builtin/cat-file.c: support NUL-delimited input with `-z`

When callers are using `cat-file` via one of the stdin-driven `--batch`
modes, all input is newline-delimited. This presents a problem when
callers wish to ask about, e.g. tree-entries that have a newline
character present in their filename.

To support this niche scenario, introduce a new `-z` mode to the
`--batch`, `--batch-check`, and `--batch-command` suite of options that
instructs `cat-file` to treat its input as NUL-delimited, allowing the
individual commands themselves to have newlines present.

The refactoring here is slightly unfortunate, since we turn loops like:

    while (strbuf_getline(&buf, stdin) != EOF)

into:

    while (1) {
        int ret;
        if (opt->nul_terminated)
            ret = strbuf_getline_nul(&input, stdin);
        else
            ret = strbuf_getline(&input, stdin);

        if (ret == EOF)
            break;
    }

It's tempting to think that we could use `strbuf_getwholeline()` and
specify either `\n` or `\0` as the terminating character. But for input
on platforms that include a CR character preceeding the LF, this
wouldn't quite be the same, since `strbuf_getline(...)` will trim any
trailing CR, while `strbuf_getwholeline(&buf, stdin, '\n')` will not.

In the future, we could clean this up further by introducing a variant
of `strbuf_getwholeline()` that addresses the aforementioned gap, but
that approach felt too heavy-handed for this pair of uses.

Some tests are added in t1006 to ensure that `cat-file` produces the
same output in `--batch`, `--batch-check`, and `--batch-command` modes
with and without the new `-z` option.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/Documentation/git-cat-file.txt b/Documentation/git-cat-file.txt
index 24a811f0ef..3515350ed6 100644
--- a/Documentation/git-cat-file.txt
+++ b/Documentation/git-cat-file.txt
@@ -14,7 +14,7 @@ SYNOPSIS
 'git cat-file' (-t | -s) [--allow-unknown-type] <object>
 'git cat-file' (--batch | --batch-check | --batch-command) [--batch-all-objects]
 	     [--buffer] [--follow-symlinks] [--unordered]
-	     [--textconv | --filters]
+	     [--textconv | --filters] [-z]
 'git cat-file' (--textconv | --filters)
 	     [<rev>:<path|tree-ish> | --path=<path|tree-ish> <rev>]
 
@@ -207,6 +207,11 @@ respectively print:
 	/etc/passwd
 --
 
+-z::
+	Only meaningful with `--batch`, `--batch-check`, or
+	`--batch-command`; input is NUL-delimited instead of
+	newline-delimited.
+
 
 OUTPUT
 ------