patch-id: make it stable against hunk reordering
Patch id changes if users reorder file diffs that make up a patch. As the result is functionally equivalent, a different patch id is surprising to many users. In particular, reordering files using diff -O is helpful to make patches more readable (e.g. API header diff before implementation diff). Add an option to change patch-id behaviour making it stable against these kinds of patch change: calculate SHA1 hash for each hunk separately and sum all hashes (using a symmetrical sum) to get patch id We use a 20byte sum and not xor - since xor would give 0 output for patches that have two identical diffs, which isn't all that unlikely (e.g. append the same line in two places). The new behaviour is enabled - when patchid.stable is true - when --stable flag is present Using a new flag --unstable or setting patchid.stable to false force the historical behaviour. In the documentation, clarify that patch ID can now be a sum of hashes, not a hash. Document how command line and config options affect the behaviour. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Michael S. Tsirkin committed
Apr 27, 2014 at 21:15 UTC
30e12b924b57b15e707f1749f2e5af15f1c7fe09
2 files changed
+91
-20
Documentation/git-patch-id.txt
+32
-5
@@ -8,14 +8,14 @@ git-patch-id - Compute unique ID for a patch
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SYNOPSIS
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--------
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[verse]
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-'git patch-id' < <patch>
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+'git patch-id' [--stable | --unstable] < <patch>
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DESCRIPTION
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-----------
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-A "patch ID" is nothing but a SHA-1 of the diff associated with a patch, with
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-whitespace and line numbers ignored. As such, it's "reasonably stable", but at
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-the same time also reasonably unique, i.e., two patches that have the same "patch
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-ID" are almost guaranteed to be the same thing.
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+A "patch ID" is nothing but a sum of SHA-1 of the file diffs associated with a
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+patch, with whitespace and line numbers ignored. As such, it's "reasonably
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+stable", but at the same time also reasonably unique, i.e., two patches that
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+have the same "patch ID" are almost guaranteed to be the same thing.
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IOW, you can use this thing to look for likely duplicate commits.
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@@ -27,6 +27,33 @@ This can be used to make a mapping from patch ID to commit ID.
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OPTIONS
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-------
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+
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+--stable::
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+ Use a "stable" sum of hashes as the patch ID. With this option:
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+ - Reordering file diffs that make up a patch does not affect the ID.
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+ In particular, two patches produced by comparing the same two trees
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+ with two different settings for "-O<orderfile>" result in the same
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+ patch ID signature, thereby allowing the computed result to be used
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+ as a key to index some meta-information about the change between
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+ the two trees;
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+
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+ - Result is different from the value produced by git 1.9 and older
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+ or produced when an "unstable" hash (see --unstable below) is
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+ configured - even when used on a diff output taken without any use
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+ of "-O<orderfile>", thereby making existing databases storing such
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+ "unstable" or historical patch-ids unusable.
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+
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+ This is the default if patchid.stable is set to true.
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+
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+--unstable::
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+ Use an "unstable" hash as the patch ID. With this option,
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+ the result produced is compatible with the patch-id value produced
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+ by git 1.9 and older. Users with pre-existing databases storing
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+ patch-ids produced by git 1.9 and older (who do not deal with reordered
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+ patches) may want to use this option.
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+
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+ This is the default.
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+
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<patch>::
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The diff to create the ID of.
