Subject: Re: [PATCH 1/5] fast-import: Let importers retrieve blobs

From: Ramkumar Ramachandra <[email-masked]>
Date: Mon, 18 Oct 2010 13:06:08 +0530

Hi David,

David Barr writes:
> As the description of the "progress" option in git-fast-import.1
> hints, there is no convenient way to immediately access the blobs
> written to a new repository through fast-import.  Until a checkpoint
> has been started and finishes writing the pack index, any new blobs
> will not be accessible using standard git tools.
> 
> So introduce another way: a "cat-blob" command introduced in the
> command stream requests for fast-import to print a blob to stdout
> or a file descriptor specified by the argument --cat-blob-fd.
> 
> The output uses the same format as "git cat-file --batch".

Nice :) It looks like we finally have a nice polished version.
Caution: Most of the review are just notes to self, or style
nitpicks. Feel free to ignore.

> Cc: Shawn O. Pearce <[email-masked]>
> Cc: Ramkumar Ramachandra <[email-masked]>
> Helped-by: Sverre Rabbelier <[email-masked]>
> Based-on-patch-by: Jonathan Nieder <[email-masked]>
> Signed-off-by: David Barr <[email-masked]>
> ---
>  Documentation/git-fast-import.txt |   34 ++++++++++++++
>  fast-import.c                     |   92 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 126 insertions(+), 0 deletions(-)
> 
> diff --git a/Documentation/git-fast-import.txt b/Documentation/git-fast-import.txt
> index 966ba4f..42a100b 100644
> --- a/Documentation/git-fast-import.txt
> +++ b/Documentation/git-fast-import.txt
> @@ -92,6 +92,17 @@ OPTIONS
>  	--(no-)-relative-marks= with the --(import|export)-marks=
>  	options.
>  
> +--cat-blob-fd=<fd>::
> +	Specify the file descriptor that will be written to
> +	when the `cat-blob` command is encountered in the stream.
> +	The default behaviour is to write to `stdout`.
> ++
> +The described objects are not necessarily accessible
> +using standard git plumbing tools until a little while
> +after the next checkpoint.  To request access to the
> +blobs before then, use `cat-blob` lines in the command
> +stream.
> +
>  --export-pack-edges=<file>::
>  	After creating a packfile, print a line of data to
>  	<file> listing the filename of the packfile and the last
> @@ -320,6 +331,11 @@ and control the current import process.  More detailed discussion
>  	standard output.  This command is optional and is not needed
>  	to perform an import.
>  
> +`cat-blob`::
> +	Causes fast-import to print a blob in 'cat-file --batch'
> +	format to the file descriptor set with `--cat-blob-fd` or
> +	`stdout` if unspecified.
> +
>  `feature`::
>  	Require that fast-import supports the specified feature, or
>  	abort if it does not.
> @@ -876,6 +892,23 @@ Placing a `progress` command immediately after a `checkpoint` will
>  inform the reader when the `checkpoint` has been completed and it
>  can safely access the refs that fast-import updated.
>  
> +`cat-blob`
> +~~~~~
> +Causes fast-import to print a blob to a file descriptor previously
> +arranged with the `--cat-blob-fd` argument.  The command otherwise
> +has no impact on the current import; its main purpose is to
> +retrieve blobs that may be in fast-import's memory but not
> +accessible from the target repository a little quicker than by the
> +method suggested by the description of the `progress` option.
> +
> +....
> +	'cat-blob' SP <dataref> LF
> +....
> +
> +The `<dataref>` can be either a mark reference (`:<idnum>`)
> +set previously, or a full 40-byte SHA-1 of any Git blob,
> +preexisting or ready to be written.
> +
>  `feature`
>  ~~~~~~~~~
>  Require that fast-import supports the specified feature, or abort if
> @@ -896,6 +929,7 @@ The following features are currently supported:
>  * date-format
>  * import-marks
>  * export-marks
> +* cat-blob
>  * relative-marks
>  * no-relative-marks
>  * force

Looks perfect.

> diff --git a/fast-import.c b/fast-import.c
> index 2317b0f..ea3e529 100644
> --- a/fast-import.c
> +++ b/fast-import.c
> @@ -55,6 +55,8 @@ Format of STDIN stream:
>      ('from' sp committish lf)?
>      lf?;
>  
> +  cat_blob ::= 'cat-blob' sp (hexsha1 | idnum) lf;
> +
>    checkpoint ::= 'checkpoint' lf
>      lf?;
>  
> @@ -361,6 +363,9 @@ static uintmax_t next_mark;
>  static struct strbuf new_data = STRBUF_INIT;
>  static int seen_data_command;
>  
> +/* Where to write output of cat-blob commands */
> +static int cat_blob_fd = 1;
> +

Right. Defaults to stdout, as described in documentation.

>  static void parse_argv(void);
>  
>  static void write_branch_report(FILE *rpt, struct branch *b)
> @@ -2680,6 +2685,77 @@ static void parse_reset_branch(void)
>  		unread_command_buf = 1;
>  }
>  
> +static void cat_blob_write(const char *buf, unsigned long size)
> +{
> +	if (write_in_full(cat_blob_fd, buf, size) != size)
> +		die_errno("Write to frontend failed");
> +}

Note to self: I didn't notice write_in_full in wrapper.c
earlier. Returns the actual size written to the fd from the buffer.

