Re: [PATCH 3/5] match-trees: use hashcpy to splice trees

From: Jeff King <peff@peff.net>
To: "brian m. carlson" <sandals@crustytoothpaste.net>,
	git@vger.kernel.org, "Martin Ågren" <martin.agren@gmail.com>,
	"Junio C Hamano" <gitster@pobox.com>
Subject: Re: [PATCH 3/5] match-trees: use hashcpy to splice trees
Date: Fri, 11 Jan 2019 09:54:46 -0500	[thread overview]
Message-ID: <20190111145446.GA31518@sigill.intra.peff.net> (raw)
In-Reply-To: <20190111145106.GB16754@sigill.intra.peff.net>

On Fri, Jan 11, 2019 at 09:51:06AM -0500, Jeff King wrote:

> I think rewrite_here needs to be a direct pointer into the buffer, since
> we plan to modify it.
> 
> I think rewrite_with is correct to be an object_id. It's either the oid
> passed in from the caller, or the subtree we generated (in which case
> it's the result from write_object_file).
> 
> So the "most correct" thing is probably something like this:

Of course, it would be nice if what I sent compiled. ;)

rewrite_here does double duty: it's the pointer in the tree that we need
to rewrite, and it's also the oid we pass down recursively. So we have
to handle both cases, like so:

diff --git a/match-trees.c b/match-trees.c
index 03e81b56e1..3e0ed889b4 100644
--- a/match-trees.c
+++ b/match-trees.c
@@ -179,7 +179,7 @@ static int splice_tree(const struct object_id *oid1, const char *prefix,
 	char *buf;
 	unsigned long sz;
 	struct tree_desc desc;
-	struct object_id *rewrite_here;
+	unsigned char *rewrite_here;
 	const struct object_id *rewrite_with;
 	struct object_id subtree;
 	enum object_type type;
@@ -206,9 +206,16 @@ static int splice_tree(const struct object_id *oid1, const char *prefix,
 			if (!S_ISDIR(mode))
 				die("entry %s in tree %s is not a tree", name,
 				    oid_to_hex(oid1));
-			rewrite_here = (struct object_id *)(desc.entry.path +
-							    strlen(desc.entry.path) +
-							    1);
+			/*
+			 * We cast here for two reasons:
+			 *
+			 *   - to flip the "char *" (for the path) to "unsigned
+			 *     char *" (for the hash stored after it)
+			 *
+			 *   - to discard the "const"; this is OK because we
+			 *     know it points into our non-const "buf"
+			 */
+			rewrite_here = (unsigned char *)desc.entry.path + strlen(desc.entry.path) + 1;
 			break;
 		}
 		update_tree_entry(&desc);
@@ -217,14 +224,16 @@ static int splice_tree(const struct object_id *oid1, const char *prefix,
 		die("entry %.*s not found in tree %s", toplen, prefix,
 		    oid_to_hex(oid1));
 	if (*subpath) {
-		status = splice_tree(rewrite_here, subpath, oid2, &subtree);
+		struct object_id tree_oid;
+		hashcpy(tree_oid.hash, rewrite_here);
+		status = splice_tree(&tree_oid, subpath, oid2, &subtree);
 		if (status)
 			return status;
 		rewrite_with = &subtree;
 	} else {
 		rewrite_with = oid2;
 	}
-	hashcpy(rewrite_here->hash, rewrite_with->hash);
+	hashcpy(rewrite_here, rewrite_with->hash);
 	status = write_object_file(buf, sz, tree_type, result);
 	free(buf);
 	return status;
