Subject: Re: [PATCH v2 2/2] sub-process: refactor handshake to common function

From: Brandon Williams <[email-masked]>
Date: Tue, 25 Jul 2017 13:28:07 -0700

On 07/25, Jonathan Tan wrote:
> Refactor, into a common function, the version and capability negotiation
> done when invoking a long-running process as a clean or smudge filter.
> This will be useful for other Git code that needs to interact similarly
> with a long-running process.
> 
> As you can see in the change to t0021, this commit changes the error
> message reported when the long-running process does not introduce itself
> with the expected "server"-terminated line. Originally, the error
> message reports that the filter "does not support filter protocol
> version 2", differentiating between the old single-file filter protocol
> and the new multi-file filter protocol - I have updated it to something
> more generic and useful.
> 
> Signed-off-by: Jonathan Tan <[email-masked]>
> ---
>  convert.c             | 61 ++++-----------------------------
>  pkt-line.c            | 19 -----------
>  pkt-line.h            |  2 --
>  sub-process.c         | 94 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  sub-process.h         | 26 ++++++++++++++
>  t/t0021-conversion.sh |  2 +-
>  6 files changed, 128 insertions(+), 76 deletions(-)
> 
> diff --git a/convert.c b/convert.c
> index deaf0ba7b..ec8ecc2ea 100644
> --- a/convert.c
> +++ b/convert.c
> @@ -512,62 +512,15 @@ static struct hashmap subprocess_map;
>  
>  static int start_multi_file_filter_fn(struct subprocess_entry *subprocess)
>  {
> -	int err;
> +	static int versions[] = {2, 0};
> +	static struct subprocess_capability capabilities[] = {
> +		{"clean", CAP_CLEAN}, {"smudge", CAP_SMUDGE}, {NULL, 0}
> +	};
>  	struct cmd2process *entry = (struct cmd2process *)subprocess;
> -	struct string_list cap_list = STRING_LIST_INIT_NODUP;
> -	char *cap_buf;
> -	const char *cap_name;
> -	struct child_process *process = &subprocess->process;
> -	const char *cmd = subprocess->cmd;
> -
> -	sigchain_push(SIGPIPE, SIG_IGN);
> -
> -	err = packet_writel(process->in, "git-filter-client", "version=2", NULL);
> -	if (err)
> -		goto done;
> -
> -	err = strcmp(packet_read_line(process->out, NULL), "git-filter-server");
> -	if (err) {
> -		error("external filter '%s' does not support filter protocol version 2", cmd);
> -		goto done;
> -	}
> -	err = strcmp(packet_read_line(process->out, NULL), "version=2");
> -	if (err)
> -		goto done;
> -	err = packet_read_line(process->out, NULL) != NULL;
> -	if (err)
> -		goto done;
> -
> -	err = packet_writel(process->in, "capability=clean", "capability=smudge", NULL);
> -
> -	for (;;) {
> -		cap_buf = packet_read_line(process->out, NULL);
> -		if (!cap_buf)
> -			break;
> -		string_list_split_in_place(&cap_list, cap_buf, '=', 1);
> -
> -		if (cap_list.nr != 2 || strcmp(cap_list.items[0].string, "capability"))
> -			continue;
> -
> -		cap_name = cap_list.items[1].string;
> -		if (!strcmp(cap_name, "clean")) {
> -			entry->supported_capabilities |= CAP_CLEAN;
> -		} else if (!strcmp(cap_name, "smudge")) {
> -			entry->supported_capabilities |= CAP_SMUDGE;
> -		} else {
> -			warning(
> -				"external filter '%s' requested unsupported filter capability '%s'",
> -				cmd, cap_name
> -			);
> -		}
> -
> -		string_list_clear(&cap_list, 0);
> -	}
> -
> -done:
> -	sigchain_pop(SIGPIPE);
>  
> -	return err;
> +	return subprocess_handshake(subprocess, "git-filter-", versions, NULL,
> +				    capabilities,
> +				    &entry->supported_capabilities);
>  }
>  
>  static int apply_multi_file_filter(const char *path, const char *src, size_t len,
> diff --git a/pkt-line.c b/pkt-line.c
> index 9d845ecc3..7db911957 100644
> --- a/pkt-line.c
> +++ b/pkt-line.c
> @@ -171,25 +171,6 @@ int packet_write_fmt_gently(int fd, const char *fmt, ...)
>  	return status;
>  }
>  
> -int packet_writel(int fd, const char *line, ...)
> -{
> -	va_list args;
> -	int err;
> -	va_start(args, line);
> -	for (;;) {
> -		if (!line)
