hash: create union for hash context allocation

In various parts of our code, we want to allocate a structure
representing the internal state of a hash algorithm.  The original
implementation of the hash algorithm abstraction assumed we would do
that using heap allocations, and added a context size element to struct
git_hash_algo.  However, most of the existing code uses stack
allocations and conversion would needlessly complicate various parts of
the code.  Add a union for the purpose of allocating hash contexts on
the stack and a typedef for ease of use.  Use this union for defining
the init, update, and final functions to avoid casts.  Remove the ctxsz
element for struct git_hash_algo, which is no longer very useful.

This does mean that stack allocations will grow slightly as additional
hash functions are added, but this should not be a significant problem,
since we don't allocate many hash contexts.  The improved usability and
benefits from avoiding dynamic allocation outweigh this small downside.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hash.h

@@ -55,9 +55,15 @@
 /* Number of algorithms supported (including unknown). */
 #define GIT_HASH_NALGOS (GIT_HASH_SHA1 + 1)
 
-typedef void (*git_hash_init_fn)(void *ctx);
-typedef void (*git_hash_update_fn)(void *ctx, const void *in, size_t len);
-typedef void (*git_hash_final_fn)(unsigned char *hash, void *ctx);
+/* A suitably aligned type for stack allocations of hash contexts. */
+union git_hash_ctx {
+	git_SHA_CTX sha1;
+};
+typedef union git_hash_ctx git_hash_ctx;
+
+typedef void (*git_hash_init_fn)(git_hash_ctx *ctx);
+typedef void (*git_hash_update_fn)(git_hash_ctx *ctx, const void *in, size_t len);
+typedef void (*git_hash_final_fn)(unsigned char *hash, git_hash_ctx *ctx);
 
 struct git_hash_algo {
 	/*
@@ -69,9 +75,6 @@ struct git_hash_algo {
 	/* A four-byte version identifier, used in pack indices. */
 	uint32_t format_id;
 
-	/* The size of a hash context (e.g. git_SHA_CTX). */
-	size_t ctxsz;
-
 	/* The length of the hash in binary. */
 	size_t rawsz;
 

sha1_file.c

@@ -39,32 +39,32 @@ const struct object_id empty_blob_oid = {
 	EMPTY_BLOB_SHA1_BIN_LITERAL
 };
 
-static void git_hash_sha1_init(void *ctx)
+static void git_hash_sha1_init(git_hash_ctx *ctx)
 {
-	git_SHA1_Init((git_SHA_CTX *)ctx);
+	git_SHA1_Init(&ctx->sha1);
 }
 
-static void git_hash_sha1_update(void *ctx, const void *data, size_t len)
+static void git_hash_sha1_update(git_hash_ctx *ctx, const void *data, size_t len)
 {
-	git_SHA1_Update((git_SHA_CTX *)ctx, data, len);
+	git_SHA1_Update(&ctx->sha1, data, len);
 }
 
-static void git_hash_sha1_final(unsigned char *hash, void *ctx)
+static void git_hash_sha1_final(unsigned char *hash, git_hash_ctx *ctx)
 {
-	git_SHA1_Final(hash, (git_SHA_CTX *)ctx);
+	git_SHA1_Final(hash, &ctx->sha1);
 }
 
-static void git_hash_unknown_init(void *ctx)
+static void git_hash_unknown_init(git_hash_ctx *ctx)
 {
 	die("trying to init unknown hash");
 }
 
-static void git_hash_unknown_update(void *ctx, const void *data, size_t len)
+static void git_hash_unknown_update(git_hash_ctx *ctx, const void *data, size_t len)
 {
 	die("trying to update unknown hash");
 }
 
-static void git_hash_unknown_final(unsigned char *hash, void *ctx)
+static void git_hash_unknown_final(unsigned char *hash, git_hash_ctx *ctx)
 {
 	die("trying to finalize unknown hash");
 }
@@ -75,7 +75,6 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
 		0x00000000,
 		0,
 		0,
-		0,
 		git_hash_unknown_init,
 		git_hash_unknown_update,
 		git_hash_unknown_final,
@@ -86,7 +85,6 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
 		"sha-1",
 		/* "sha1", big-endian */
 		0x73686131,
-		sizeof(git_SHA_CTX),
 		GIT_SHA1_RAWSZ,
 		GIT_SHA1_HEXSZ,
 		git_hash_sha1_init,