+ Detailed Design
+=================
+
+A client can request to speak protocol v2 by sending `version=2` in the
+side-channel `GIT_PROTOCOL` in the initial request to the server.
+
+In protocol v2 communication is command oriented. When first contacting a
+server a list of capabilities will advertised. Some of these capabilities
+will be commands which a client can request be executed. Once a command
+has completed, a client can reuse the connection and request that other
+commands be executed.
+
+ Special Packets
+-----------------
+
+In protocol v2 these special packets will have the following semantics:
+
+ * '0000' Flush Packet (flush-pkt) - indicates the end of a message
+ * '0001' Delimiter Packet (delim-pkt) - separates sections of a message
+
+ Capability Advertisement
+--------------------------
+
+A server which decides to communicate (based on a request from a client)
+using protocol version 2, notifies the client by sending a version string
+in its initial response followed by an advertisement of its capabilities.
+Each capability is a key with an optional value. Clients must ignore all
+unknown keys. Semantics of unknown values are left to the definition of
+each key. Some capabilities will describe commands which can be requested
+to be executed by the client.
+
+ capability-advertisement = protocol-version
+ capability-list
+ flush-pkt
+
+ protocol-version = PKT-LINE("version 2" LF)
+ capability-list = *capability
+ capability = PKT-LINE(key[=value] LF)
+
+ key = 1*CHAR
+ value = 1*CHAR
+ CHAR = 1*(ALPHA / DIGIT / "-" / "_")
+
+A client then responds to select the command it wants with any particular
+capabilities or arguments. There is then an optional section where the
+client can provide any command specific parameters or queries.
+
+ command-request = command
+ capability-list
+ (command-args)
+ flush-pkt
+ command = PKT-LINE("command=" key LF)
+ command-args = delim-pkt
+ *arg
+ arg = 1*CHAR
+
+The server will then check to ensure that the client's request is
+comprised of a valid command as well as valid capabilities which were
+advertised. If the request is valid the server will then execute the
+command.
+
+When a command has finished a client can either request that another
+command be executed or can terminate the connection by sending an empty
+request consisting of just a flush-pkt.
+
+ Capabilities
+~~~~~~~~~~~~~~
+
+There are two different types of capabilities: normal capabilities,
+which can be used to to convey information or alter the behavior of a
+request, and command capabilities, which are the core actions that a
+client wants to perform (fetch, push, etc).
+
+ agent
+-------
+
+The server can advertise the `agent` capability with a value `X` (in the
+form `agent=X`) to notify the client that the server is running version
+`X`. The client may optionally send its own agent string by including
+the `agent` capability with a value `Y` (in the form `agent=Y`) in its
+request to the server (but it MUST NOT do so if the server did not
+advertise the agent capability). The `X` and `Y` strings may contain any
+printable ASCII characters except space (i.e., the byte range 32 < x <
+127), and are typically of the form "package/version" (e.g.,
+"git/1.8.3.1"). The agent strings are purely informative for statistics
+and debugging purposes, and MUST NOT be used to programmatically assume
+the presence or absence of particular features.
+
+ stateless-rpc
+---------------
+
+If advertised, the `stateless-rpc` capability indicates that the server
+supports running commands in a stateless-rpc mode, which means that a
+command lasts for only a single request-response round.
+
+Normally a command can last for as many rounds as are required to
+complete it (multiple for negotiation during fetch or no additional
+trips in the case of ls-refs). If the client sends the `stateless-rpc`
+capability with a value of `true` (in the form `stateless-rpc=true`)
+then the invoked command must only last a single round.
