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@xxxxxxxxxx>
---
builtin/patch-id.c | 74 +++++++++++++++++++++++++++++++++---------
Documentation/git-patch-id.txt | 37 ++++++++++++++++++---
2 files changed, 91 insertions(+), 20 deletions(-)
diff --git a/builtin/patch-id.c b/builtin/patch-id.c
index 3cfe02d..77db873 100644
--- a/builtin/patch-id.c
+++ b/builtin/patch-id.c
@@ -1,17 +1,14 @@
#include "builtin.h"
-static void flush_current_id(int patchlen, unsigned char *id, git_SHA_CTX *c)
+static void flush_current_id(int patchlen, unsigned char *id, unsigned char *result)
{
- unsigned char result[20];
char name[50];
if (!patchlen)
return;
- git_SHA1_Final(result, c);
memcpy(name, sha1_to_hex(id), 41);
printf("%s %s\n", sha1_to_hex(result), name);
- git_SHA1_Init(c);
}
static int remove_space(char *line)
@@ -56,10 +53,31 @@ static int scan_hunk_header(const char *p, int *p_before, int *p_after)
return 1;
}
-static int get_one_patchid(unsigned char *next_sha1, git_SHA_CTX *ctx, struct strbuf *line_buf)
+static void flush_one_hunk(unsigned char *result, git_SHA_CTX *ctx)
+{
+ unsigned char hash[20];
+ unsigned short carry = 0;
+ int i;
+
+ git_SHA1_Final(hash, ctx);
+ git_SHA1_Init(ctx);
+ /* 20-byte sum, with carry */
+ for (i = 0; i < 20; ++i) {
+ carry += result[i] + hash[i];
+ result[i] = carry;
+ carry >>= 8;
+ }
+}
+
+static int get_one_patchid(unsigned char *next_sha1, unsigned char *result,
+ struct strbuf *line_buf, int stable)
{
int patchlen = 0, found_next = 0;
int before = -1, after = -1;
+ git_SHA_CTX ctx;
+
+ git_SHA1_Init(&ctx);
+ hashclr(result);
while (strbuf_getwholeline(line_buf, stdin, '\n') != EOF) {
char *line = line_buf->buf;
@@ -107,6 +125,8 @@ static int get_one_patchid(unsigned char *next_sha1, git_SHA_CTX *ctx, struct st
break;
/* Else we're parsing another header. */
+ if (stable)
+ flush_one_hunk(result, &ctx);
before = after = -1;
}
@@ -119,39 +139,63 @@ static int get_one_patchid(unsigned char *next_sha1, git_SHA_CTX *ctx, struct st
/* Compute the sha without whitespace */
len = remove_space(line);
patchlen += len;
- git_SHA1_Update(ctx, line, len);
+ git_SHA1_Update(&ctx, line, len);
}
if (!found_next)
hashclr(next_sha1);
+ flush_one_hunk(result, &ctx);
+
return patchlen;
}
-static void generate_id_list(void)
+static void generate_id_list(int stable)
{
- unsigned char sha1[20], n[20];
- git_SHA_CTX ctx;
+ unsigned char sha1[20], n[20], result[20];
int patchlen;
struct strbuf line_buf = STRBUF_INIT;
- git_SHA1_Init(&ctx);
hashclr(sha1);
while (!feof(stdin)) {
- patchlen = get_one_patchid(n, &ctx, &line_buf);
- flush_current_id(patchlen, sha1, &ctx);
+ patchlen = get_one_patchid(n, result, &line_buf, stable);
+ flush_current_id(patchlen, sha1, result);
hashcpy(sha1, n);
}
strbuf_release(&line_buf);
}
-static const char patch_id_usage[] = "git patch-id < patch";
+static const char patch_id_usage[] = "git patch-id [--stable | --unstable] < patch";
+
+static int git_patch_id_config(const char *var, const char *value, void *cb)
+{
+ int *stable = cb;
+
+ if (!strcmp(var, "patchid.stable")) {
+ *stable = git_config_bool(var, value);
+ return 0;
+ }
+
+ return git_default_config(var, value, cb);
+}
int cmd_patch_id(int argc, const char **argv, const char *prefix)
{
- if (argc != 1)
+ int stable = -1;
+
+ git_config(git_patch_id_config, &stable);
+
+ /* If nothing is set, default to unstable. */
+ if (stable < 0)
+ stable = 0;
+
+ if (argc == 2 && !strcmp(argv[1], "--stable"))
+ stable = 1;
+ else if (argc == 2 && !strcmp(argv[1], "--unstable"))
+ stable = 0;
+ else if (argc != 1)
usage(patch_id_usage);
- generate_id_list();
+ generate_id_list(stable);
return 0;
}
diff --git a/Documentation/git-patch-id.txt b/Documentation/git-patch-id.txt
index 312c3b1..fa562d3 100644
--- a/Documentation/git-patch-id.txt
+++ b/Documentation/git-patch-id.txt
@@ -8,14 +8,14 @@ git-patch-id - Compute unique ID for a patch
SYNOPSIS
--------
[verse]
-'git patch-id' < <patch>
+'git patch-id' [--stable | --unstable] < <patch>
DESCRIPTION
-----------
-A "patch ID" is nothing but a SHA-1 of the diff associated with a patch, with
-whitespace and line numbers ignored. As such, it's "reasonably stable", but at
-the same time also reasonably unique, i.e., two patches that have the same "patch
-ID" are almost guaranteed to be the same thing.
+A "patch ID" is nothing but a sum of SHA-1 of the file diffs associated with a
+patch, with whitespace and line numbers ignored. As such, it's "reasonably
+stable", but at the same time also reasonably unique, i.e., two patches that
+have the same "patch ID" are almost guaranteed to be the same thing.
IOW, you can use this thing to look for likely duplicate commits.
@@ -27,6 +27,33 @@ This can be used to make a mapping from patch ID to commit ID.
OPTIONS
-------
+
+--stable::
+ Use a "stable" sum of hashes as the patch ID. With this option:
+ - Reordering file diffs that make up a patch does not affect the ID.
+ In particular, two patches produced by comparing the same two trees
+ with two different settings for "-O<orderfile>" result in the same
+ patch ID signature, thereby allowing the computed result to be used
+ as a key to index some meta-information about the change between
+ the two trees;
+
+ - Result is different from the value produced by git 1.9 and older
+ or produced when an "unstable" hash (see --unstable below) is
+ configured - even when used on a diff output taken without any use
+ of "-O<orderfile>", thereby making existing databases storing such
+ "unstable" or historical patch-ids unusable.
+
+ This is the default if patchid.stable is set to true.
+
+--unstable::
+ Use an "unstable" hash as the patch ID. With this option,
+ the result produced is compatible with the patch-id value produced
+ by git 1.9 and older. Users with pre-existing databases storing
+ patch-ids produced by git 1.9 and older (who do not deal with reordered
+ patches) may want to use this option.
+
+ This is the default.
+
<patch>::
The diff to create the ID of.
--
MST
--
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