Re: [PATCH 6/8] ssh: 'auto' variant to select between 'ssh' and 'simple'

> Android's "repo" tool is a tool for managing a large codebase
> consisting of multiple smaller repositories, similar to Git's
> submodule feature.  Starting with Git 94b8ae5a (ssh: introduce a
> 'simple' ssh variant, 2017-10-16), users noticed that it stopped
> handling the port in ssh:// URLs.
> 
> The cause: when it encounters ssh:// URLs, repo pre-connects to the
> server and sets GIT_SSH to a helper ".repo/repo/git_ssh" that reuses
> that connection.  Before 94b8ae5a, the helper was assumed to support
> OpenSSH options for lack of a better guess and got passed a -p option
> to set the port.  After that patch, it uses the new default of a
> simple helper that does not accept an option to set the port.
> 
> The next release of "repo" will set GIT_SSH_VARIANT to "ssh" to avoid
> that.  But users of old versions and of other similar GIT_SSH
> implementations would not get the benefit of that fix.
> 
> So update the default to use OpenSSH options again, with a twist.  As
> observed in 94b8ae5a, we cannot assume that $GIT_SSH always handles
> OpenSSH options: common helpers such as travis-ci's dpl[*] are
> configured using GIT_SSH and do not accept OpenSSH options.  So make
> the default a new variant "auto", with the following behavior:
> 
>  1. First, check for a recognized basename, like today.
> 
>  2. If the basename is not recognized, check whether $GIT_SSH supports
>     OpenSSH options by running
> 
> 	$GIT_SSH -G <options> <host>
> 
>     This returns status 0 and prints configuration in OpenSSH if it
>     recognizes all <options> and returns status 255 if it encounters
>     an unrecognized option.  A wrapper script like
> 
> 	exec ssh -- "$@"
> 
>     would fail with
> 
> 	ssh: Could not resolve hostname -g: Name or service not known
> 
>     , correctly reflecting that it does not support OpenSSH options.
>     The command is run with stdin, stdout, and stderr redirected to
>     /dev/null so even a command that expects a terminal would exit
>     immediately.
> 
>  3. Based on the result from step (2), behave like "ssh" (if it
>     succeeded) or "simple" (if it failed).
> 
> This way, the default ssh variant for unrecognized commands can handle
> both the repo and dpl cases as intended.
> 
> This autodetection has been running on Google workstations since
> 2017-10-23 with no reported negative effects.
> 
> [*] https://github.com/travis-ci/dpl/blob/6c3fddfda1f2a85944c544446b068bac0a77c049/lib/dpl/provider.rb#L215
> 
> Reported-by: William Yan <wyan@google.com>
> Improved-by: Jonathan Tan <jonathantanmy@google.com>
> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
> ---
> Added two notes to the commit message:
>  - describing the real-world testing this patch has undergone
>  - stdin, stdout, and stderr go to /dev/null, preventing a
>    hypothetical ssh variant that *ignores* -G from hanging waiting for
>    input from the terminal.
> 
> This is to address the worries at
> https://public-inbox.org/git/xmqq60b59toe.fsf@gitster.mtv.corp.google.com/
> and https://public-inbox.org/git/CAGZ79kZTjUvcq_hKHCqTDoaBxt2x+9XcqYc6ao1bhcET2SM-PQ@mail.gmail.com/
> about hanging.
> 
> No change to the code from last time.
> 

>  Documentation/config.txt | 24 +++++++++++++++---------
>  connect.c                | 32 +++++++++++++++++++++++++-------
>  t/t5601-clone.sh         | 21 +++++++++++++++++++++
>  3 files changed, 61 insertions(+), 16 deletions(-)
> 
> diff --git a/Documentation/config.txt b/Documentation/config.txt
> index 0460af37e2..0c371ad786 100644
> --- a/Documentation/config.txt
> +++ b/Documentation/config.txt
> @@ -2081,16 +2081,22 @@ matched against are those given directly to Git commands.  This means any URLs
>  visited as a result of a redirection do not participate in matching.
>  
>  ssh.variant::
> -	Depending on the value of the environment variables `GIT_SSH` or
> -	`GIT_SSH_COMMAND`, or the config setting `core.sshCommand`, Git
> -	auto-detects whether to adjust its command-line parameters for use
> -	with ssh (OpenSSH), plink or tortoiseplink, as opposed to the default
> -	(simple).
> +	By default, Git determines the command line arguments to use
> +	based on the basename of the configured SSH command (configured
> +	using the environment variable `GIT_SSH` or `GIT_SSH_COMMAND` or
> +	the config setting `core.sshCommand`). If the basename is
> +	unrecognized, Git will attempt to detect support of OpenSSH
> +	options by first invoking the configured SSH command with the
> +	`-G` (print configuration) option and will subsequently use
> +	OpenSSH options (if that is successful) or no options besides
> +	the host and remote command (if it fails).
>  +
> -The config variable `ssh.variant` can be set to override this auto-detection;
> -valid values are `ssh`, `simple`, `plink`, `putty` or `tortoiseplink`. Any
> -other value will be treated as normal ssh. This setting can be overridden via
> -the environment variable `GIT_SSH_VARIANT`.
> +The config variable `ssh.variant` can be set to override this detection.
> +Valid values are `ssh` (to use OpenSSH options), `plink`, `putty`,
> +`tortoiseplink`, `simple` (no options except the host and remote command).