diff --git a/Makefile b/Makefile
index 3b849c060..18c255428 100644
--- a/Makefile
+++ b/Makefile
@@ -881,6 +881,7 @@ LIB_OBJS += revision.o
LIB_OBJS += run-command.o
LIB_OBJS += send-pack.o
LIB_OBJS += sequencer.o
+LIB_OBJS += serve.o
LIB_OBJS += server-info.o
LIB_OBJS += setup.o
LIB_OBJS += sha1-array.o
@@ -1014,6 +1015,7 @@ BUILTIN_OBJS += builtin/rev-parse.o
BUILTIN_OBJS += builtin/revert.o
BUILTIN_OBJS += builtin/rm.o
BUILTIN_OBJS += builtin/send-pack.o
+BUILTIN_OBJS += builtin/serve.o
BUILTIN_OBJS += builtin/shortlog.o
BUILTIN_OBJS += builtin/show-branch.o
BUILTIN_OBJS += builtin/show-ref.o
diff --git a/builtin.h b/builtin.h
index f332a1257..3f3fdfc28 100644
--- a/builtin.h
+++ b/builtin.h
@@ -215,6 +215,7 @@ extern int cmd_rev_parse(int argc, const char **argv, const char *prefix);
extern int cmd_revert(int argc, const char **argv, const char *prefix);
extern int cmd_rm(int argc, const char **argv, const char *prefix);
extern int cmd_send_pack(int argc, const char **argv, const char *prefix);
+extern int cmd_serve(int argc, const char **argv, const char *prefix);
extern int cmd_shortlog(int argc, const char **argv, const char *prefix);
extern int cmd_show(int argc, const char **argv, const char *prefix);
extern int cmd_show_branch(int argc, const char **argv, const char *prefix);
diff --git a/builtin/serve.c b/builtin/serve.c
new file mode 100644
index 000000000..d3fd240bb
--- /dev/null
+++ b/builtin/serve.c
@@ -0,0 +1,30 @@
+#include "cache.h"
+#include "builtin.h"
+#include "parse-options.h"
+#include "serve.h"
+
+static char const * const serve_usage[] = {
+ N_("git serve [<options>]"),
+ NULL
+};
+
+int cmd_serve(int argc, const char **argv, const char *prefix)
+{
+ struct serve_options opts = SERVE_OPTIONS_INIT;
+
+ struct option options[] = {
+ OPT_BOOL(0, "stateless-rpc", &opts.stateless_rpc,
+ N_("quit after a single request/response exchange")),
+ OPT_BOOL(0, "advertise-capabilities", &opts.advertise_capabilities,
+ N_("exit immediately after advertising capabilities")),
+ OPT_END()
+ };
+
+ /* ignore all unknown cmdline switches for now */
+ argc = parse_options(argc, argv, prefix, options, serve_usage,
+ PARSE_OPT_KEEP_DASHDASH |
+ PARSE_OPT_KEEP_UNKNOWN);
+ serve(&opts);
+
+ return 0;
+}
diff --git a/git.c b/git.c
index f71073dc8..f85d682b6 100644
--- a/git.c
+++ b/git.c
@@ -461,6 +461,7 @@ static struct cmd_struct commands[] = {
{ "revert", cmd_revert, RUN_SETUP | NEED_WORK_TREE },
{ "rm", cmd_rm, RUN_SETUP },
{ "send-pack", cmd_send_pack, RUN_SETUP },
+ { "serve", cmd_serve, RUN_SETUP },
{ "shortlog", cmd_shortlog, RUN_SETUP_GENTLY | USE_PAGER },
{ "show", cmd_show, RUN_SETUP },
{ "show-branch", cmd_show_branch, RUN_SETUP },
diff --git a/serve.c b/serve.c
new file mode 100644
index 000000000..90e3defe8
--- /dev/null
+++ b/serve.c
@@ -0,0 +1,249 @@
+#include "cache.h"
+#include "repository.h"
+#include "config.h"
+#include "pkt-line.h"
+#include "version.h"
+#include "argv-array.h"
+#include "serve.h"
+
+static int always_advertise(struct repository *r,
+ struct strbuf *value)
+{
+ return 1;
+}
+
+static int agent_advertise(struct repository *r,
+ struct strbuf *value)
+{
+ if (value)
+ strbuf_addstr(value, git_user_agent_sanitized());
+ return 1;
+}
+
+struct protocol_capability {
+ /*
+ * The name of the capability. The server uses this name when
+ * advertising this capability, and the client uses this name to
+ * specify this capability.
+ */
+ const char *name;
+
+ /*
+ * Function queried to see if a capability should be advertised.
+ * Optionally a value can be specified by adding it to 'value'.
+ * If a value is added to 'value', the server will advertise this
+ * capability as "<name>=<value>" instead of "<name>".
+ */
+ int (*advertise)(struct repository *r, struct strbuf *value);
+
+ /*
+ * Function called when a client requests the capability as a command.
