Merge branch 'jk/http-server-protocol-versions'

Taking advantage of the CGI interface, http-backend has been
updated to enable protocol v2 automatically when the other side
asks for it.

* jk/http-server-protocol-versions:
  docs/protocol-v2: point readers transport config discussion
  docs/git: discuss server-side config for GIT_PROTOCOL
  docs/http-backend: mention v2 protocol
  http-backend: handle HTTP_GIT_PROTOCOL CGI variable
  t5551: test v2-to-v0 http protocol fallback

7 files changed, 73 insertions, 4 deletions

diff --git a/Documentation/git-http-backend.txt b/Documentation/git-http-backend.txt
index 558966aa83..0c5c0dde19 100644
--- a/Documentation/git-http-backend.txt
+++ b/Documentation/git-http-backend.txt
@@ -16,7 +16,9 @@ A simple CGI program to serve the contents of a Git repository to Git
 clients accessing the repository over http:// and https:// protocols.
 The program supports clients fetching using both the smart HTTP protocol
 and the backwards-compatible dumb HTTP protocol, as well as clients
-pushing using the smart HTTP protocol.
+pushing using the smart HTTP protocol. It also supports Git's
+more-efficient "v2" protocol if properly configured; see the
+discussion of `GIT_PROTOCOL` in the ENVIRONMENT section below.
 
 It verifies that the directory has the magic file
 "git-daemon-export-ok", and it will refuse to export any Git directory
@@ -77,6 +79,18 @@ Apache 2.x::
 SetEnv GIT_PROJECT_ROOT /var/www/git
 SetEnv GIT_HTTP_EXPORT_ALL
 ScriptAlias /git/ /usr/libexec/git-core/git-http-backend/
+
+# This is not strictly necessary using Apache and a modern version of
+# git-http-backend, as the webserver will pass along the header in the
+# environment as HTTP_GIT_PROTOCOL, and http-backend will copy that into
+# GIT_PROTOCOL. But you may need this line (or something similar if you
+# are using a different webserver), or if you want to support older Git
+# versions that did not do that copying.
+#
+# Having the webserver set up GIT_PROTOCOL is perfectly fine even with
+# modern versions (and will take precedence over HTTP_GIT_PROTOCOL,
+# which means it can be used to override the client's request).
+SetEnvIf Git-Protocol ".*" GIT_PROTOCOL=$0
 ----------------------------------------------------------------
 +
 To enable anonymous read access but authenticated write access,
@@ -264,6 +278,16 @@ a repository with an extremely large number of refs.  The value can be
 specified with a unit (e.g., `100M` for 100 megabytes). The default is
 10 megabytes.
 
+Clients may probe for optional protocol capabilities (like the v2
+protocol) using the `Git-Protocol` HTTP header. In order to support
+these, the contents of that header must appear in the `GIT_PROTOCOL`
+environment variable. Most webservers will pass this header to the CGI
+via the `HTTP_GIT_PROTOCOL` variable, and `git-http-backend` will
+automatically copy that to `GIT_PROTOCOL`. However, some webservers may
+be more selective about which headers they'll pass, in which case they
+need to be configured explicitly (see the mention of `Git-Protocol` in
+the Apache config from the earlier EXAMPLES section).
+
 The backend process sets GIT_COMMITTER_NAME to '$REMOTE_USER' and
 GIT_COMMITTER_EMAIL to '$\{REMOTE_USER}@http.$\{REMOTE_ADDR\}',
 ensuring that any reflogs created by 'git-receive-pack' contain some
diff --git a/Documentation/git-upload-pack.txt b/Documentation/git-upload-pack.txt
index 739416ec83..8f87b23ea8 100644
--- a/Documentation/git-upload-pack.txt
+++ b/Documentation/git-upload-pack.txt
@@ -48,6 +48,14 @@ OPTIONS
 <directory>::
 	The repository to sync from.
 
+ENVIRONMENT
+-----------
+
+`GIT_PROTOCOL`::
+	Internal variable used for handshaking the wire protocol. Server
+	admins may need to configure some transports to allow this
+	variable to be passed. See the discussion in linkgit:git[1].
+
 SEE ALSO
 --------
 linkgit:gitnamespaces[7]
diff --git a/Documentation/git.txt b/Documentation/git.txt
index 6dd241ef83..e4b82599fc 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -894,6 +894,21 @@ for full details.
 	Contains a colon ':' separated list of keys with optional values
 	'key[=value]'.  Presence of unknown keys and values must be
 	ignored.
++
+Note that servers may need to be configured to allow this variable to
+pass over some transports. It will be propagated automatically when
+accessing local repositories (i.e., `file://` or a filesystem path), as
+well as over the `git://` protocol. For git-over-http, it should work
+automatically in most configurations, but see the discussion in
+linkgit:git-http-backend[1]. For git-over-ssh, the ssh server may need
+to be configured to allow clients to pass this variable (e.g., by using
+`AcceptEnv GIT_PROTOCOL` with OpenSSH).
++
+This configuration is optional. If the variable is not propagated, then
+clients will fall back to the original "v0" protocol (but may miss out
+on some performance improvements or features). This variable currently
+only affects clones and fetches; it is not yet used for pushes (but may
+be in the future).
 