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builtin/patch-id.c
+59
-15
@@ -1,17 +1,14 @@
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#include "builtin.h"
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-static void flush_current_id(int patchlen, unsigned char *id, git_SHA_CTX *c)
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+static void flush_current_id(int patchlen, unsigned char *id, unsigned char *result)
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{
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- unsigned char result[20];
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char name[50];
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if (!patchlen)
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return;
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- git_SHA1_Final(result, c);
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memcpy(name, sha1_to_hex(id), 41);
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printf("%s %s\n", sha1_to_hex(result), name);
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- git_SHA1_Init(c);
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}
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static int remove_space(char *line)
@@ -56,10 +53,31 @@ static int scan_hunk_header(const char *p, int *p_before, int *p_after)
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return 1;
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}
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-static int get_one_patchid(unsigned char *next_sha1, git_SHA_CTX *ctx, struct strbuf *line_buf)
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+static void flush_one_hunk(unsigned char *result, git_SHA_CTX *ctx)
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+{
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+ unsigned char hash[20];
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+ unsigned short carry = 0;
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+ int i;
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+
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+ git_SHA1_Final(hash, ctx);
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+ git_SHA1_Init(ctx);
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+ /* 20-byte sum, with carry */
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+ for (i = 0; i < 20; ++i) {
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+ carry += result[i] + hash[i];
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+ result[i] = carry;
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+ carry >>= 8;
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+ }
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+}
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+
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+static int get_one_patchid(unsigned char *next_sha1, unsigned char *result,
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+ struct strbuf *line_buf, int stable)
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{
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int patchlen = 0, found_next = 0;
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int before = -1, after = -1;
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+ git_SHA_CTX ctx;
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+
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+ git_SHA1_Init(&ctx);
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+ hashclr(result);
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while (strbuf_getwholeline(line_buf, stdin, '\n') != EOF) {
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char *line = line_buf->buf;
@@ -107,6 +125,8 @@ static int get_one_patchid(unsigned char *next_sha1, git_SHA_CTX *ctx, struct st
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break;
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/* Else we're parsing another header. */
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+ if (stable)
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+ flush_one_hunk(result, &ctx);
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before = after = -1;
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}
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@@ -119,39 +139,63 @@ static int get_one_patchid(unsigned char *next_sha1, git_SHA_CTX *ctx, struct st
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/* Compute the sha without whitespace */
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len = remove_space(line);
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patchlen += len;
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- git_SHA1_Update(ctx, line, len);
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+ git_SHA1_Update(&ctx, line, len);
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}
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if (!found_next)
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hashclr(next_sha1);
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+ flush_one_hunk(result, &ctx);
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+
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return patchlen;
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}
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-static void generate_id_list(void)
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+static void generate_id_list(int stable)
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{
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- unsigned char sha1[20], n[20];
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- git_SHA_CTX ctx;
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+ unsigned char sha1[20], n[20], result[20];
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int patchlen;
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struct strbuf line_buf = STRBUF_INIT;
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- git_SHA1_Init(&ctx);
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hashclr(sha1);
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while (!feof(stdin)) {
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- patchlen = get_one_patchid(n, &ctx, &line_buf);
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- flush_current_id(patchlen, sha1, &ctx);
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+ patchlen = get_one_patchid(n, result, &line_buf, stable);
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+ flush_current_id(patchlen, sha1, result);
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hashcpy(sha1, n);
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}
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strbuf_release(&line_buf);
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}
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-static const char patch_id_usage[] = "git patch-id < patch";
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+static const char patch_id_usage[] = "git patch-id [--stable | --unstable] < patch";
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+
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+static int git_patch_id_config(const char *var, const char *value, void *cb)
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+{
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+ int *stable = cb;
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+
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+ if (!strcmp(var, "patchid.stable")) {
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+ *stable = git_config_bool(var, value);
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+ return 0;
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+ }
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+
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+ return git_default_config(var, value, cb);
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+}
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int cmd_patch_id(int argc, const char **argv, const char *prefix)
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{
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- if (argc != 1)
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+ int stable = -1;
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+
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+ git_config(git_patch_id_config, &stable);
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+
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+ /* If nothing is set, default to unstable. */
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+ if (stable < 0)
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+ stable = 0;
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+
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+ if (argc == 2 && !strcmp(argv[1], "--stable"))
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+ stable = 1;
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+ else if (argc == 2 && !strcmp(argv[1], "--unstable"))
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+ stable = 0;
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+ else if (argc != 1)
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usage(patch_id_usage);
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- generate_id_list();
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+ generate_id_list(stable);
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return 0;
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}