(+cc: Sam)
Hi,
David Barr wrote:
> 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.
Yes, please!
> Cc: Shawn O. Pearce <spearce@xxxxxxxxxxx>
> Cc: Ramkumar Ramachandra <artagnon@xxxxxxxxx>
It turns out these Cc tags are not supposed to be used except in
some very weird circumstances. See [1] if curious.
[...]
> --- 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`.
Sounds good.
> ++
> +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.
This is stale explanation from --report-fd, I think, to explain
why the commit ids it printed were not very useful. It would
be possible to reword it to describe cat-blob-fd but since the
frontend does not have easy access to blob names as it is, I
think cat-blob motivates itself on its own.
[...]
> @@ -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`
> +~~~~~
~~~~~~~~~~
> @@ -896,6 +929,7 @@ The following features are currently supported:
> * date-format
> * import-marks
> * export-marks
> +* cat-blob
The explanation says (paraphrased) "Features work identically to their
option counterparts, with the exception of import-marks as described
below".
Maybe ought to be reworded?
date-format::
export-marks::
relative-marks::
no-relative-marks::
force::
See the corresponding command-line option.
import-marks::
Like --import-marks, except in two respects. First, only one
"feature import-marks" command is allowed per stream. Second,
an --import-marks= specified on the command line will override it.
cat-blob::
No-op to check that the importer supports the cat-blob command.
By the way, it might be nice to make cat-blob not just check the importer
but the environment in which it was invoked, like this:
exporter says:
feature this
feature that
feature cat-blob
feature another
...
importer says:
feature cat-blob
That is, after writing "feature cat-blob\n", an exporter could tell if
the backchannel was set up correctly by reading for "feature cat-blob\n"
from the importer.
> --- a/fast-import.c
> +++ b/fast-import.c
> @@ -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");
> +}
An odd operation, since if the pipe_buf gets filled then it blocks
until the exporter finds time to read. Maybe in some future version
this would write to a private ring buffer and there would be an
event loop or seperate thread to flush it out when the exporter is
ready.
Upshot: I am happy with this as a separate function.
[...]
> @@ -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 */
Maybe if (!strcmp(...?
[...]
> @@ -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);
> + }
> +
> if (parse_one_option(a + 2))
> continue;
Probably worth mentioning in the manual, under the option command:
The following command-line option describes the environment
in which fast-import was executed and may not be passed to
'option':
* cat-blob-fd
> @@ -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();
Thanks. That was simple. :)
Tests?
[1] http://thread.gmane.org/gmane.comp.version-control.git/157711
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