Since Linus added auto-sizing for abbreviations in
e6c587c733 ("abbrev: auto size the default abbreviation", 2016-09-30)
we've been less likely to produce a short SHA-1 today that'll collide
on the same repository tomorrow, since before we'd always pick the
bare minimum we could get away with.
But we still do a full disambiguation check, which can be very
expensive in some cases. There's a work-in-progress MIDX
implementation to address that[1].
This change adds an alternate method of achieving some of the same
ends (but possibly not all, see [2] and replies to the original thread
at [1]).
Now, as described in the docs the user can set a relative abbreviation
length via core.abbrev, e.g. on linux.git core.abbrev=+2 will produce
SHA-1s that are 14 characters (as opposed to the implicit 12).
This in combination with core.validateAbbrev=false (off by default)
allows for picking a trade-off between performance, and the odds that
future or remote (or current and local) short SHA-1 will be ambiguous.
1. https://public-inbox.org/git/20180107181459.222909-1-dstolee@xxxxxxxxxxxxx/
2. https://public-inbox.org/git/87lgbsz61p.fsf@xxxxxxxxxxxxxxxxxxx/
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>
---
Documentation/config.txt | 46 ++++++++++++++++++++++++++++++++++++++++
cache.h | 2 ++
config.c | 14 ++++++++++++
environment.c | 2 ++
sha1-name.c | 15 +++++++++++++
5 files changed, 79 insertions(+)
diff --git a/Documentation/config.txt b/Documentation/config.txt
index ab641bf5a9..8624110818 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -919,6 +919,52 @@ core.abbrev::
in your repository, which hopefully is enough for
abbreviated object names to stay unique for some time.
The minimum length is 4.
++
+This can also be set to relative values such as `+2` or `-2`, which
+means to add or subtract N characters from the SHA-1 that Git would
+otherwise print. This is useful in combination with the
+`core.validateAbbrev` setting, or to get more future-proof hashes to
+reference in the future in a repository whose number of objects is
+expected to grow.
+
+core.validateAbbrev::
+ If set to false (true by default) don't do any validation when
+ printing abbreviated object names to see if they're really
+ unique. This makes printing objects more performant at the
+ cost of potentially printing object names that aren't unique
+ within the current repository.
++
+When printing abbreviated object names Git needs to look through the
+local object store. This is an `O(log N)` operation assuming all the
+objects are in a single pack file, but `X * O(log N)` given `X` pack
+files, which can get expensive on some larger repositories.
++
+This setting changes that to `O(1)`, but with the trade-off that
+depending on the value of `core.abbrev` way may be printing
+abbreviated hashes that collide. Too see how likely this is, try
+running:
++
+-----------------------------------------------------------------------------------------------------------
+git log --all --pretty=format:%h --abbrev=4 | perl -nE 'chomp; say length' | sort | uniq -c | sort -nr
+-----------------------------------------------------------------------------------------------------------
++
+This shows how many commits were found at each abbreviation length. On
+linux.git in June 2018 this shows a bit more than 750,000 commits,
+with just 4 needing 11 characters to be fully abbreviated, and the
+default heuristic picks a length of 12.
++
+Even without `core.validateAbbrev=false` the results abbreviation
+already a bit of a probability game. They're guaranteed at the moment
+of generation, but as more objects are added, ambiguities may be
+introduced. Likewise, what's unambiguous for you may not be for
+somebody else you're communicating with, if they have their own clone.
++
+Therefore the default of `core.validateAbbrev=true` may not save you
+in practice if you're sharing the SHA-1 or noting it now to use after
+a `git fetch`. You may be better off setting `core.abbrev` to
+e.g. `+2` to add 2 extra characters to the SHA-1 in combination with
+`core.validateAbbrev=false` to get a reasonable trade-off between
+safety and performance.
add.ignoreErrors::
add.ignore-errors (deprecated)::
diff --git a/cache.h b/cache.h
index 89a107a7f7..6dc5af9482 100644
--- a/cache.h
+++ b/cache.h
@@ -772,6 +772,8 @@ extern int check_stat;
extern int quote_path_fully;
extern int has_symlinks;
extern int minimum_abbrev, default_abbrev;
+extern int default_abbrev_relative;
+extern int validate_abbrev;
extern int ignore_case;
extern int assume_unchanged;
extern int prefer_symlink_refs;
diff --git a/config.c b/config.c
index 12f762ad92..b6e0d17af1 100644
--- a/config.c
+++ b/config.c
@@ -1146,11 +1146,25 @@ static int git_default_core_config(const char *var, const char *value)
return 0;
}
+ if (!strcmp(var, "core.validateabbrev")) {
+ if (!value)
+ return config_error_nonbool(var);
+ validate_abbrev = git_config_bool(var, value);
+ return 0;
+ }
+
if (!strcmp(var, "core.abbrev")) {
if (!value)
return config_error_nonbool(var);
if (!strcasecmp(value, "auto")) {
default_abbrev = -1;
+ } else if (*value == '+' || *value == '-') {
+ int relative = git_config_int(var, value);
+ if (relative == 0)
+ die(_("bad core.abbrev value %s. "
+ "relative values must be non-zero"),
+ value);
+ default_abbrev_relative = relative;
} else {
int abbrev = git_config_int(var, value);
if (abbrev < minimum_abbrev || abbrev > 40)
diff --git a/environment.c b/environment.c
index 2a6de2330b..4a24d8126b 100644
--- a/environment.c
+++ b/environment.c
@@ -22,6 +22,8 @@ int trust_ctime = 1;
int check_stat = 1;
int has_symlinks = 1;
int minimum_abbrev = 4, default_abbrev = -1;
+int default_abbrev_relative = 0;
+int validate_abbrev = 1;
int ignore_case;
int assume_unchanged;
int prefer_symlink_refs;
diff --git a/sha1-name.c b/sha1-name.c
index 60d9ef3c7e..aa7ccea14d 100644
--- a/sha1-name.c
+++ b/sha1-name.c
@@ -576,6 +576,7 @@ int find_unique_abbrev_r(char *hex, const struct object_id *oid, int len)
struct disambiguate_state ds;
struct min_abbrev_data mad;
struct object_id oid_ret;
+ int dar = default_abbrev_relative;
if (len < 0) {
unsigned long count = approximate_object_count();
/*
@@ -602,6 +603,20 @@ int find_unique_abbrev_r(char *hex, const struct object_id *oid, int len)
if (len == GIT_SHA1_HEXSZ || !len)
return GIT_SHA1_HEXSZ;
+ if (dar) {
+ if (len + dar < MINIMUM_ABBREV) {
+ len = MINIMUM_ABBREV;
+ dar = 0;
+ }
+
+ if (validate_abbrev) {
+ len += dar;
+ } else {
+ hex[len + dar] = 0;
+ return len + dar;
+ }
+ }
+
mad.init_len = len;
mad.cur_len = len;
mad.hex = hex;
--
2.17.0.290.gded63e768a