> -			break;
> -		if (strlen(line) > LARGE_PACKET_DATA_MAX)
> -			return -1;
> -		err = packet_write_fmt_gently(fd, "%s\n", line);
> -		if (err)
> -			return err;
> -		line = va_arg(args, const char*);
> -	}
> -	va_end(args);
> -	return packet_flush_gently(fd);
> -}
> -
>  static int packet_write_gently(const int fd_out, const char *buf, size_t size)
>  {
>  	static char packet_write_buffer[LARGE_PACKET_MAX];
> diff --git a/pkt-line.h b/pkt-line.h
> index 450183b64..66ef610fc 100644
> --- a/pkt-line.h
> +++ b/pkt-line.h
> @@ -25,8 +25,6 @@ void packet_buf_flush(struct strbuf *buf);
>  void packet_buf_write(struct strbuf *buf, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
>  int packet_flush_gently(int fd);
>  int packet_write_fmt_gently(int fd, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
> -LAST_ARG_MUST_BE_NULL
> -int packet_writel(int fd, const char *line, ...);
>  int write_packetized_from_fd(int fd_in, int fd_out);
>  int write_packetized_from_buf(const char *src_in, size_t len, int fd_out);
>  
> diff --git a/sub-process.c b/sub-process.c
> index a3cfab1a9..1a3f39bdf 100644
> --- a/sub-process.c
> +++ b/sub-process.c
> @@ -105,3 +105,97 @@ int subprocess_start(struct hashmap *hashmap, struct subprocess_entry *entry, co
>  	hashmap_add(hashmap, entry);
>  	return 0;
>  }
> +
> +int subprocess_handshake(struct subprocess_entry *entry,
> +			 const char *welcome_prefix,
> +			 int *versions,
> +			 int *chosen_version,
> +			 struct subprocess_capability *capabilities,
> +			 unsigned int *supported_capabilities) {
> +	int version_scratch;
> +	unsigned int capabilities_scratch;
> +	struct child_process *process = &entry->process;
> +	int i;
> +	char *line;
> +	const char *p;
> +
> +	if (!chosen_version)
> +		chosen_version = &version_scratch;
> +	if (!supported_capabilities)
> +		supported_capabilities = &capabilities_scratch;
> +
> +	sigchain_push(SIGPIPE, SIG_IGN);
> +
> +	if (packet_write_fmt_gently(process->in, "%sclient\n",
> +				    welcome_prefix)) {
> +		error("Could not write client identification");
> +		goto error;
> +	}
> +	for (i = 0; versions[i]; i++) {
> +		if (packet_write_fmt_gently(process->in, "version=%d\n",
> +					    versions[i])) {
> +			error("Could not write requested version");
> +			goto error;
> +		}
> +	}
> +	if (packet_flush_gently(process->in))
> +		goto error;
> +
> +	if (!(line = packet_read_line(process->out, NULL)) ||
> +	    !skip_prefix(line, welcome_prefix, &p) ||
> +	    strcmp(p, "server")) {
> +		error("Unexpected line '%s', expected %sserver",
> +		      line ? line : "<flush packet>", welcome_prefix);
> +		goto error;
> +	}
> +	if (!(line = packet_read_line(process->out, NULL)) ||
> +	    !skip_prefix(line, "version=", &p) ||
> +	    strtol_i(p, 10, chosen_version)) {
> +		error("Unexpected line '%s', expected version",
> +		      line ? line : "<flush packet>");
> +		goto error;
> +	}
> +	for (i = 0; versions[i]; i++) {
> +		if (versions[i] == *chosen_version)
> +			goto version_found;
> +	}
> +	error("Version %d not supported", *chosen_version);
> +	goto error;
> +version_found:
> +	if ((line = packet_read_line(process->out, NULL))) {
> +		error("Unexpected line '%s', expected flush", line);
> +		goto error;
> +	}
> +
> +	for (i = 0; capabilities[i].name; i++) {
> +		if (packet_write_fmt_gently(process->in, "capability=%s\n",
> +					    capabilities[i].name)) {
> +			error("Could not write requested capability");
> +			goto error;
> +		}
> +	}
> +	if (packet_flush_gently(process->in))
> +		goto error;
> +
> +	while ((line = packet_read_line(process->out, NULL))) {
> +		if (!skip_prefix(line, "capability=", &p))
> +			continue;
> +
> +		for (i = 0; capabilities[i].name; i++) {
> +			if (!strcmp(p, capabilities[i].name)) {
> +				*supported_capabilities |= capabilities[i].flag;
> +				goto capability_found;
> +			}
> +		}
> +		warning("external filter requested unsupported filter capability '%s'",
> +			p);
> +capability_found:
> +		;
> +	}