> +The default auto-detection can be explicitly requested using the value
> +`auto`.  Any other value is treated as `ssh`.  This setting can also be
> +overridden via the environment variable `GIT_SSH_VARIANT`.
>  +
>  The current command-line parameters used for each variant are as
>  follows:
> diff --git a/connect.c b/connect.c
> index d2fbb15cc5..455c54a2ec 100644
> --- a/connect.c
> +++ b/connect.c
> @@ -788,6 +788,7 @@ static const char *get_ssh_command(void)
>  }
>  
>  enum ssh_variant {
> +	VARIANT_AUTO,
>  	VARIANT_SIMPLE,
>  	VARIANT_SSH,
>  	VARIANT_PLINK,
> @@ -795,14 +796,16 @@ enum ssh_variant {
>  	VARIANT_TORTOISEPLINK,
>  };
>  
> -static int override_ssh_variant(enum ssh_variant *ssh_variant)
> +static void override_ssh_variant(enum ssh_variant *ssh_variant)
>  {
>  	const char *variant = getenv("GIT_SSH_VARIANT");
>  
>  	if (!variant && git_config_get_string_const("ssh.variant", &variant))
> -		return 0;
> +		return;
>  
> -	if (!strcmp(variant, "plink"))
> +	if (!strcmp(variant, "auto"))
> +		*ssh_variant = VARIANT_AUTO;
> +	else if (!strcmp(variant, "plink"))
>  		*ssh_variant = VARIANT_PLINK;
>  	else if (!strcmp(variant, "putty"))
>  		*ssh_variant = VARIANT_PUTTY;
> @@ -812,18 +815,18 @@ static int override_ssh_variant(enum ssh_variant *ssh_variant)
>  		*ssh_variant = VARIANT_SIMPLE;
>  	else
>  		*ssh_variant = VARIANT_SSH;
> -
> -	return 1;
>  }
>  
>  static enum ssh_variant determine_ssh_variant(const char *ssh_command,
>  					      int is_cmdline)
>  {
> -	enum ssh_variant ssh_variant = VARIANT_SIMPLE;
> +	enum ssh_variant ssh_variant = VARIANT_AUTO;
>  	const char *variant;
>  	char *p = NULL;
>  
> -	if (override_ssh_variant(&ssh_variant))
> +	override_ssh_variant(&ssh_variant);
> +
> +	if (ssh_variant != VARIANT_AUTO)
>  		return ssh_variant;
>  
>  	if (!is_cmdline) {
> @@ -982,6 +985,21 @@ static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
>  		variant = determine_ssh_variant(ssh, 0);
>  	}
>  
> +	if (variant == VARIANT_AUTO) {
> +		struct child_process detect = CHILD_PROCESS_INIT;
> +
> +		detect.use_shell = conn->use_shell;
> +		detect.no_stdin = detect.no_stdout = detect.no_stderr = 1;
> +
> +		argv_array_push(&detect.args, ssh);
> +		argv_array_push(&detect.args, "-G");
> +		push_ssh_options(&detect.args, &detect.env_array,
> +				 VARIANT_SSH, port, flags);
> +		argv_array_push(&detect.args, ssh_host);
> +
> +		variant = run_command(&detect) ? VARIANT_SIMPLE : VARIANT_SSH;
> +	}
> +
>  	argv_array_push(&conn->args, ssh);
>  	push_ssh_options(&conn->args, &conn->env_array, variant, port, flags);
>  	argv_array_push(&conn->args, ssh_host);
> diff --git a/t/t5601-clone.sh b/t/t5601-clone.sh
> index 9d007c0f8d..209e2d5604 100755
> --- a/t/t5601-clone.sh
> +++ b/t/t5601-clone.sh
> @@ -369,6 +369,12 @@ test_expect_success 'variant can be overriden' '
>  	expect_ssh myhost src
>  '
>  
> +test_expect_success 'variant=auto picks based on basename' '
> +	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
> +	git -c ssh.variant=auto clone -4 "[myhost:123]:src" ssh-auto-clone &&
> +	expect_ssh "-4 -P 123" myhost src
> +'
> +
>  test_expect_success 'simple is treated as simple' '
>  	copy_ssh_wrapper_as "$TRASH_DIRECTORY/simple" &&
>  	git clone -4 "[myhost:123]:src" ssh-bracket-clone-simple &&
> @@ -381,6 +387,21 @@ test_expect_success 'uplink is treated as simple' '
>  	expect_ssh myhost src
>  '
>  
> +test_expect_success 'OpenSSH-like uplink is treated as ssh' '
> +	write_script "$TRASH_DIRECTORY/uplink" <<-EOF &&
> +	if test "\$1" = "-G"
> +	then
> +		exit 0
> +	fi &&
> +	exec "\$TRASH_DIRECTORY/ssh$X" "\$@"
> +	EOF
> +	test_when_finished "rm -f \"\$TRASH_DIRECTORY/uplink\"" &&
> +	GIT_SSH="$TRASH_DIRECTORY/uplink" &&
> +	test_when_finished "GIT_SSH=\"\$TRASH_DIRECTORY/ssh\$X\"" &&
> +	git clone "[myhost:123]:src" ssh-bracket-clone-sshlike-uplink &&
> +	expect_ssh "-p 123" myhost src
> +'
> +
>  test_expect_success 'plink is treated specially (as putty)' '
>  	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
>  	git clone "[myhost:123]:src" ssh-bracket-clone-plink-0 &&
> -- 
> 2.15.0.448.gf294e3d99a
>