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[RFC/PATCH] connect: add GIT_SSH_{SEND,RECEIVE}{,_COMMAND} env variables




Amend the long-standing logic for overriding the ssh command with
GIT_SSH or GIT_SSH_COMMAND to also support
e.g. GIT_SSH_SEND_COMMAND. The new specific send/receive variables
take priority over the old ones, and fall back to the older ones if
they exist.

This is useful for talking to systems such as Github or Gitlab that
identify user accounts (or deploy keys) by ssh keys. Normally, ssh
could do this itself by supplying multiple keys via -i, but that trick
doesn't work on these systems as the connection will have already been
accepted when the "wrong" key gets rejected.

This new feature is redundant to and less general than setting the
GIT_SSH_COMMAND to the path of a script that's going to dispatch to
ssh depending on what the second argument to the script is, e.g.:

    $ cat ./git-ssh-command
    #!/usr/bin/perl
    if ($ARGV[1] =~ /^git-upload-pack /) {
       system qw(ssh -i /some/ro/key) => @ARGV;
    } elsif ($ARGV[1] =~ /^git-receive-pack /) {
       system qw(ssh -i /some/rw/key) => @ARGV;
    } else { ... }
    $ GIT_TRACE=1 GIT_SSH_COMMAND="./git-ssh-command" git fetch
    10:22:39.415684 git.c:344               trace: built-in: git 'fetch'
    10:22:39.432192 run-command.c:627       trace: run_command: './git-ssh-command' '-G' 'git@xxxxxxxxxx'
    10:22:39.434156 run-command.c:627       trace: run_command: './git-ssh-command' 'git@xxxxxxxxxx' 'git-upload-pack '\''git/git.git'\'''
    Warning: Identity file /some/ro/key not accessible: No such file or directory.

However, I feel that this is a common enough case to be worth
supporting explicitly, and such a script will also need to deal with
arbitrary arguments fed via git-fetch's --upload-pack="...", and
git-push's corresponding --receive-pack argument.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>
---

I'm not 100% sure about this one myself, but am leaning towards
inclusion for the reasons explained above, and the patch is trivial
enough that I think we can discuss whether it's worthwhile without
test / documentation.

 builtin/fetch-pack.c |  2 +-
 builtin/send-pack.c  |  3 ++-
 connect.c            | 21 ++++++++++++++++++---
 connect.h            |  2 ++
 4 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/builtin/fetch-pack.c b/builtin/fetch-pack.c
index 366b9d13f9..dae10f8419 100644
--- a/builtin/fetch-pack.c
+++ b/builtin/fetch-pack.c
@@ -189,7 +189,7 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 		if (args.diag_url)
 			flags |= CONNECT_DIAG_URL;
 		conn = git_connect(fd, dest, args.uploadpack,
-				   flags);
+				   flags | CONNECT_RECEIVE);
 		if (!conn)
 			return args.diag_url ? 0 : 1;
 	}
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index fc4f0bb5fb..2374d2b29c 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -252,8 +252,9 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 		fd[0] = 0;
 		fd[1] = 1;
 	} else {
+		int flags = args.verbose ? CONNECT_VERBOSE : 0;
 		conn = git_connect(fd, dest, receivepack,
-			args.verbose ? CONNECT_VERBOSE : 0);
+			flags | CONNECT_SEND);
 	}
 
 	get_remote_heads(fd[0], NULL, 0, &remote_refs, REF_NORMAL,
diff --git a/connect.c b/connect.c
index c3a014c5ba..2a35924292 100644
--- a/connect.c
+++ b/connect.c
@@ -774,13 +774,23 @@ static enum protocol parse_connect_url(const char *url_orig, char **ret_host,
 	return protocol;
 }
 
-static const char *get_ssh_command(void)
+static const char *get_ssh_command(int flags)
 {
 	const char *ssh;
 
+	if (flags & CONNECT_SEND && (ssh = getenv("GIT_SSH_SEND_COMMAND")))
+		return ssh;
+	else if (flags & CONNECT_RECEIVE && (ssh = getenv("GIT_SSH_RECEIVE_COMMAND")))
+		return ssh;
 	if ((ssh = getenv("GIT_SSH_COMMAND")))
 		return ssh;
 
+	if (flags & CONNECT_SEND &&
+	    !git_config_get_string_const("core.sshsendcommand", &ssh))
+		return ssh;
+	else if (flags & CONNECT_RECEIVE &&
+	    !git_config_get_string_const("core.sshreceivecommand", &ssh))
+		return ssh;
 	if (!git_config_get_string_const("core.sshcommand", &ssh))
 		return ssh;
 
@@ -997,7 +1007,7 @@ static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
 	if (looks_like_command_line_option(ssh_host))
 		die("strange hostname '%s' blocked", ssh_host);
 
-	ssh = get_ssh_command();
+	ssh = get_ssh_command(flags);
 	if (ssh) {
 		variant = determine_ssh_variant(ssh, 1);
 	} else {
@@ -1008,7 +1018,12 @@ static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
 		 */
 		conn->use_shell = 0;
 
-		ssh = getenv("GIT_SSH");
+		if (flags & CONNECT_SEND)
+			ssh = getenv("GIT_SSH_SEND");
+		else if (flags & CONNECT_RECEIVE)
+			ssh = getenv("GIT_SSH_RECEIVE");
+		if (!ssh)
+			ssh = getenv("GIT_SSH");
 		if (!ssh)
 			ssh = "ssh";
 		variant = determine_ssh_variant(ssh, 0);
diff --git a/connect.h b/connect.h
index 01f14cdf3f..e3ff0d5838 100644
--- a/connect.h
+++ b/connect.h
@@ -5,6 +5,8 @@
 #define CONNECT_DIAG_URL      (1u << 1)
 #define CONNECT_IPV4          (1u << 2)
 #define CONNECT_IPV6          (1u << 3)
+#define CONNECT_SEND          (1u << 4)
+#define CONNECT_RECEIVE       (1u << 5)
 extern struct child_process *git_connect(int fd[2], const char *url, const char *prog, int flags);
 extern int finish_connect(struct child_process *conn);
 extern int git_connection_is_socket(struct child_process *conn);
-- 
2.15.1.424.g9478a66081