Resending this as my last message accidentally included some html and got rejected by the listserv


On 27 Jan 2022, at 16:04, John Cai wrote:

Hey Phillip,

First of all thank you for the thorough review—it really helps someone who’s learning how to contribute!

On 27 Jan 2022, at 6:25, Phillip Wood wrote:

quoted

Hi John

On 25/01/2022 22:50, John Cai wrote:

quoted

This RFC patch proposes a new flag --stdin-cmd that works with
git-cat-file --batch. Similar to git-update-ref --stdin, it will accept
commands and arguments from stdin.

The start of this idea was discussed in [1], where the original
motivation was to be able to control when the buffer was flushed to
stdout in --buffer mode.

However, this can actually be much more useful in situations when
git-cat-file --batch is being used as a long lived backend query
process. At GitLab, we use a pair of cat-file processes. One for
iterating over object metadata with --batch-check, and the other to grab
object contents with --batch. However, if we had --stdin-cmd, we could
get rid of the second --batch-check process, and just have one progress
where we can flip between getting object info, and getting object contents.
This can lead to huge savings.

git cat-file --batch --stdin-cmd

$ <command> [arg1] [arg2] NL

We can also add a -z mode to allow for NUL-terminated lines

$ <command> [arg1] [arg2] NUL

This patch adds three commands: object, info, fflush

$ object <sha1> NL
$ info <sha1> NL
$ fflush NL

These three would be immediately useful in GitLab's context, but one can
imagine this mode to be further extended for other things.

For instance, a non-trivial part of "cat-file --batch" time is spent
on parsing its argument and seeing if it's a revision, ref etc. So we
could add a command that only accepts a full-length 40
character SHA-1.

This would be the first step in adding such an interface to
git-cat-file.

[1] https://lore.kernel.org/git/pull.1124.git.git.1636149400.gitgitgadget@gmail.com/ (local)

Helped-by: Ævar Arnfjörð Bjarmason [off-list ref]
Signed-off-by: John Cai <redacted>
---
Changes from v1:

- changed option name to batch-command.
- changed command function interface to receive the whole line after the command
   name to put the onus of parsing arguments to each individual command function.
- pass in whole line to batch_one_object in both parse_cmd_object and
   parse_cmd_info to support spaces in the object reference.
- removed addition of -z to include in a separate patch series
- added documentation.

I've left some comments below, they're mostly small details, I like the new option name and the changes you've made to the command parsing.

quoted

---
  Documentation/git-cat-file.txt |  15 +++++
  builtin/cat-file.c             | 114 ++++++++++++++++++++++++++++++++-
  strvec.c                       |  23 +++++++
  strvec.h                       |   8 +++
  t/t1006-cat-file.sh            |  32 +++++++++
  5 files changed, 191 insertions(+), 1 deletion(-)

diff --git a/Documentation/git-cat-file.txt b/Documentation/git-cat-file.txt
index bef76f4dd0..8aefa45e4c 100644
--- a/Documentation/git-cat-file.txt
+++ b/Documentation/git-cat-file.txt

@@ -96,6 +96,21 @@ OPTIONS
  	need to specify the path, separated by whitespace.  See the
  	section `BATCH OUTPUT` below for details.
 +-batch-command::

is this missing a '-'?

quoted

+	Enter a command mode that reads from stdin. May not be combined with any
+	other options or arguments except `--textconv` or `--filters`, in which
+	case the input lines also need to specify the path, separated by
+	whitespace.  See the section `BATCH OUTPUT` below for details.
+
+object <object>::
+	Print object contents for object reference <object>
+
+info <object>::
+	Print object info for object reference <object>
+
+flush::
+	Flush to stdout immediately when used with --buffer
+
  --batch-all-objects::
  	Instead of reading a list of objects on stdin, perform the
  	requested batch operation on all objects in the repository and

diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index 7b3f42950e..30794284d5 100644
--- a/builtin/cat-file.c
+++ b/builtin/cat-file.c

@@ -16,6 +16,7 @@
  #include "packfile.h"
  #include "object-store.h"
  #include "promisor-remote.h"
+#include "strvec.h"
   struct batch_options {
  	int enabled;

@@ -26,7 +27,10 @@ struct batch_options {
  	int unordered;
  	int cmdmode; /* may be 'w' or 'c' for --filters or --textconv */
  	const char *format;
+	int stdin_cmd;

Now that the option has been renamed it would be nice to rename the corresponding variable to match

I was trying to find a good name. There is already a cmdmode variable. I’m thinking stdin_cmd is not such a
bad name since we are receiving commands from stdin. suggestions welcome!

quoted

quoted

+	int end_null;

If you're not adding '-z' here then you don't need this or the addition below.

quoted

  };
+static char line_termination = '\n';
   static const char *force_path;
 @@ -508,6 +512,102 @@ static int batch_unordered_packed(const struct object_id *oid,
  				      data);
  }
 +enum batch_state {
+	/* Non-transactional state open for commands. */
+	BATCH_STATE_OPEN,
+};

I forgot to ask what the idea behind the batch state is last time - what's it for?

