Subject: Re: [PATCH v7] ls-files: introduce "--format" option

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Mon, 18 Jul 2022 10:09:15 +0200

On Wed, Jul 13 2022, ZheNing Hu via GitGitGadget wrote:

> From: ZheNing Hu <[email-masked]>
>
> Add a new option "--format" that outputs index entries
> informations in a custom format, taking inspiration
> from the option with the same name in the `git ls-tree`
> command.
>
> "--format" cannot used with "-s", "-o", "-k", "-t",
> " --resolve-undo","--deduplicate" and "--eol".
>
> Signed-off-by: ZheNing Hu <[email-masked]>
> ---
>     ls-files: introduce "--format" options
>     
>     v6->v7:
>     
>      1. Change documents helped by Junio.
>      2. Fix bug of parsing format.
>      3. Add more test cases for other mode index entries (120000 and
>         160000).
>
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1262%2Fadlternative%2Fzh%2Fls-file-format-v7
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1262/adlternative/zh/ls-file-format-v7
> Pull-Request: https://github.com/gitgitgadget/git/pull/1262
>
> Range-diff vs v6:
>
>  1:  57ed2c15728 ! 1:  9ca22edba94 ls-files: introduce "--format" option
>      @@ Metadata
>        ## Commit message ##
>           ls-files: introduce "--format" option
>       
>      -    Add a new option --format that output index enties
>      -    informations with custom format, taking inspiration
>      +    Add a new option "--format" that outputs index entries
>      +    informations in a custom format, taking inspiration
>           from the option with the same name in the `git ls-tree`
>           command.
>       
>      -    --format cannot used with -s, -o, -k, -t, --resolve-undo,
>      -    --deduplicate and --eol.
>      +    "--format" cannot used with "-s", "-o", "-k", "-t",
>      +    " --resolve-undo","--deduplicate" and "--eol".
>       
>           Signed-off-by: ZheNing Hu <[email-masked]>
>       
>      @@ Documentation/git-ls-files.txt: quoted as explained for the configuration variab
>       +
>       +FIELD NAMES
>       +-----------
>      -+Various values from structured fields can be used to interpolate
>      -+into the resulting output. For each outputting line, the following
>      -+names can be used:
>      ++The way each path is shown can be customized by using the
>      ++`--format=<format>` option, where the %(fieldname) in the
>      ++<format> string for various aspects of the index entry are
>      ++interpolated.  The following "fieldname" are understood:
>       +
>       +objectmode::
>       +	The mode of the file which is recorded in the index.
>      @@ builtin/ls-files.c: static void write_name(const char *name)
>       +static void write_name_to_buf(struct strbuf *sb, const char *name)
>       +{
>       +	const char *rel = relative_path(name, prefix_len ? prefix : NULL, sb);
>      ++
>       +	if (line_terminator)
>       +		quote_c_style(rel, sb, NULL, 0);
>       +	else
>      -+		strbuf_add(sb, rel, strlen(rel));
>      ++		strbuf_addstr(sb, rel);
>       +}
>       +
>        static const char *get_tag(const struct cache_entry *ce, const char *tag)
>      @@ builtin/ls-files.c: static void show_submodule(struct repository *superproject,
>       +};
>       +
>       +static size_t expand_show_index(struct strbuf *sb, const char *start,
>      -+			       void *context)
>      ++				void *context)
>       +{
>       +	struct show_index_data *data = context;
>       +	const char *end;
>      @@ builtin/ls-files.c: static void show_submodule(struct repository *superproject,
>       +		strbuf_add_unique_abbrev(sb, &data->ce->oid, abbrev);
>       +	else if (skip_prefix(start, "(stage)", &p))
>       +		strbuf_addf(sb, "%d", ce_stage(data->ce));
>      -+	else if (skip_prefix(start, "(eolinfo:index)", &p) &&
>      -+		 S_ISREG(data->ce->ce_mode))
>      -+		strbuf_addstr(sb, get_cached_convert_stats_ascii(data->istate,
>      -+								 data->ce->name));
>      -+	else if (skip_prefix(start, "(eolinfo:worktree)", &p) &&
>      -+		 !lstat(data->pathname, &st) && S_ISREG(st.st_mode))
>      -+		strbuf_addstr(sb, get_wt_convert_stats_ascii(data->pathname));
>      ++	else if (skip_prefix(start, "(eolinfo:index)", &p))
>      ++		strbuf_addstr(sb, S_ISREG(data->ce->ce_mode) ?
