Import: Use absurd `git apply' emulation if gbp pq import fails

gbp import can fail due to git apply not understanding patches.
This is #841867 (against dgit).

The underlying problem is #841866 (in gbp pq) which exposes things
like #841865 and #829067 (in git).  I imagine there are other lurking
incompatibilities between git-apply and dpkg-source.

We could in principle reimplement the gbp patch metadata extraction.
But that would be quite tiresome and have its own compatibility
problems.

The real problem is just `git apply'.  (Indeed gbp already tries git
apply without, and then with, a whitespace fix option.)  We work
around the trouble by providing our own implementation of `git apply'.

Specifically:

We try to do things the sane way (just running gbp pq import) first.
If that works, great.  If it doesn't, we put /usr/share/dgit/absurd on
the PATH.  That contains only a sh script called `git'.  This sh
script figures out whether the caller is trying to invoke `git apply'.
If not, it runs the real git.

If the caller wanted git-apply, the absurd git script emulates it
using dpkg-source --before-build.  Conveniently, we know that the
series file will not be touched by patches.  So we can write just the
patch we care about into the series file, and run --before-build,
which applies just that one patch.

The results are committed (minus the .pc), and for the next patch,
dpkg-source sees again a tree with simply a single patch to apply.

We try ordinary gbp pq first because our absurd approach is very slow
on a big tree.  Also we would like to maximise our chances of the
import working.  If git and/or gbp ever work better by themselves, all
of this craziness will simply not happen.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

3 files changed, 109 insertions, 13 deletions

diff --git a/Makefile b/Makefile
index b72a1e9..9491e4d 100644
--- a/Makefile
+++ b/Makefile
@@ -37,6 +37,7 @@ MAN1PAGES=dgit.1
 MAN7PAGES=dgit.7 dgit-maint-merge.7
 TXTDOCS=README.dsc-import
 PERLMODULES=Debian/Dgit.pm
+ABSURDITIES=git
 
 INFRA_PROGRAMS=dgit-repos-server dgit-ssh-dispatch \
 	dgit-repos-policy-debian dgit-repos-admin-debian \
@@ -57,6 +58,8 @@ substituted/%:	%
 install:	installdirs all
 	$(INSTALL_PROGRAM) $(addprefix substituted/,$(PROGRAMS)) \
 		$(DESTDIR)$(bindir)
+	$(INSTALL_PROGRAM) $(addprefix absurd/,$(ABSURDITIES)) \
+		$(DESTDIR)$(absurddir)
 	$(INSTALL_DATA) $(MAN1PAGES) $(DESTDIR)$(man1dir)
 	$(INSTALL_DATA) $(MAN7PAGES) $(DESTDIR)$(man7dir)
 	$(INSTALL_DATA) $(TXTDOCS) $(DESTDIR)$(txtdocdir)
diff --git a/absurd/git b/absurd/git
new file mode 100755
index 0000000..039bbf0
--- /dev/null
+++ b/absurd/git
@@ -0,0 +1,70 @@
+#!/bin/sh
+set -e
+
+fail () {
+	echo >&2 "DGIT ABSURD GIT APPLY - FAILED: $*"
+	exit 127
+}
+
+self=${0%/*}
+npath=${PATH#$self:}
+if test "x$PATH" = "x$npath"; then
+	fail "PATH FILTER FAIL ($0 $self $PATH)"
+fi
+
+bypass=true
+for arg in "$@"; do
+	case "$arg" in
+	apply)	bypass=false; break	;;
+	-*)				;;
+	*)	bypass=true; break	;;
+	esac
+done
+
+if $bypass; then
+	PATH=$npath
+	exec git "$@"
+fi
+
+echo >&2 "DGIT ABSURD GIT APPLY - NO BYPASS: $*"
+
+#exec >/dev/tty 2>&1
+
+index=0
+noo=0
+
+for arg in "$@"; do
+	case "$noo.$arg" in
+	1.--index)
+		index=1
+		continue
+		;;
+	?.-*)
+		fail "UNKNOWN OPTION $arg ($*)"
+		;;
+	0.apply)
+		;;
+	1.*)	patch="$arg"
+		;;
+	*)
+		fail "BAD USAGE $arg ($noo $*)"
+	esac
+	noo=$(( $noo + 1 ))
+done
+
+if [ $noo != 2 ]; then
+	fail "NO PATCH ($*)"
+fi
+
+pwd=`pwd`
+patch=${patch#$pwd/debian/patches/}
+printf "%s\n" "$patch" >debian/patches/series
+
+dpkg-source --before-build .
+
+rm -rf .pc
+git checkout debian/patches/series
+git add -Af .
+
+echo >&2 "DGIT ABSURD GIT APPLY - APPLIED $patch"
+#printf 'APPLIED '; date --iso-8601=ns
diff --git a/dgit b/dgit
index 344d006..f821299 100755
--- a/dgit
+++ b/dgit
@@ -1954,25 +1954,48 @@ END
 	local $ENV{GIT_AUTHOR_EMAIL} = $authline[1];
 	local $ENV{GIT_AUTHOR_DATE} =  $authline[2];
 
-	eval {
-	    runcmd shell_cmd 'exec >/dev/null 2>../../gbp-pq-output',
-		gbp_pq, qw(import);
-	};
-	if ($@) {
-	    { local $@; eval { runcmd qw(cat ../../gbp-pq-output); }; }
-	    die $@;
-	}
+	my $path = $ENV{PATH} or die;
+
+	foreach my $use_absurd (qw(0 1)) {
+	    local $ENV{PATH} = $path;
+	    if ($use_absurd) {
+		chomp $@;
+		progress "warning: $@";
+		$path = "$absurdity:$path";
+		progress "$us: trying slow absurd-git-apply...";
+		rename "../../gbp-pq-output","../../gbp-pq-output.0"
+		    or die $!;
+	    }
+	    eval {
+		local $ENV{PATH} = $path if $use_absurd;
+
+		my @showcmd = (gbp_pq, qw(import));
+		my @realcmd = shell_cmd
+		    'exec >/dev/null 2>../../gbp-pq-output', @showcmd;
+		debugcmd "+",@realcmd;
+		if (system @realcmd) {
+		    die +(shellquote @showcmd).
+			" failed: ".
+			failedcmd_waitstatus()."\n";
+		}
 
-	my $gapplied = git_rev_parse('HEAD');
-	my $gappliedtree = cmdoutput @git, qw(rev-parse HEAD:);
-	$gappliedtree eq $dappliedtree or
-	    fail <<END;
+		my $gapplied = git_rev_parse('HEAD');
+		my $gappliedtree = cmdoutput @git, qw(rev-parse HEAD:);
+		$gappliedtree eq $dappliedtree or
+		    fail <<END;
 gbp-pq import and dpkg-source disagree!
  gbp-pq import gave commit $gapplied
  gbp-pq import gave tree $gappliedtree
  dpkg-source --before-build gave tree $dappliedtree
 END
-	$rawimport_hash = $gapplied;
+		$rawimport_hash = $gapplied;
+	    };
+	    last unless $@;
+	}
+	if ($@) {
+	    { local $@; eval { runcmd qw(cat ../../gbp-pq-output); }; }
+	    die $@;
+	}
     }
 
     progress "synthesised git commit from .dsc $cversion";