I'm not familiar with this section of code so I can't comment on what
the logic is doing.  But I think that this whole function can be written
better.  Its quite a lengthy function so you may benefit from breaking
it up into smaller functions to improve readability.  My biggest
complaint is the use of goto's to jump around the function, in and out
of loops and the like.  IMO its completely fine to have a 'goto error'
where you can do any necessary cleanup but it just makes things very
hard to follow when they are structured using gotos to break out of
loops or jump to other parts of the function as you have here.

It should be pretty easy to eliminate the two non-essential goto's by
making a helper function to determine if a version is found as well as
finding a capability.  That way you should be able to restructure the
code using if's instead of gotos.

> +
> +	sigchain_pop(SIGPIPE);
> +	return 0;
> +error:
> +	sigchain_pop(SIGPIPE);
> +	return 1;
> +}
> diff --git a/sub-process.h b/sub-process.h
> index 9e6975b5e..28863fabc 100644
> --- a/sub-process.h
> +++ b/sub-process.h
> @@ -29,6 +29,16 @@ struct subprocess_entry {
>  	struct child_process process;
>  };
>  
> +struct subprocess_capability {
> +	const char *name;
> +
> +	/*
> +	 * subprocess_handshake will "|=" this value to supported_capabilities
> +	 * if the server reports that it supports this capability.
> +	 */
> +	unsigned int flag;
> +};
> +
>  /* subprocess functions */
>  
>  /* Function to test two subprocess hashmap entries for equality. */
> @@ -62,6 +72,22 @@ static inline struct child_process *subprocess_get_child_process(
>  	return &entry->process;
>  }
>  
> +/*
> + * Perform the version and capability negotiation as described in the "Long
> + * Running Filter Process" section of the gitattributes documentation using the
> + * given requested versions and capabilities. The "versions" and "capabilities"
> + * parameters are arrays terminated by a 0 or blank struct.
> + *
> + * This function is typically called when a subprocess is started (as part of
> + * the "startfn" passed to subprocess_start).
> + */
> +int subprocess_handshake(struct subprocess_entry *entry,
> +			 const char *welcome_prefix,
> +			 int *versions,
> +			 int *chosen_version,
> +			 struct subprocess_capability *capabilities,
> +			 unsigned int *supported_capabilities);
> +
>  /*
>   * Helper function that will read packets looking for "status=<foo>"
>   * key/value pairs and return the value from the last "status" packet
> diff --git a/t/t0021-conversion.sh b/t/t0021-conversion.sh
> index 161f56044..d191a7228 100755
> --- a/t/t0021-conversion.sh
> +++ b/t/t0021-conversion.sh
> @@ -697,7 +697,7 @@ test_expect_success PERL 'invalid process filter must fail (and not hang!)' '
>  
>  		cp "$TEST_ROOT/test.o" test.r &&
>  		test_must_fail git add . 2>git-stderr.log &&
> -		grep "does not support filter protocol version" git-stderr.log
> +		grep "expected git-filter-server" git-stderr.log
>  	)
>  '
>  
> -- 
> 2.14.0.rc0.400.g1c36432dff-goog
> 

-- 
Brandon Williams