[PATCH v2 3/9] protocol: introduce protocol extention mechanisms

[Brandon Williams <bmwill@google.com>]

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>
---
 Documentation/config.txt | 17 ++++++++++++
 Documentation/git.txt    |  5 ++++
 Makefile                 |  1 +
 cache.h                  |  7 +++++
 protocol.c               | 72 ++++++++++++++++++++++++++++++++++++++++++++++++
 protocol.h               | 14 ++++++++++
 6 files changed, 116 insertions(+)
 create mode 100644 protocol.c
 create mode 100644 protocol.h

diff --git a/Documentation/config.txt b/Documentation/config.txt
index dc4e3f58a..b78747abc 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -2517,6 +2517,23 @@ The protocol names currently used by git are:
     `hg` to allow the `git-remote-hg` helper)
 --
 
+protocol.version::
+	Experimental. If set, clients will attempt to communicate with a
+	server using the specified protocol version.  If unset, no
+	attempt will be made by the client to communicate using a
+	particular protocol version, this results in protocol version 0
+	being used.
+	Supported versions:
++
+--
+
+* `0` - the original wire protocol.
+
+* `1` - the original wire protocol with the addition of a version string
+  in the initial response from the server.
+
+--
+
 pull.ff::
 	By default, Git does not create an extra merge commit when merging
 	a commit that is a descendant of the current commit. Instead, the
diff --git a/Documentation/git.txt b/Documentation/git.txt
index 6e3a6767e..299f75c7b 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -697,6 +697,11 @@ of clones and fetches.
 	which feed potentially-untrusted URLS to git commands.  See
 	linkgit:git-config[1] for more details.
 
+`GIT_PROTOCOL`::
+	For internal use only.  Used in handshaking the wire protocol.
+	Contains a colon ':' separated list of keys with optional values
+	'key[=value]'.  Presence of unknown keys must be tolerated.
+
 Discussion[[Discussion]]
 ------------------------
 
diff --git a/Makefile b/Makefile
index ed4ca438b..9ce68cded 100644
--- a/Makefile
+++ b/Makefile
@@ -842,6 +842,7 @@ LIB_OBJS += pretty.o
 LIB_OBJS += prio-queue.o
 LIB_OBJS += progress.o
 LIB_OBJS += prompt.o
+LIB_OBJS += protocol.o
 LIB_OBJS += quote.o
 LIB_OBJS += reachable.o
 LIB_OBJS += read-cache.o
diff --git a/cache.h b/cache.h
index 49b083ee0..0c792545f 100644
--- a/cache.h
+++ b/cache.h
@@ -445,6 +445,13 @@ static inline enum object_type object_type(unsigned int mode)
 #define GIT_ICASE_PATHSPECS_ENVIRONMENT "GIT_ICASE_PATHSPECS"
 #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 must be tolerated.
+ */
+#define GIT_PROTOCOL_ENVIRONMENT "GIT_PROTOCOL"
+
 /*
  * This environment variable is expected to contain a boolean indicating
  * whether we should or should not treat:
diff --git a/protocol.c b/protocol.c
new file mode 100644
index 000000000..369503065
--- /dev/null
+++ b/protocol.c
@@ -0,0 +1,72 @@
+#include "cache.h"
+#include "config.h"
+#include "protocol.h"
+
+static enum protocol_version parse_protocol_version(const char *value)
+{
+	if (!strcmp(value, "0"))
+		return protocol_v0;
+	else if (!strcmp(value, "1"))
+		return protocol_v1;
+	else
+		return protocol_unknown_version;
+}
+
+enum protocol_version get_protocol_version_config(void)
+{
+	const char *value;
+	if (!git_config_get_string_const("protocol.version", &value)) {
+		enum protocol_version version = parse_protocol_version(value);
+
+		if (version == protocol_unknown_version)
+			die("unknown value for config 'protocol.version': %s",
+			    value);
+
+		return version;
+	}
+
+	return protocol_v0;
+}
+
+enum protocol_version determine_protocol_version_server(void)
+{
+	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
+	enum protocol_version version = protocol_v0;
+
+	if (git_protocol) {
+		struct string_list list = STRING_LIST_INIT_DUP;
+		const struct string_list_item *item;
+		string_list_split(&list, git_protocol, ':', -1);
+
+		for_each_string_list_item(item, &list) {
+			const char *value;
+			enum protocol_version v;
+
+			if (skip_prefix(item->string, "version=", &value)) {
+				v = parse_protocol_version(value);
+				if (v > version)
+					version = v;
+			}
+		}
+
+		string_list_clear(&list, 0);
+	}
+
+	return version;
+}
+
+enum protocol_version determine_protocol_version_client(const char *server_response)
+{
+	enum protocol_version version = protocol_v0;
+
+	if (skip_prefix(server_response, "version ", &server_response)) {
+		version = parse_protocol_version(server_response);
+
+		if (version == protocol_unknown_version)
+			die("server is speaking an unknown protocol");
+		if (version == protocol_v0)
+			die("protocol error: server explicitly said version 0");
+	}
+
+	return version;
+}
diff --git a/protocol.h b/protocol.h
new file mode 100644
index 000000000..18f9a5235
--- /dev/null
+++ b/protocol.h
@@ -0,0 +1,14 @@
+#ifndef PROTOCOL_H
+#define PROTOCOL_H
+
+enum protocol_version {
+	protocol_unknown_version = -1,
+	protocol_v0 = 0,
+	protocol_v1 = 1,
+};
+
+extern enum protocol_version get_protocol_version_config(void);
+extern enum protocol_version determine_protocol_version_server(void);
+extern enum protocol_version determine_protocol_version_client(const char *server_response);
+
+#endif /* PROTOCOL_H */
-- 
2.14.1.992.g2c7b836f3a-goog