protocol: introduce protocol extension mechanisms

Create protocol.{c,h} and provide functions which future servers and
clients can use to determine which protocol to use or is being used.

Also introduce the 'GIT_PROTOCOL' environment variable which will be
used to communicate a colon separated list of keys with optional values
to a server.  Unknown keys and values must be tolerated.  This mechanism
is used to communicate which version of the wire protocol a client would
like to use with a server.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/cache.h b/cache.h
index 49b083e..c74b736 100644
--- a/cache.h
+++ b/cache.h
@@ -446,6 +446,14 @@ static inline enum object_type object_type(unsigned int mode)
 #define GIT_QUARANTINE_ENVIRONMENT "GIT_QUARANTINE_PATH"
 
 /*
+ * Environment variable used in handshaking the wire protocol.
+ * Contains a colon ':' separated list of keys with optional values
+ * 'key[=value]'.  Presence of unknown keys and values must be
+ * ignored.
+ */
+#define GIT_PROTOCOL_ENVIRONMENT "GIT_PROTOCOL"
+
+/*
  * This environment variable is expected to contain a boolean indicating
  * whether we should or should not treat:
  *