path: root/Documentation
diff options
authorMichael S. Tsirkin <>2014-04-27 18:15:44 (GMT)
committerJunio C Hamano <>2014-06-10 20:09:24 (GMT)
commit30e12b924b57b15e707f1749f2e5af15f1c7fe09 (patch)
treeb1b51fe49bb067dd3d50c1ebd4dc26a3272283ab /Documentation
parentbb98b01ee8c5c6f66a6d70aa453b70184dd3d96f (diff)
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 <> Signed-off-by: Junio C Hamano <>
Diffstat (limited to 'Documentation')
1 files changed, 32 insertions, 5 deletions
diff --git a/Documentation/git-patch-id.txt b/Documentation/git-patch-id.txt
index 312c3b1..31efc58 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
-'git patch-id' < <patch>
+'git patch-id' [--stable | --unstable] < <patch>
-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.
+ 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.
+ 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.
The diff to create the ID of.