Merge branch 'tb/pack-revindex-on-disk'

Introduce an on-disk file to record revindex for packdata, which
traditionally was always created on the fly and only in-core.

* tb/pack-revindex-on-disk:
  t5325: check both on-disk and in-memory reverse index
  pack-revindex: ensure that on-disk reverse indexes are given precedence
  t: support GIT_TEST_WRITE_REV_INDEX
  t: prepare for GIT_TEST_WRITE_REV_INDEX
  Documentation/config/pack.txt: advertise 'pack.writeReverseIndex'
  builtin/pack-objects.c: respect 'pack.writeReverseIndex'
  builtin/index-pack.c: write reverse indexes
  builtin/index-pack.c: allow stripping arbitrary extensions
  pack-write.c: prepare to write 'pack-*.rev' files
  packfile: prepare for the existence of '*.rev' files

3 files changed, 40 insertions, 5 deletions

diff --git a/Documentation/config/pack.txt b/Documentation/config/pack.txt
index 837f1b1679..3da4ea98e2 100644
--- a/Documentation/config/pack.txt
+++ b/Documentation/config/pack.txt
@@ -133,3 +133,10 @@ pack.writeBitmapHashCache::
 	between an older, bitmapped pack and objects that have been
 	pushed since the last gc). The downside is that it consumes 4
 	bytes per object of disk space. Defaults to true.
+
+pack.writeReverseIndex::
+	When true, git will write a corresponding .rev file (see:
+	link:../technical/pack-format.html[Documentation/technical/pack-format.txt])
+	for each new packfile that it writes in all places except for
+	linkgit:git-fast-import[1] and in the bulk checkin mechanism.
+	Defaults to false.
diff --git a/Documentation/git-index-pack.txt b/Documentation/git-index-pack.txt
index af0c26232c..69ba904d44 100644
--- a/Documentation/git-index-pack.txt
+++ b/Documentation/git-index-pack.txt
@@ -9,17 +9,18 @@ git-index-pack - Build pack index file for an existing packed archive
 SYNOPSIS
 --------
 [verse]
-'git index-pack' [-v] [-o <index-file>] <pack-file>
+'git index-pack' [-v] [-o <index-file>] [--[no-]rev-index] <pack-file>
 'git index-pack' --stdin [--fix-thin] [--keep] [-v] [-o <index-file>]
-                 [<pack-file>]
+		  [--[no-]rev-index] [<pack-file>]
 
 
 DESCRIPTION
 -----------
 Reads a packed archive (.pack) from the specified file, and
-builds a pack index file (.idx) for it.  The packed archive
-together with the pack index can then be placed in the
-objects/pack/ directory of a Git repository.
+builds a pack index file (.idx) for it. Optionally writes a
+reverse-index (.rev) for the specified pack. The packed
+archive together with the pack index can then be placed in
+the objects/pack/ directory of a Git repository.
 
 
 OPTIONS
@@ -35,6 +36,13 @@ OPTIONS
 	fails if the name of packed archive does not end
 	with .pack).
 
+--[no-]rev-index::
+	When this flag is provided, generate a reverse index
+	(a `.rev` file) corresponding to the given pack. If
+	`--verify` is given, ensure that the existing
+	reverse index is correct. Takes precedence over
+	`pack.writeReverseIndex`.
+
 --stdin::
 	When this flag is provided, the pack is read from stdin
 	instead and a copy is then written to <pack-file>. If
diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
index 96d2fc589f..8833b71c8b 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -274,6 +274,26 @@ Pack file entry: <+
 
     Index checksum of all of the above.
 
+== pack-*.rev files have the format:
+
+  - A 4-byte magic number '0x52494458' ('RIDX').
+
+  - A 4-byte version identifier (= 1).
+
+  - A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).
+
+  - A table of index positions (one per packed object, num_objects in
+    total, each a 4-byte unsigned integer in network order), sorted by
+    their corresponding offsets in the packfile.
+
+  - A trailer, containing a:
+
+    checksum of the corresponding packfile, and
+
+    a checksum of all of the above.
+
+All 4-byte numbers are in network order.
+
 == multi-pack-index (MIDX) files have the following format:
 
 The multi-pack-index files refer to multiple pack-files and loose objects.