Merge branch 'jt/cdn-offload'

The "fetch/clone" protocol has been updated to allow the server to
instruct the clients to grab pre-packaged packfile(s) in addition
to the packed object data coming over the wire.

* jt/cdn-offload:
  upload-pack: fix a sparse '0 as NULL pointer' warning
  upload-pack: send part of packfile response as uri
  fetch-pack: support more than one pack lockfile
  upload-pack: refactor reading of pack-objects out
  Documentation: add Packfile URIs design doc
  Documentation: order protocol v2 sections
  http-fetch: support fetching packfiles by URL
  http-fetch: refactor into function
  http: refactor finish_http_pack_request()
  http: use --stdin when indexing dumb HTTP pack

3 files changed, 124 insertions, 11 deletions

diff --git a/Documentation/git-http-fetch.txt b/Documentation/git-http-fetch.txt
index 666b042679..4deb4893f5 100644
--- a/Documentation/git-http-fetch.txt
+++ b/Documentation/git-http-fetch.txt
@@ -9,7 +9,7 @@ git-http-fetch - Download from a remote Git repository via HTTP
 SYNOPSIS
 --------
 [verse]
-'git http-fetch' [-c] [-t] [-a] [-d] [-v] [-w filename] [--recover] [--stdin] <commit> <url>
+'git http-fetch' [-c] [-t] [-a] [-d] [-v] [-w filename] [--recover] [--stdin | --packfile=<hash> | <commit>] <url>
 
 DESCRIPTION
 -----------
@@ -40,6 +40,13 @@ commit-id::
 
 		<commit-id>['\t'<filename-as-in--w>]
 
+--packfile=<hash>::
+	Instead of a commit id on the command line (which is not expected in
+	this case), 'git http-fetch' fetches the packfile directly at the given
+	URL and uses index-pack to generate corresponding .idx and .keep files.
+	The hash is used to determine the name of the temporary file and is
+	arbitrary. The output of index-pack is printed to stdout.
+
 --recover::
 	Verify that everything reachable from target is fetched.  Used after
 	an earlier fetch is interrupted.
diff --git a/Documentation/technical/packfile-uri.txt b/Documentation/technical/packfile-uri.txt
new file mode 100644
index 0000000000..318713abc3
--- /dev/null
+++ b/Documentation/technical/packfile-uri.txt
@@ -0,0 +1,78 @@
+Packfile URIs
+=============
+
+This feature allows servers to serve part of their packfile response as URIs.
+This allows server designs that improve scalability in bandwidth and CPU usage
+(for example, by serving some data through a CDN), and (in the future) provides
+some measure of resumability to clients.
+
+This feature is available only in protocol version 2.
+
+Protocol
+--------
+
+The server advertises the `packfile-uris` capability.
+
+If the client then communicates which protocols (HTTPS, etc.) it supports with
+a `packfile-uris` argument, the server MAY send a `packfile-uris` section
+directly before the `packfile` section (right after `wanted-refs` if it is
+sent) containing URIs of any of the given protocols. The URIs point to
+packfiles that use only features that the client has declared that it supports
+(e.g. ofs-delta and thin-pack). See protocol-v2.txt for the documentation of
+this section.
+
+Clients should then download and index all the given URIs (in addition to
+downloading and indexing the packfile given in the `packfile` section of the
+response) before performing the connectivity check.
+
+Server design
+-------------
+
+The server can be trivially made compatible with the proposed protocol by
+having it advertise `packfile-uris`, tolerating the client sending
+`packfile-uris`, and never sending any `packfile-uris` section. But we should
+include some sort of non-trivial implementation in the Minimum Viable Product,
+at least so that we can test the client.
+
+This is the implementation: a feature, marked experimental, that allows the
+server to be configured by one or more `uploadpack.blobPackfileUri=<sha1>
+<uri>` entries. Whenever the list of objects to be sent is assembled, all such
+blobs are excluded, replaced with URIs. The client will download those URIs,
+expecting them to each point to packfiles containing single blobs.
+
+Client design
+-------------
+
+The client has a config variable `fetch.uriprotocols` that determines which
+protocols the end user is willing to use. By default, this is empty.
+
+When the client downloads the given URIs, it should store them with "keep"
+files, just like it does with the packfile in the `packfile` section. These
+additional "keep" files can only be removed after the refs have been updated -
+just like the "keep" file for the packfile in the `packfile` section.
+
+The division of work (initial fetch + additional URIs) introduces convenient
+points for resumption of an interrupted clone - such resumption can be done
+after the Minimum Viable Product (see "Future work").
+
+Future work
+-----------
+
+The protocol design allows some evolution of the server and client without any
+need for protocol changes, so only a small-scoped design is included here to
+form the MVP. For example, the following can be done:
+
+ * On the server, more sophisticated means of excluding objects (e.g. by
+   specifying a commit to represent that commit and all objects that it
+   references).
+ * On the client, resumption of clone. If a clone is interrupted, information
+   could be recorded in the repository's config and a "clone-resume" command
+   can resume the clone in progress. (Resumption of subsequent fetches is more
+   difficult because that must deal with the user wanting to use the repository
+   even after the fetch was interrupted.)
+
+There are some possible features that will require a change in protocol:
+
+ * Additional HTTP headers (e.g. authentication)
+ * Byte range support
+ * Different file formats referenced by URIs (e.g. raw object)
diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index 3996d70891..5852f499a6 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -325,13 +325,26 @@ included in the client's request:
 	indicating its sideband (1, 2, or 3), and the server may send "0005\2"
 	(a PKT-LINE of sideband 2 with no payload) as a keepalive packet.
 