+ * The command request will be provided to the function via 'keys', the
+ * capabilities requested, and 'args', the command specific parameters.
+ *
+ * This field should be NULL for capabilities which are not commands.
+ */
+ int (*command)(struct repository *r,
+ struct argv_array *keys,
+ struct argv_array *args);
+};
+
+static struct protocol_capability capabilities[] = {
+ { "agent", agent_advertise, NULL },
+ { "stateless-rpc", always_advertise, NULL },
+};
+
+static void advertise_capabilities(void)
+{
+ struct strbuf capability = STRBUF_INIT;
+ struct strbuf value = STRBUF_INIT;
+ int i;
+
+ for (i = 0; i < ARRAY_SIZE(capabilities); i++) {
+ struct protocol_capability *c = &capabilities[i];
+
+ if (c->advertise(the_repository, &value)) {
+ strbuf_addstr(&capability, c->name);
+
+ if (value.len) {
+ strbuf_addch(&capability, '=');
+ strbuf_addbuf(&capability, &value);
+ }
+
+ strbuf_addch(&capability, '\n');
+ packet_write(1, capability.buf, capability.len);
+ }
+
+ strbuf_reset(&capability);
+ strbuf_reset(&value);
+ }
+
+ packet_flush(1);
+ strbuf_release(&capability);
+ strbuf_release(&value);
+}
+
+static struct protocol_capability *get_capability(const char *key)
+{
+ int i;
+
+ if (!key)
+ return NULL;
+
+ for (i = 0; i < ARRAY_SIZE(capabilities); i++) {
+ struct protocol_capability *c = &capabilities[i];
+ const char *out;
+ if (skip_prefix(key, c->name, &out) && (!*out || *out == '='))
+ return c;
+ }
+
+ return NULL;
+}
+
+static int is_valid_capability(const char *key)
+{
+ const struct protocol_capability *c = get_capability(key);
+
+ return c && c->advertise(the_repository, NULL);
+}
+
+static int is_command(const char *key, struct protocol_capability **command)
+{
+ const char *out;
+
+ if (skip_prefix(key, "command=", &out)) {
+ struct protocol_capability *cmd = get_capability(out);
+
+ if (!cmd || !cmd->advertise(the_repository, NULL) || !cmd->command)
+ die("invalid command '%s'", out);
+ if (*command)
+ die("command already requested");
+
+ *command = cmd;
+ return 1;
+ }
+
+ return 0;
+}
+
+int has_capability(const struct argv_array *keys, const char *capability,
+ const char **value)
+{
+ int i;
+ for (i = 0; i < keys->argc; i++) {
+ const char *out;
+ if (skip_prefix(keys->argv[i], capability, &out) &&
+ (!*out || *out == '=')) {
+ if (value) {
+ if (*out == '=')
+ out++;
+ *value = out;
+ }
+ return 1;
+ }
+ }
+
+ return 0;
+}
+
+enum request_state {
+ PROCESS_REQUEST_KEYS = 0,
+ PROCESS_REQUEST_ARGS,
+ PROCESS_REQUEST_DONE,
+};
+
+static int process_request(void)
+{
+ enum request_state state = PROCESS_REQUEST_KEYS;
+ char *buffer = packet_buffer;
+ unsigned buffer_size = sizeof(packet_buffer);
+ int pktlen;
+ struct argv_array keys = ARGV_ARRAY_INIT;
+ struct argv_array args = ARGV_ARRAY_INIT;
+ struct protocol_capability *command = NULL;
+
+ while (state != PROCESS_REQUEST_DONE) {
+ switch (packet_read_with_status(0, NULL, NULL, buffer,
+ buffer_size, &pktlen,
+ PACKET_READ_CHOMP_NEWLINE)) {
+ case PACKET_READ_EOF:
+ BUG("Should have already died when seeing EOF");
+ case PACKET_READ_NORMAL:
+ break;
+ case PACKET_READ_FLUSH:
+ state = PROCESS_REQUEST_DONE;
+ continue;
+ case PACKET_READ_DELIM:
+ if (state != PROCESS_REQUEST_KEYS)
+ die("protocol error");
+ state = PROCESS_REQUEST_ARGS;
+ /*
+ * maybe include a check to make sure that a
+ * command/capabilities were given.
