Merge branch 'jk/write-broken-index-with-nul-sha1'

Earlier we started rejecting an attempt to add 0{40} object name to
the index and to tree objects, but it sometimes is necessary to
allow so to be able to use tools like filter-branch to correct such
broken tree objects.

* jk/write-broken-index-with-nul-sha1:
  write_index: optionally allow broken null sha1s

git-filter-branch.sh

@@ -283,11 +283,12 @@ while read commit parents; do
 
 	case "$filter_subdir" in
 	"")
-		git read-tree -i -m $commit
+		GIT_ALLOW_NULL_SHA1=1 git read-tree -i -m $commit
 		;;
 	*)
 		# The commit may not have the subdirectory at all
-		err=$(git read-tree -i -m $commit:"$filter_subdir" 2>&1) || {
+		err=$(GIT_ALLOW_NULL_SHA1=1 \
+		      git read-tree -i -m $commit:"$filter_subdir" 2>&1) || {
 			if ! git rev-parse -q --verify $commit:"$filter_subdir"
 			then
 				rm -f "$GIT_INDEX_FILE"

read-cache.c

@@ -1818,8 +1818,17 @@ int write_index(struct index_state *istate, int newfd)
 			continue;
 		if (!ce_uptodate(ce) && is_racy_timestamp(istate, ce))
 			ce_smudge_racily_clean_entry(ce);
-		if (is_null_sha1(ce->sha1))
+		if (is_null_sha1(ce->sha1)) {
-			return error("cache entry has null sha1: %s", ce->name);
+			static const char msg[] = "cache entry has null sha1: %s";
+			static int allow = -1;
+
+			if (allow < 0)
+				allow = git_env_bool("GIT_ALLOW_NULL_SHA1", 0);
+			if (allow)
+				warning(msg, ce->name);
+			else
+				return error(msg, ce->name);
+		}
 		if (ce_write_entry(&c, newfd, ce, previous_name) < 0)
 			return -1;
 	}

t/t7009-filter-branch-null-sha1.sh

@@ -0,0 +1,49 @@
+#!/bin/sh
+
+test_description='filter-branch removal of trees with null sha1'
+. ./test-lib.sh
+
+test_expect_success 'setup: base commits' '
+	test_commit one &&
+	test_commit two &&
+	test_commit three
+'
+
+test_expect_success 'setup: a commit with a bogus null sha1 in the tree' '
+	{
+		git ls-tree HEAD &&
+		printf "160000 commit $_z40\\tbroken\\n"
+	} >broken-tree
+	echo "add broken entry" >msg &&
+
+	tree=$(git mktree <broken-tree) &&
+	test_tick &&
+	commit=$(git commit-tree $tree -p HEAD <msg) &&
+	git update-ref HEAD "$commit"
+'
+
+# we have to make one more commit on top removing the broken
+# entry, since otherwise our index does not match HEAD (and filter-branch will
+# complain). We could make the index match HEAD, but doing so would involve
+# writing a null sha1 into the index.
+test_expect_success 'setup: bring HEAD and index in sync' '
+	test_tick &&
+	git commit -a -m "back to normal"
+'
+
+test_expect_success 'filter commands are still checked' '
+	test_must_fail git filter-branch \
+		--force --prune-empty \
+		--index-filter "git rm --cached --ignore-unmatch three.t"
+'
+
+test_expect_success 'removing the broken entry works' '
+	echo three >expect &&
+	git filter-branch \
+		--force --prune-empty \
+		--index-filter "git rm --cached --ignore-unmatch broken" &&
+	git log -1 --format=%s >actual &&
+	test_cmp expect actual
+'
+
+test_done