 `GIT_OPTIONAL_LOCKS`::
 	If set to `0`, Git will complete any requested operation without
diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index 213538f1d0..59b86fcf97 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -42,7 +42,8 @@ Initial Client Request
 In general a client can request to speak protocol v2 by sending
 `version=2` through the respective side-channel for the transport being
 used which inevitably sets `GIT_PROTOCOL`.  More information can be
-found in `pack-protocol.txt` and `http-protocol.txt`.  In all cases the
+found in `pack-protocol.txt` and `http-protocol.txt`, as well as the
+`GIT_PROTOCOL` definition in `git.txt`. In all cases the
 response from the server is the capability advertisement.
 
 Git Transport
@@ -58,6 +59,8 @@ SSH and File Transport
 
 When using either the ssh:// or file:// transport, the GIT_PROTOCOL
 environment variable must be set explicitly to include "version=2".
+The server may need to be configured to allow this environment variable
+to pass.
 
 HTTP Transport
 ~~~~~~~~~~~~~~
@@ -84,6 +87,9 @@ Subsequent requests are then made directly to the service
 Uses the `--http-backend-info-refs` option to
 linkgit:git-upload-pack[1].
 
+The server may need to be configured to pass this header's contents via
+the `GIT_PROTOCOL` variable. See the discussion in `git-http-backend.txt`.
+
 Capability Advertisement
 ------------------------
 
diff --git a/http-backend.c b/http-backend.c
index 838374edb9..e7c0eeab23 100644
--- a/http-backend.c
+++ b/http-backend.c
@@ -739,6 +739,7 @@ static int bad_request(struct strbuf *hdr, const struct service_cmd *c)
 int cmd_main(int argc, const char **argv)
 {
 	char *method = getenv("REQUEST_METHOD");
+	const char *proto_header;
 	char *dir;
 	struct service_cmd *cmd = NULL;
 	char *cmd_arg = NULL;
@@ -789,6 +790,9 @@ int cmd_main(int argc, const char **argv)
 	http_config();
 	max_request_buffer = git_env_ulong("GIT_HTTP_MAX_REQUEST_BUFFER",
 					   max_request_buffer);
+	proto_header = getenv("HTTP_GIT_PROTOCOL");
+	if (proto_header)
+		setenv(GIT_PROTOCOL_ENVIRONMENT, proto_header, 0);
 
 	cmd->imp(&hdr, cmd_arg);
 	return 0;
diff --git a/t/lib-httpd/apache.conf b/t/lib-httpd/apache.conf
index afa91e38b0..180a41fe96 100644
--- a/t/lib-httpd/apache.conf
+++ b/t/lib-httpd/apache.conf
@@ -81,8 +81,6 @@ PassEnv GIT_TRACE
 PassEnv GIT_CONFIG_NOSYSTEM
 PassEnv GIT_TEST_SIDEBAND_ALL
 
-SetEnvIf Git-Protocol ".*" GIT_PROTOCOL=$0
-
 Alias /dumb/ www/
 Alias /auth/dumb/ www/auth/dumb/
 
@@ -117,6 +115,11 @@ Alias /auth/dumb/ www/auth/dumb/
 	SetEnv GIT_EXEC_PATH ${GIT_EXEC_PATH}
 	SetEnv GIT_HTTP_EXPORT_ALL
 </LocationMatch>
+<LocationMatch /smart_v0/>
+	SetEnv GIT_EXEC_PATH ${GIT_EXEC_PATH}
+	SetEnv GIT_HTTP_EXPORT_ALL
+	SetEnv GIT_PROTOCOL
+</LocationMatch>
 ScriptAlias /smart/incomplete_length/git-upload-pack incomplete-length-upload-pack-v2-http.sh/
 ScriptAlias /smart/incomplete_body/git-upload-pack incomplete-body-upload-pack-v2-http.sh/
 ScriptAliasMatch /error_git_upload_pack/(.*)/git-upload-pack error.sh/
diff --git a/t/t5551-http-fetch-smart.sh b/t/t5551-http-fetch-smart.sh
index 4f87d90c5b..cffc47a8e3 100755
--- a/t/t5551-http-fetch-smart.sh
+++ b/t/t5551-http-fetch-smart.sh
@@ -558,4 +558,13 @@ test_expect_success 'http auth forgets bogus credentials' '
 	expect_askpass both user@host
 '
 
+test_expect_success 'client falls back from v2 to v0 to match server' '
+	GIT_TRACE_PACKET=$PWD/trace \
+	GIT_TEST_PROTOCOL_VERSION=2 \
+	git clone $HTTPD_URL/smart_v0/repo.git repo-v0 &&
+	# check for v0; there the HEAD symref is communicated in the capability
+	# line; v2 uses a different syntax on each ref advertisement line
+	grep symref=HEAD:refs/heads/ trace
+'
+
 test_done