>      ++			      get_cached_convert_stats_ascii(data->istate,
>      ++			      data->ce->name) : "");
>      ++	else if (skip_prefix(start, "(eolinfo:worktree)", &p))
>      ++		strbuf_addstr(sb, !lstat(data->pathname, &st) &&
>      ++			      S_ISREG(st.st_mode) ?
>      ++			      get_wt_convert_stats_ascii(data->pathname) : "");
>       +	else if (skip_prefix(start, "(eolattr)", &p))
>       +		strbuf_addstr(sb, get_convert_attr_ascii(data->istate,
>       +			      data->pathname));
>      @@ t/t3013-ls-files-format.sh (new)
>       +done
>       +
>       +test_expect_success 'setup' '
>      -+	echo o1 >o1 &&
>      -+	echo o2 >o2 &&
>      -+	git add o1 o2 &&
>      -+	git add --chmod +x o1 &&
>      ++	printf "LINEONE\nLINETWO\nLINETHREE\n" >o1.txt &&
>      ++	printf "LINEONE\r\nLINETWO\r\nLINETHREE\r\n" >o2.txt &&
>      ++	printf "LINEONE\r\nLINETWO\nLINETHREE\n" >o3.txt &&
>      ++	ln -s o3.txt o4.txt &&
>      ++	git add "*.txt" &&
>      ++	git add --chmod +x o1.txt &&
>      ++	git update-index --add --cacheinfo 160000 $(git hash-object o1.txt) o5.txt &&
>       +	git commit -m base
>       +'
>       +
>      -+test_expect_success 'git ls-files --format objectmode' '
>      -+	cat >expect <<-\EOF &&
>      -+	100755
>      -+	100644
>      -+	EOF
>      ++test_expect_success 'git ls-files --format objectmode v.s. -s' '
>      ++	git ls-files -s | awk "{print \$1}" >expect &&
>       +	git ls-files --format="%(objectmode)" >actual &&
>       +	test_cmp expect actual
>       +'
>       +
>      -+test_expect_success 'git ls-files --format objectname' '
>      -+	oid1=$(git hash-object o1) &&
>      -+	oid2=$(git hash-object o2) &&
>      -+	cat >expect <<-EOF &&
>      -+	$oid1
>      -+	$oid2
>      -+	EOF
>      ++test_expect_success 'git ls-files --format objectname v.s. -s' '
>      ++	git ls-files -s | awk "{print \$2}" >expect &&
>       +	git ls-files --format="%(objectname)" >actual &&
>       +	test_cmp expect actual
>       +'
>       +
>      -+HT='	'
>      -+WS='    '
>       +test_expect_success 'git ls-files --format v.s. --eol' '
>      -+	git ls-files --eol >expect 2>err &&
>      ++	git ls-files --eol >tmp &&
>      ++	sed -e "s/	/ /g" -e "s/  */ /g" tmp >expect 2>err &&
>       +	test_must_be_empty err &&
>      -+	git ls-files --format="i/%(eolinfo:index)${WS}w/%(eolinfo:worktree)${WS}attr/${WS}${WS}${WS}${WS} ${HT}%(path)" >actual 2>err &&
>      ++	git ls-files --format="i/%(eolinfo:index) w/%(eolinfo:worktree) attr/%(eolattr) %(path)" >actual 2>err &&
>       +	test_must_be_empty err &&
>       +	test_cmp expect actual
>       +'
>       +
>      -+test_expect_success 'git ls-files --format path' '
>      -+	cat >expect <<-\EOF &&
>      -+	o1
>      -+	o2
>      -+	EOF
>      ++test_expect_success 'git ls-files --format path v.s. -s' '
>      ++	git ls-files -s | awk "{print \$4}" >expect &&
>       +	git ls-files --format="%(path)" >actual &&
>       +	test_cmp expect actual