This is to support transactional semantics for commands we want to support in the future, but
since this is already a biggish change, we can leave this out of this series.

quoted

quoted

+static void parse_cmd_object(struct batch_options *opt,
+			     const char *line,
+			     struct strbuf *output,
+			     struct expand_data *data)
+{
+	opt->print_contents = 1;
+	batch_one_object(line, output, opt, data);
+}
+
+static void parse_cmd_info(struct batch_options *opt,
+			   const char *line,
+			   struct strbuf *output,
+			   struct expand_data *data)
+{
+	opt->print_contents = 0;
+	batch_one_object(line, output, opt, data);
+}
+
+static void parse_cmd_fflush(struct batch_options *opt,
+			     const char *line,
+			     struct strbuf *output,
+			     struct expand_data *data)
+{
+	fflush(stdout);
+}
+
+typedef void (*parse_cmd_fn_t)(struct batch_options *, const char *,
+			       struct strbuf *, struct expand_data *);
+
+static const struct parse_cmd {
+	const char *prefix;
+	parse_cmd_fn_t fn;
+	unsigned args;

This is now a flag so maybe 'takes_args' would better describe its purpose.

quoted

+	enum batch_state state;
+} commands[] = {
+	{ "object", parse_cmd_object, 1, BATCH_STATE_OPEN },
+	{ "info", parse_cmd_info, 1, BATCH_STATE_OPEN },
+	{ "fflush", parse_cmd_fflush, 0, BATCH_STATE_OPEN },
+};
+
+static void batch_objects_stdin_cmd(struct batch_options *opt,
+				    struct strbuf *output,
+				    struct expand_data *data)
+{
+	struct strbuf input = STRBUF_INIT;
+	enum batch_state state = BATCH_STATE_OPEN;
+
+	/* Read each line dispatch its command */
+	while (!strbuf_getwholeline(&input, stdin, line_termination)) {
+		int i;
+		const struct parse_cmd *cmd = NULL;
+		const char *p;
+
+		if (*input.buf == line_termination)
+			die("empty command in input");
+		else if (isspace(*input.buf))
+			die("whitespace before command: %s", input.buf);
+
+		for (i = 0; i < ARRAY_SIZE(commands); i++) {
+			const char *prefix = commands[i].prefix;
+			char c;
+			const char *cmd_name;

skip_prefix() sets this to the end of the name so maybe 'cmd_end' would be clearer?

quoted

+			if (!skip_prefix(input.buf, prefix, &cmd_name))
+				continue;
+			/*
+			 * If the command has arguments, verify that it's
+			 * followed by a space. Otherwise, it shall be followed
+			 * by a line terminator.
+			 */
+			c = commands[i].args ? ' ' : line_termination;
+			if (input.buf[strlen(prefix)] != c)

Now that you're using skip_prefix() you can write this as
    		if (*cmd_end != c)

quoted

+				continue;
+
+			cmd = &commands[i];
+			break;
+		}
+		if (!cmd)
+			die("unknown command: %s", input.buf);
+
+		p = input.buf + strlen(cmd->prefix) + 1;

This can be simplified to
    	p = cmd_end + 1;

quoted

+		const char *pos = strstr(p, &line_termination);

This isn't needed without '-z'. If it were required then using strchrnul() would prevent a NULL pointer dereference when the last input line does not end with a terminator. I think we typically call a pointer to the end of the line 'eol' or 'end'. Also variables should be declared at the top of the function.

quoted

+		switch (state) {
+		case BATCH_STATE_OPEN:
+			break;
+		}
+		cmd->fn(opt, xstrndup(p, pos-p), output, data);

Is there a reason this is passing a copy of the string?

quoted

+	}
+	strbuf_release(&input);
+}
+
  static int batch_objects(struct batch_options *opt)
  {
  	struct strbuf input = STRBUF_INIT;

@@ -515,6 +615,7 @@ static int batch_objects(struct batch_options *opt)
  	struct expand_data data;
  	int save_warning;
  	int retval = 0;
+	const int stdin_cmd = opt->stdin_cmd;
   	if (!opt->format)
  		opt->format = "%(objectname) %(objecttype) %(objectsize)";

@@ -590,7 +691,8 @@ static int batch_objects(struct batch_options *opt)
  	save_warning = warn_on_object_refname_ambiguity;
  	warn_on_object_refname_ambiguity = 0;
 -	while (strbuf_getline(&input, stdin) != EOF) {
+	while (!stdin_cmd &&

