Fix rare segfault in sparse-index (#690)

An internal customer reported a segfault when running `git
sparse-checkout set` with the `index.sparse` config enabled. I was
unable to reproduce it locally, but with their help we debugged into the
failing process and discovered the following stacktrace:

```
#0  0x00007ff6318fb7b0 in rehash (map=0x3dfb00d0440, newsize=1048576) at hashmap.c:125
#1  0x00007ff6318fbc66 in hashmap_add (map=0x3dfb00d0440, entry=0x3dfb5c58bc8) at hashmap.c:247
#2  0x00007ff631937a70 in hash_index_entry (istate=0x3dfb00d0400, ce=0x3dfb5c58bc8) at name-hash.c:122
#3  0x00007ff631938a2f in add_name_hash (istate=0x3dfb00d0400, ce=0x3dfb5c58bc8) at name-hash.c:638
#4  0x00007ff631a064de in set_index_entry (istate=0x3dfb00d0400, nr=8291, ce=0x3dfb5c58bc8) at sparse-index.c:255
#5  0x00007ff631a06692 in add_path_to_index (oid=0x5ff130, base=0x5ff580, path=0x3dfb4b725da "<redacted>", mode=33188, context=0x5ff570)    at sparse-index.c:307
#6  0x00007ff631a3b48c in read_tree_at (r=0x7ff631c026a0 <the_repo>, tree=0x3dfb5b41f60, base=0x5ff580, depth=2, pathspec=0x5ff5a0,    fn=0x7ff631a064e5 <add_path_to_index>, context=0x5ff570) at tree.c:46
#7  0x00007ff631a3b60b in read_tree_at (r=0x7ff631c026a0 <the_repo>, tree=0x3dfb5b41e80, base=0x5ff580, depth=1, pathspec=0x5ff5a0,    fn=0x7ff631a064e5 <add_path_to_index>, context=0x5ff570) at tree.c:80
#8  0x00007ff631a3b60b in read_tree_at (r=0x7ff631c026a0 <the_repo>, tree=0x3dfb5b41ac8, base=0x5ff580, depth=0, pathspec=0x5ff5a0,    fn=0x7ff631a064e5 <add_path_to_index>, context=0x5ff570) at tree.c:80
#9  0x00007ff631a06a95 in expand_index (istate=0x3dfb00d0100, pl=0x0) at sparse-index.c:422
#10 0x00007ff631a06cbd in ensure_full_index (istate=0x3dfb00d0100) at sparse-index.c:456
#11 0x00007ff631990d08 in index_name_stage_pos (istate=0x3dfb00d0100, name=0x3dfb0020080 "algorithm/levenshtein", namelen=21, stage=0,    search_mode=EXPAND_SPARSE) at read-cache.c:556
#12 0x00007ff631990d6c in index_name_pos (istate=0x3dfb00d0100, name=0x3dfb0020080 "algorithm/levenshtein", namelen=21) at read-cache.c:566
#13 0x00007ff63180dbb5 in sanitize_paths (argc=185, argv=0x3dfb0030018, prefix=0x0, skip_checks=0) at builtin/sparse-checkout.c:756
#14 0x00007ff63180de50 in sparse_checkout_set (argc=185, argv=0x3dfb0030018, prefix=0x0) at builtin/sparse-checkout.c:860
#15 0x00007ff63180e6c5 in cmd_sparse_checkout (argc=186, argv=0x3dfb0030018, prefix=0x0) at builtin/sparse-checkout.c:1063
#16 0x00007ff6317234cb in run_builtin (p=0x7ff631ad9b38 <commands+2808>, argc=187, argv=0x3dfb0030018) at git.c:548
#17 0x00007ff6317239c0 in handle_builtin (argc=187, argv=0x3dfb0030018) at git.c:808
#18 0x00007ff631723c7d in run_argv (argcp=0x5ffdd0, argv=0x5ffd78) at git.c:877
#19 0x00007ff6317241d1 in cmd_main (argc=187, argv=0x3dfb0030018) at git.c:1017
#20 0x00007ff631838b60 in main (argc=190, argv=0x3dfb0030000) at common-main.c:64 
```

The very bottom of the stack being the `rehash()` method from
`hashmap.c` as called within the `name-hash` API made me look at where
these hashmaps were being used in the sparse index logic. These were
being copied across indexes, which seems dangerous. Indeed, clearing
these hashmaps and setting them as not initialized fixes the segfault.

The second commit is a response to a test failure that happens in
`t1092-sparse-checkout-compatibility.sh` where `git stash pop` starts to
fail because the underlying `git checkout-index` process fails due to
colliding files. Passing the `-f` flag appears to work, but it's unclear
why this name-hash change causes that change in behavior.

builtin/stash.c

@@ -401,7 +401,7 @@ static int restore_untracked(struct object_id *u_tree)
 
 	child_process_init(&cp);
 	cp.git_cmd = 1;
-	strvec_pushl(&cp.args, "checkout-index", "--all", NULL);
+	strvec_pushl(&cp.args, "checkout-index", "--all", "-f", NULL);
 	strvec_pushf(&cp.env, "GIT_INDEX_FILE=%s",
 		     stash_index_path.buf);
 

sparse-index.c

@@ -372,6 +372,10 @@ void expand_index(struct index_state *istate, struct pattern_list *pl)
 	full = xcalloc(1, sizeof(struct index_state));
 	memcpy(full, istate, sizeof(struct index_state));
 
+	full->name_hash_initialized = 0;
+	memset(&full->name_hash, 0, sizeof(full->name_hash));
+	memset(&full->dir_hash, 0, sizeof(full->dir_hash));
+
 	/*
 	 * This slightly-misnamed 'full' index might still be sparse if we
 	 * are only modifying the list of sparse directories. This hinges
@@ -430,9 +434,15 @@ void expand_index(struct index_state *istate, struct pattern_list *pl)
 	}
 
 	/* Copy back into original index. */
+	if (istate->name_hash_initialized) {
+		hashmap_clear(&istate->name_hash);
+		hashmap_clear(&istate->dir_hash);
+	}
+
 	istate->name_hash_initialized = full->name_hash_initialized;
 	memcpy(&istate->name_hash, &full->name_hash, sizeof(full->name_hash));
 	memcpy(&istate->dir_hash, &full->dir_hash, sizeof(full->dir_hash));
+
 	istate->sparse_index = pl ? INDEX_PARTIALLY_SPARSE : INDEX_EXPANDED;
 	free(istate->cache);
 	istate->cache = full->cache;