+ */
+ continue;
+ }
+
+ switch (state) {
+ case PROCESS_REQUEST_KEYS:
+ /* collect request; a sequence of keys and values */
+ if (is_command(buffer, &command) ||
+ is_valid_capability(buffer))
+ argv_array_push(&keys, buffer);
+ else
+ die("unknown capability '%s'", buffer);
+ break;
+ case PROCESS_REQUEST_ARGS:
+ /* collect arguments for the requested command */
+ argv_array_push(&args, buffer);
+ break;
+ case PROCESS_REQUEST_DONE:
+ continue;
+ }
+ }
+
+ /*
+ * If no command and no keys were given then the client wanted to
+ * terminate the connection.
+ */
+ if (!keys.argc && !args.argc)
+ return 1;
+
+ if (!command)
+ die("no command requested");
+
+ command->command(the_repository, &keys, &args);
+
+ argv_array_clear(&keys);
+ argv_array_clear(&args);
+ return 0;
+}
+
+/* Main serve loop for protocol version 2 */
+void serve(struct serve_options *options)
+{
+ if (options->advertise_capabilities || !options->stateless_rpc) {
+ /* serve by default supports v2 */
+ packet_write_fmt(1, "version 2\n");
+
+ advertise_capabilities();
+ /*
+ * If only the list of capabilities was requested exit
+ * immediately after advertising capabilities
+ */
+ if (options->advertise_capabilities)
+ return;
+ }
+
+ /*
+ * If stateless-rpc was requested then exit after
+ * a single request/response exchange
+ */
+ if (options->stateless_rpc) {
+ process_request();
+ } else {
+ for (;;)
+ if (process_request())
+ break;
+ }
+}
diff --git a/serve.h b/serve.h
new file mode 100644
index 000000000..fe65ba9f4
--- /dev/null
+++ b/serve.h
@@ -0,0 +1,15 @@
+#ifndef SERVE_H
+#define SERVE_H
+
+struct argv_array;
+extern int has_capability(const struct argv_array *keys, const char *capability,
+ const char **value);
+
+struct serve_options {
+ unsigned advertise_capabilities;
+ unsigned stateless_rpc;
+};
+#define SERVE_OPTIONS_INIT { 0 }
+extern void serve(struct serve_options *options);
+
+#endif /* SERVE_H */
diff --git a/t/t5701-git-serve.sh b/t/t5701-git-serve.sh
new file mode 100755
index 000000000..b5cc049e5
--- /dev/null
+++ b/t/t5701-git-serve.sh
@@ -0,0 +1,56 @@
+#!/bin/sh
+
+test_description='test git-serve and server commands'
+
+. ./test-lib.sh
+
+test_expect_success 'test capability advertisement' '
+ cat >expect <<-EOF &&
+ version 2
+ agent=git/$(git version | cut -d" " -f3)
+ stateless-rpc
+ 0000
+ EOF
+
+ git serve --advertise-capabilities >out &&
+ test-pkt-line unpack <out >actual &&
+ test_cmp actual expect
+'
+
+test_expect_success 'stateless-rpc flag does not list capabilities' '
+ test-pkt-line pack >in <<-EOF &&
+ 0000
+ EOF
+ git serve --stateless-rpc >out <in &&
+ test_must_be_empty out
+'
+
+test_expect_success 'request invalid capability' '
+ test-pkt-line pack >in <<-EOF &&
+ foobar
+ 0000
+ EOF
+ test_must_fail git serve --stateless-rpc 2>err <in &&
+ test_i18ngrep "unknown capability" err
+'
+
+test_expect_success 'request with no command' '
+ test-pkt-line pack >in <<-EOF &&
+ agent=git/test
+ 0000
+ EOF
+ test_must_fail git serve --stateless-rpc 2>err <in &&
+ test_i18ngrep "no command requested" err
+'
+
+test_expect_success 'request invalid command' '
+ test-pkt-line pack >in <<-EOF &&
+ command=foo
+ agent=git/test
+ 0000
+ EOF
+ test_must_fail git serve --stateless-rpc 2>err <in &&
+ test_i18ngrep "invalid command" err
+'
+
+test_done