If you moved the 'if (stdin_cmd)' block above this block we could loose this change. I'm not sure if that is possible without looking at the whole function though.

quoted

+	       strbuf_getline(&input, stdin) != EOF) {
  		if (data.split_on_whitespace) {
  			/*
  			 * Split at first whitespace, tying off the beginning

@@ -608,6 +710,9 @@ static int batch_objects(struct batch_options *opt)
  		batch_one_object(input.buf, &output, opt, &data);
  	}
 +	if (stdin_cmd)
+		batch_objects_stdin_cmd(opt, &output, &data);
+
  	strbuf_release(&input);
  	strbuf_release(&output);
  	warn_on_object_refname_ambiguity = save_warning;

@@ -636,6 +741,7 @@ static int batch_option_callback(const struct option *opt,
   	bo->enabled = 1;
  	bo->print_contents = !strcmp(opt->long_name, "batch");
+	bo->stdin_cmd = !strcmp(opt->long_name, "batch-command");
  	bo->format = arg;
   	return 0;

@@ -683,6 +789,10 @@ int cmd_cat_file(int argc, const char **argv, const char *prefix)
  			N_("like --batch, but don't emit <contents>"),
  			PARSE_OPT_OPTARG | PARSE_OPT_NONEG,
  			batch_option_callback),
+		OPT_CALLBACK_F(0, "batch-command", &batch, N_(""),
+			 N_("enters batch mode that accepts commands"),
+			 PARSE_OPT_NOARG | PARSE_OPT_NONEG,
+			 batch_option_callback),
  		OPT_CMDMODE(0, "batch-all-objects", &opt,
  			    N_("with --batch[-check]: ignores stdin, batches all known objects"), 'b'),
  		/* Batch-specific options */

@@ -738,6 +848,8 @@ int cmd_cat_file(int argc, const char **argv, const char *prefix)
  	/* Batch defaults */
  	if (batch.buffer_output < 0)
  		batch.buffer_output = batch.all_objects;
+	if (batch.end_null)
+		line_termination = '\0';
   	/* Return early if we're in batch mode? */
  	if (batch.enabled) {

diff --git a/strvec.c b/strvec.c
index 61a76ce6cb..7dca04bf7a 100644
--- a/strvec.c
+++ b/strvec.c

[...]

We don't need any strvec changes now that we don't split the input lines to --bactch-command

quoted

+F='%s\0'

This isn't used now

quoted

+test_expect_success 'batch-command unknown command' '
+	echo unknown_command >cmd &&
+	test_expect_code 128 git cat-file --batch-command < cmd 2>err &&
+	grep -E "^fatal:.*unknown command.*" err
+'
+
+test_expect_success 'setup object data' '
+	content="Object Data" &&
+	size=$(strlen "$content") &&
+	sha1=$(echo_without_newline "$content" | git hash-object -w --stdin)
+'
+
+test_expect_success 'batch-command calling object works' '
+	echo "object $sha1" | git cat-file --batch-command >actual &&
+	echo "$sha1 blob $size" >expect &&
+	echo `git cat-file -p "$sha1"` >>expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'batch-command calling info works' '
+	echo "info $sha1" | git cat-file --batch-command >actual &&
+	echo "$sha1 blob $size" >expect &&
+	test_cmp expect actual
+'

I had a quick look at this test file and there is a loop at the top that runs some --batch tests on various inputs, I wonder if these two tests could go in there.

quoted

+test_expect_success 'batch-command fflush works' '
+	printf "fflush\n" > cmd &&
+	test_expect_code 0 git cat-file --batch-command < cmd 2>err
+'

It'd be nice to check this actually flushes the output.

could you give me some ideas on how to do this?

Ok, after filling around I think it can be something like this (copied over from an example in t9300-fast-import.sh

run_buffer_test () {
	type=$1
	sha1=$2
	size=$3

	mkfifo V.input
	exec 8<>V.input
	rm V.input

	mkfifo V.output
	exec 9<>V.output
	rm V.output
	
	(
		git cat-file --buffer --batch-command <&8 >&9 &
		echo $! >&9 &&
		wait $!
	) &
	sh_pid=$!
	read fi_pid <&9
	test_when_finished "
		exec 8>&-; exec 9>&-;
		kill $sh_pid && wait $sh_pid
		kill $fi_pid && wait $fi_pid
		true"
	expect=$(echo "$sha1 $type $size")
	echo "info $sha1" >&8
	echo "fflush" >&8
	read actual <&9
	test "$actual" = "$expect"
}

Not sure if there's a simpler way since we need to simulate writing to and reading from the process.

Thanks

quoted

Best Wishes

Phillip

quoted

  test_done