numbers, build
    times)

Let's provide a script that builds and installs the manpages
for two commits, renders the results using "man", and diffs
the result. Since this is time-consuming, we'll also do our
best to avoid repeated work, keeping intermediate results
between runs.

Some of this could probably be made a little less ugly if we
built support into Documentation/Makefile. But by relying
only on "make install-man" working, this script should work
for generating a diff between any two versions, whether they
include this script or not.

Signed-off-by: Jeff King <peff@peff.net>
---
I wrote this up for my own use after our discussion in [1]. I'm not sure
if it's too ugly for inclusion, or if it might be helpful to others.
I've only just written it, but my plan is to try to run it on anything I
submit to check the formatting. So it _seems_ useful and appears to
work, but only after a few minutes of playing with it. :)

[1] https://public-inbox.org/git/20180720223608.GE18502@genre.crustytoothpaste.net/

 Documentation/.gitignore |   1 +
 Documentation/doc-diff   | 100 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 101 insertions(+)
 create mode 100755 Documentation/doc-diff

diff --git a/Documentation/.gitignore b/Documentation/.gitignore
index c7096f11f1..3ef54e0adb 100644
--- a/Documentation/.gitignore
+++ b/Documentation/.gitignore
@@ -12,3 +12,4 @@ cmds-*.txt
 mergetools-*.txt
 manpage-base-url.xsl
 SubmittingPatches.txt
+tmp-doc-diff/
diff --git a/Documentation/doc-diff b/Documentation/doc-diff
new file mode 100755
index 0000000000..61deb2579e
--- /dev/null
+++ b/Documentation/doc-diff
@@ -0,0 +1,100 @@
+#!/bin/sh
+
+OPTIONS_SPEC="\
+doc-diff <from> <to> [-- diff options]
+--
+j	parallel argument to pass to make
+f	force rebuild; do not rely on cached results
+"
+SUBDIRECTORY_OK=1
+. "$(git --exec-path)/git-sh-setup"
+
+parallel=8
+force=
+while test $# -gt 0
+do
+	case "$1" in
+	-j)
+		parallel=${1#-j} ;;
+	-f)
+		force=t ;;
+	--)
+		shift; break ;;
+	*)
+		usage ;;
+	esac
+	shift
+done
+
+test $# -gt 1 || usage
+from=$1; shift
+to=$1; shift
+
+from_oid=$(git rev-parse --verify "$from") || exit 1
+to_oid=$(git rev-parse --verify "$to") || exit 1
+
+cd_to_toplevel
+tmp=Documentation/tmp-doc-diff
+
+if test -n "$force"
+then
+	rm -rf "$tmp"
+fi
+
+# We'll do both builds in a single worktree, which lets make reuse
+# results that don't differ between the two trees.
+if ! test -d "$tmp/worktree"
+then
+	git worktree add --detach "$tmp/worktree" "$from" &&
+	dots=$(echo "$tmp/worktree" | sed 's#[^/]*#..#g') &&
+	ln -s "$dots/config.mak" "$tmp/worktree/config.mak"
+fi
+
+# generate_render_makefile <srcdir> <dstdir>
+generate_render_makefile () {
+	find "$1" -type f |
+	while read src
+	do
+		dst=$2/${src#$1/}
+		printf 'all:: %s\n' "$dst"
+		printf '%s: %s\n' "$dst" "$src"
+		printf '\t@echo >&2 "  RENDER $(notdir $@)" && \\\n'
+		printf '\tmkdir -p $(dir $@) && \\\n'
+		printf '\tMANWIDTH=80 man -l $< >$@+ && \\\n'
+		printf '\tmv $@+ $@\n'
+	done
+}
+
+# render_tree <dirname> <committish>
+render_tree () {
+	# Skip install-man entirely if we already have an installed directory.
+	# We can't rely on make here, since "install-man" unconditionally
+	# copies the files (spending effort, but also updating timestamps that
+	# we then can't rely on during the render step). We use "mv" to make
+	# sure we don't get confused by a previous run that failed partway
+	# through.
+	if ! test -d "$tmp/installed/$1"
+	then
+		git -C "$tmp/worktree" checkout "$2" &&
+		make -j$parallel -C "$tmp/worktree" \
+			GIT_VERSION=omitted \
+			SOURCE_DATE_EPOCH=0 \
+			DESTDIR="$PWD/$tmp/installed/$1+" \
+			install-man &&
+		mv "$tmp/installed/$1+" "$tmp/installed/$1"
+	fi &&
+
+	# As with "installed" above, we skip the render if it's already been
+	# done.  So using make here is primarily just about running in
+	# parallel.
+	if ! test -d "$tmp/rendered/$1"
+	then
+		generate_render_makefile "$tmp/installed/$1" "$tmp/rendered/$1+" |
+		make -j$parallel -f - &&
+		mv "$tmp/rendered/$1+" "$tmp/rendered/$1"
+	fi
+}
+
+render_tree $from_oid "$from" &&
+render_tree $to_oid "$to" &&
+git -C $tmp/rendered diff --no-index "$@" $from_oid $to_oid
-- 
2.18.0.912.g3ccaa4d859