+If the 'packfile-uris' feature is advertised, the following argument
+can be included in the client's request as well as the potential
+addition of the 'packfile-uris' section in the server's response as
+explained below.
+
+    packfile-uris <comma-separated list of protocols>
+	Indicates to the server that the client is willing to receive
+	URIs of any of the given protocols in place of objects in the
+	sent packfile. Before performing the connectivity check, the
+	client should download from all given URIs. Currently, the
+	protocols supported are "http" and "https".
+
 The response of `fetch` is broken into a number of sections separated by
 delimiter packets (0001), with each section beginning with its section
-header.
+header. Most sections are sent only when the packfile is sent.
 
-    output = *section
-    section = (acknowledgments | shallow-info | wanted-refs | packfile)
-	      (flush-pkt | delim-pkt)
+    output = acknowledgements flush-pkt |
+	     [acknowledgments delim-pkt] [shallow-info delim-pkt]
+	     [wanted-refs delim-pkt] [packfile-uris delim-pkt]
+	     packfile flush-pkt
 
     acknowledgments = PKT-LINE("acknowledgments" LF)
 		      (nak | *ack)
@@ -349,13 +362,17 @@ header.
 		  *PKT-LINE(wanted-ref LF)
     wanted-ref = obj-id SP refname
 
+    packfile-uris = PKT-LINE("packfile-uris" LF) *packfile-uri
+    packfile-uri = PKT-LINE(40*(HEXDIGIT) SP *%x20-ff LF)
+
     packfile = PKT-LINE("packfile" LF)
 	       *PKT-LINE(%x01-03 *%x00-ff)
 
     acknowledgments section
-	* If the client determines that it is finished with negotiations
-	  by sending a "done" line, the acknowledgments sections MUST be
-	  omitted from the server's response.
+	* If the client determines that it is finished with negotiations by
+	  sending a "done" line (thus requiring the server to send a packfile),
+	  the acknowledgments sections MUST be omitted from the server's
+	  response.
 
 	* Always begins with the section header "acknowledgments"
 
@@ -406,9 +423,6 @@ header.
 	  which the client has not indicated was shallow as a part of
 	  its request.
 
-	* This section is only included if a packfile section is also
-	  included in the response.
-
     wanted-refs section
 	* This section is only included if the client has requested a
 	  ref using a 'want-ref' line and if a packfile section is also
@@ -422,6 +436,20 @@ header.
 	* The server MUST NOT send any refs which were not requested
 	  using 'want-ref' lines.
 
+    packfile-uris section
+	* This section is only included if the client sent
+	  'packfile-uris' and the server has at least one such URI to
+	  send.
+
+	* Always begins with the section header "packfile-uris".
+
+	* For each URI the server sends, it sends a hash of the pack's
+	  contents (as output by git index-pack) followed by the URI.
+
+	* The hashes are 40 hex characters long. When Git upgrades to a new
+	  hash algorithm, this might need to be updated. (It should match
+	  whatever index-pack outputs after "pack\t" or "keep\t".
+
     packfile section
 	* This section is only included if the client has sent 'want'
 	  lines in its request and either requested that no more