dgit: absurd: Work around mangled env metadata from gbp

Some versions of gbp pq will pass broken metadata from patch files
into the env vars for git-commit-tree.  Eg, #950446/#950326 where gbp
pq from buster cannot cope with binutils_2.33.90.20200122-2.dsc,
setting GIT_AUTHOR_DATE to "???" (literally, three ? marks, as copied
from the patch file libctf-soname.diff).

Work around this as follows:
 * Try git-commit-tree of an empty commit message.  Does it work?
   If so, workaround is not needed.
 * Try resetting GIT_AUTHOR_* to GIT_COMMITTER_*.  Does it work now?
   Ah, that is the fix!
 * Otherwise, just run git-commit-tree as usual.

This code makes some perhaps-unreasonable assumptions:
 * gbp's git commit-tree has the tree object as 2nd argument
   (really shonky command line parsing).  However, if this
   assumption is violated, the result is that the git-commit-tree
   attempts either fail harmlessly, or succeed harmlessly, defeating
   the workaround but with no other implications.
 * if a workaround is needed, it is best to throw away all the
   author information, rather that try to pick and choose, or to
   fix it up.
These are IMO tolerable because I think it's gbp's job to get this
right.  The output from dgit (if it succeeds) will be right in terms
of tree and even in terms of commit structure.

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

1 files changed, 26 insertions, 0 deletions

diff --git a/absurd/git b/absurd/git
index d8ac24e..d9e2d4e 100755
--- a/absurd/git
+++ b/absurd/git
@@ -29,10 +29,35 @@ filter_path () {
 	PATH=$npath
 }
 
+squash_author () {
+	author_vars='NAME EMAIL DATE'
+	for var in in $author_vars; do
+		eval "GIT_AUTHOR_$var=\$GIT_COMMITTER_$var"
+	done
+}
+
+try_commit () { git commit-tree "$try_commit_obj" </dev/null >/dev/null; }
+
+maybe_squash_author () {
+	if   (set -e; filter_path;                try_commit); then return; fi
+	if ! (set -e; filter_path; squash_author; try_commit); then return; fi
+	read last <../../absurd-apply-applied
+	echo >&4 \
+ "dgit: warning: suppressed corrupted metadata! ($last)"
+	squash_author
+}
+
 bypass=true
 for arg in "$@"; do
 	case "$arg" in
 	apply)	bypass=false; break	;;
+	commit-tree)
+		# Work around #950446 in gbp
+		echo >&3 "DGIT ABSURD GIT APPLY - COMMIT-TREE: $*"
+		try_commit_obj="$2"
+		maybe_squash_author
+		bypass=true; break
+		;;
 	-*)				;;
 	*)	bypass=true; break	;;
 	esac
@@ -118,4 +143,5 @@ git checkout debian/patches/series
 git add -Af .
 
 log "APPLIED $patch"
+printf >../../absurd-apply-applied '%s\n' "$patch"
 #printf 'APPLIED '; date --iso-8601=ns