Merge branch 'tb/idx-midx-race-fix'

Processes that access packdata while the .idx file gets removed
(e.g. while repacking) did not fail or fall back gracefully as they
could.

* tb/idx-midx-race-fix:
  midx.c: protect against disappearing packs
  packfile.c: protect against disappearing indexes

t/t5319-multi-pack-index.sh

@@ -138,7 +138,7 @@ test_expect_success 'write midx with one v2 pack' '
 
 compare_results_with_midx "one v2 pack"
 
-test_expect_success 'corrupt idx not opened' '
+test_expect_success 'corrupt idx reports errors' '
 	idx=$(test-tool read-midx $objdir | grep "\.idx\$") &&
 	mv $objdir/pack/$idx backup-$idx &&
 	test_when_finished "mv backup-\$idx \$objdir/pack/\$idx" &&
@@ -149,7 +149,7 @@ test_expect_success 'corrupt idx not opened' '
 	test_copy_bytes 1064 <backup-$idx >$objdir/pack/$idx &&
 
 	git -c core.multiPackIndex=true rev-list --objects --all 2>err &&
-	test_must_be_empty err
+	grep "index unavailable" err
 '
 
 test_expect_success 'add more objects' '
@@ -755,4 +755,30 @@ test_expect_success 'repack --batch-size=<large> repacks everything' '
 	)
 '
 
+test_expect_success 'load reverse index when missing .idx, .pack' '
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+
+		git config core.multiPackIndex true &&
+
+		test_commit base &&
+		git repack -ad &&
+		git multi-pack-index write &&
+
+		git rev-parse HEAD >tip &&
+		pack=$(ls .git/objects/pack/pack-*.pack) &&
+		idx=$(ls .git/objects/pack/pack-*.idx) &&
+
+		mv $idx $idx.bak &&
+		git cat-file --batch-check="%(objectsize:disk)" <tip &&
+
+		mv $idx.bak $idx &&
+
+		mv $pack $pack.bak &&
+		git cat-file --batch-check="%(objectsize:disk)" <tip
+	)
+'
+
 test_done