> +static void cat_blob(struct object_entry *oe, unsigned char sha1[20])
> +{
> +	struct strbuf line = STRBUF_INIT;
> +	unsigned long size;
> +	enum object_type type = 0;
> +	char *buf;
> +
> +	if (oe && oe->pack_id != MAX_PACK_ID) {
> +		type = oe->type;
> +		buf = gfi_unpack_entry(oe, &size);
> +	} else {
> +		buf = read_sha1_file(sha1, &type, &size);
> +	}
> +	if (!buf)
> +		die("Can't read object %s", sha1_to_hex(sha1));

Ok. Looks similar enough to the code in the other functions.

> +
> +	/*
> +	 * Output based on batch_one_object() from cat-file.c.
> +	 */
> +	if (type <= 0) {
> +		strbuf_reset(&line);
> +		strbuf_addf(&line, "%s missing\n", sha1_to_hex(sha1));
> +		cat_blob_write(line.buf, line.len);
> +		return;
> +	} else if (type != OBJ_BLOB) {
> +		die("Object %s is a %s but a blob was expected.",
> +		    sha1_to_hex(sha1), typename(type));
> +	}
> +	strbuf_reset(&line);
> +	strbuf_addf(&line, "%s %s %lu\n", sha1_to_hex(sha1),
> +						typename(type), size);
> +	cat_blob_write(line.buf, line.len);
> +	cat_blob_write(buf, size);
> +	cat_blob_write("\n", 1);
> +	free(buf);

Cool.

> +}
> +
> +
> +static void parse_cat_blob(void)
> +{
> +	const char *p;
> +	struct object_entry *oe = oe;
> +	unsigned char sha1[20];
> +
> +	/* cat SP <object> */
> +	p = command_buf.buf + strlen("cat-blob ");
> +	if (*p == ':') {
> +		char *x;
> +		oe = find_mark(strtoumax(p + 1, &x, 10));
> +		if (x == p + 1)
> +			die("Invalid mark: %s", command_buf.buf);
> +		if (!oe)
> +			die("Unknown mark: %s", command_buf.buf);
> +		p = x;
> +		hashcpy(sha1, oe->idx.sha1);

The mark case.

> +	} else {
> +		if (get_sha1_hex(p, sha1))
> +			die("Invalid SHA1: %s", command_buf.buf);
> +		p += 40;
> +		oe = find_object(sha1);
> +	}

The SHA1 case.
Looks similar enough to the code in the other functions.

> +
> +	cat_blob(oe, sha1);
> +}
> +
>  static void parse_checkpoint(void)
>  {
>  	if (object_count) {
> @@ -2755,6 +2831,14 @@ static void option_export_marks(const char *marks)
>  	safe_create_leading_directories_const(export_marks_file);
>  }
>  
> +static void option_cat_blob_fd(const char *fd)
> +{
> +	unsigned long n = strtoul(fd, NULL, 0);
> +	if (n > (unsigned long) INT_MAX)
> +		die("--cat-blob-fd cannot exceed %d", INT_MAX);
> +	cat_blob_fd = (int) n;
> +}
> +

You don't display an appropriate error when n < 0.

>  static void option_export_pack_edges(const char *edges)
>  {
>  	if (pack_edges)
> @@ -2808,6 +2892,8 @@ static int parse_one_feature(const char *feature, int from_stream)
>  		option_import_marks(feature + 13, from_stream);
>  	} else if (!prefixcmp(feature, "export-marks=")) {
>  		option_export_marks(feature + 13);
> +	} else if (!prefixcmp(feature, "cat-blob")) {
> +		/* Don't die - this feature is supported */

Style nit: There are no statements in this branch. Maybe put a
semicolon?

>  	} else if (!prefixcmp(feature, "relative-marks")) {
>  		relative_marks_paths = 1;
>  	} else if (!prefixcmp(feature, "no-relative-marks")) {
> @@ -2896,6 +2982,10 @@ static void parse_argv(void)
>  		if (*a != '-' || !strcmp(a, "--"))
>  			break;
>  
> +		if (!prefixcmp(a + 2, "cat-blob-fd=")) {
> +			option_cat_blob_fd(a + 2 + 12);
> +		}
> +

Style nit: Unnecessary braces around `if` statement.

>  		if (parse_one_option(a + 2))
>  			continue;
>  
> @@ -2953,6 +3043,8 @@ int main(int argc, const char **argv)
>  			parse_new_tag();
>  		else if (!prefixcmp(command_buf.buf, "reset "))
>  			parse_reset_branch();
> +		else if (!prefixcmp(command_buf.buf, "cat-blob "))
> +			parse_cat_blob();
>  		else if (!strcmp("checkpoint", command_buf.buf))
>  			parse_checkpoint();
>  		else if (!prefixcmp(command_buf.buf, "progress "))
> -- 
> 1.7.3.32.g634ef
> 

Ok. I'm eager to see this go through to `master`.
Reviewed-by: Ramkumar Ramachandra <[email-masked]>

-- Ram