fast-import: add 'ls' command

Lazy fast-import frontend authors that want to rely on the backend to
keep track of the content of the imported trees _almost_ have what
they need in the 'cat-blob' command (v1.7.4-rc0~30^2~3, 2010-11-28).
But it is not quite enough, since

 (1) cat-blob can be used to retrieve the content of files, but
     not their mode, and

 (2) using cat-blob requires the frontend to keep track of a name
     (mark number or object id) for each blob to be retrieved

Introduce an 'ls' command to complement cat-blob and take care of the
remaining needs.  The 'ls' command finds what is at a given path
within a given tree-ish (tag, commit, or tree):

	'ls' SP <dataref> SP <path> LF

or in fast-import's active commit:

	'ls' SP <path> LF

The response is a single line sent through the cat-blob channel,
imitating ls-tree output.  So for example:

	FE> ls :1 Documentation
	gfi> 040000 tree 9e6c2b599341d28a2a375f8207507e0a2a627fe9	Documentation
	FE> ls 9e6c2b599341d28a2a375f8207507e0a2a627fe9 git-fast-import.txt
	gfi> 100644 blob 4f92954396e3f0f97e75b6838a5635b583708870	git-fast-import.txt
	FE> ls :1 RelNotes
	gfi> 120000 blob b942e499449d97aeb50c73ca2bdc1c6e6d528743	RelNotes
	FE> cat-blob b942e499449d97aeb50c73ca2bdc1c6e6d528743
	gfi> b942e499449d97aeb50c73ca2bdc1c6e6d528743 blob 32
	gfi> Documentation/RelNotes/1.7.4.txt

The most interesting parts of the reply are the first word, which is
a 6-digit octal mode (regular file, executable, symlink, directory,
or submodule), and the part from the second space to the tab, which is
a <dataref> that can be used in later cat-blob, ls, and filemodify (M)
commands to refer to the content (blob, tree, or commit) at that path.

If there is nothing there, the response is "missing some/path".

The intent is for this command to be used to read files from the
active commit, so a frontend can apply patches to them, and to copy
files and directories from previous revisions.

For example, proposed updates to svn-fe use this command in place of
its internal representation of the repository directory structure.
This simplifies the frontend a great deal and means support for
resuming an import in a separate fast-import run (i.e., incremental
import) is basically free.

Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Improved-by: Junio C Hamano <gitster@pobox.com>
Improved-by: Sverre Rabbelier <srabbelier@gmail.com>

1 files changed, 60 insertions, 3 deletions

diff --git a/Documentation/git-fast-import.txt b/Documentation/git-fast-import.txt
index c3a2766b20..e1b7a0f9ec 100644
--- a/Documentation/git-fast-import.txt
+++ b/Documentation/git-fast-import.txt
@@ -196,7 +196,8 @@ especially when a higher level language such as Perl, Python or
 Ruby is being used.
 
 fast-import is very strict about its input.  Where we say SP below we mean
-*exactly* one space.  Likewise LF means one (and only one) linefeed.
+*exactly* one space.  Likewise LF means one (and only one) linefeed
+and HT one (and only one) horizontal tab.
 Supplying additional whitespace characters will cause unexpected
 results, such as branch names or file names with leading or trailing
 spaces in their name, or early termination of fast-import when it encounters
@@ -334,6 +335,11 @@ and control the current import process.  More detailed discussion
 	format to the file descriptor set with `--cat-blob-fd` or
 	`stdout` if unspecified.
 
+`ls`::
+	Causes fast-import to print a line describing a directory
+	entry in 'ls-tree' 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.
@@ -919,6 +925,55 @@ This command can be used anywhere in the stream that comments are
 accepted.  In particular, the `cat-blob` command can be used in the
 middle of a commit but not in the middle of a `data` command.
 
+`ls`
+~~~~
+Prints information about the object at a path to a file descriptor
+previously arranged with the `--cat-blob-fd` argument.  This allows
+printing a blob from the active commit (with `cat-blob`) or copying a
+blob or tree from a previous commit for use in the current one (with
+`filemodify`).
+
+The `ls` command can be used anywhere in the stream that comments are
+accepted, including the middle of a commit.
+
+Reading from the active commit::
+	This form can only be used in the middle of a `commit`.
+	The path names a directory entry within fast-import's
+	active commit.  The path must be quoted in this case.
++
+....
+	'ls' SP <path> LF
+....
+
+Reading from a named tree::
+	The `<dataref>` can be a mark reference (`:<idnum>`) or the
+	full 40-byte SHA-1 of a Git tag, commit, or tree object,
+	preexisting or waiting to be written.
+	The path is relative to the top level of the tree
+	named by `<dataref>`.
++
+....
+	'ls' SP <dataref> SP <path> LF
+....
+
+See `filemodify` above for a detailed description of `<path>`.
+
+Output uses the same format as `git ls-tree <tree> {litdd} <path>`:
+
+====
+	<mode> SP ('blob' | 'tree' | 'commit') SP <dataref> HT <path> LF
+====
+
+The <dataref> represents the blob, tree, or commit object at <path>
+and can be used in later 'cat-blob', 'filemodify', or 'ls' commands.
+
+If there is no file or subtree at that path, 'git fast-import' will
+instead report
+
+====
+	missing SP <path> LF
+====
+
 `feature`
 ~~~~~~~~~
 Require that fast-import supports the specified feature, or abort if
@@ -946,8 +1001,10 @@ import-marks::
 	any "feature import-marks" command in the stream.
 
 cat-blob::
-	Ignored.  Versions of fast-import not supporting the
-	"cat-blob" command will exit with a message indicating so.
+ls::
+	Require that the backend support the 'cat-blob' or 'ls' command.
+	Versions of fast-import not supporting the specified command
+	will exit with a message indicating so.
 	This lets the import error out early with a clear message,
 	rather than wasting time on the early part of an import
 	before the unsupported command is detected.