>       +'
>       +
>       +test_expect_success 'git ls-files --format with -m' '
>      -+	echo change >o1 &&
>      ++	echo change >o1.txt &&
>       +	cat >expect <<-\EOF &&
>      -+	o1
>      ++	o1.txt
>      ++	o5.txt
>       +	EOF
>       +	git ls-files --format="%(path)" -m >actual &&
>       +	test_cmp expect actual
>       +'
>       +
>       +test_expect_success 'git ls-files --format with -d' '
>      -+	echo o3 >o3 &&
>      -+	git add o3 &&
>      -+	rm o3 &&
>      ++	echo o6 >o6.txt &&
>      ++	git add o6.txt &&
>      ++	rm o6.txt &&
>       +	cat >expect <<-\EOF &&
>      -+	o3
>      ++	o5.txt
>      ++	o6.txt
>       +	EOF
>       +	git ls-files --format="%(path)" -d >actual &&
>       +	test_cmp expect actual
>
>
>  Documentation/git-ls-files.txt | 39 +++++++++++++-
>  builtin/ls-files.c             | 95 ++++++++++++++++++++++++++++++++++
>  t/t3013-ls-files-format.sh     | 87 +++++++++++++++++++++++++++++++
>  3 files changed, 220 insertions(+), 1 deletion(-)
>  create mode 100755 t/t3013-ls-files-format.sh
>
> diff --git a/Documentation/git-ls-files.txt b/Documentation/git-ls-files.txt
> index 0dabf3f0ddc..d7986419c25 100644
> --- a/Documentation/git-ls-files.txt
> +++ b/Documentation/git-ls-files.txt
> @@ -20,7 +20,7 @@ SYNOPSIS
>  		[--exclude-standard]
>  		[--error-unmatch] [--with-tree=<tree-ish>]
>  		[--full-name] [--recurse-submodules]
> -		[--abbrev[=<n>]] [--] [<file>...]
> +		[--abbrev[=<n>]] [--format=<format>] [--] [<file>...]
>  
>  DESCRIPTION
>  -----------
> @@ -192,6 +192,13 @@ followed by the  ("attr/<eolattr>").
>  	to the contained files. Sparse directories will be shown with a
>  	trailing slash, such as "x/" for a sparse directory "x".
>  
> +--format=<format>::
> +	A string that interpolates `%(fieldname)` from the result being shown.
> +	It also interpolates `%%` to `%`, and `%xx` where `xx` are hex digits
> +	interpolates to character with hex code `xx`; for example `%00`
> +	interpolates to `\0` (NUL), `%09` to `\t` (TAB) and %0a to `\n` (LF).
> +	--format cannot be combined with `-s`, `-o`, `-k`, `-t`, `--resolve-undo`
> +	and `--eol`.
>  \--::
>  	Do not interpret any more arguments as options.
>  
> @@ -223,6 +230,36 @@ quoted as explained for the configuration variable `core.quotePath`
>  (see linkgit:git-config[1]).  Using `-z` the filename is output
>  verbatim and the line is terminated by a NUL byte.
>  
> +It is possible to print in a custom format by using the `--format`
> +option, which is able to interpolate different fields using
> +a `%(fieldname)` notation. For example, if you only care about the
> +"objectname" and "path" fields, you can execute with a specific
> +"--format" like
> +
> +	git ls-files --format='%(objectname) %(path)'
> +
> +FIELD NAMES
> +-----------
> +The way each path is shown can be customized by using the
> +`--format=<format>` option, where the %(fieldname) in the
> +<format> string for various aspects of the index entry are
> +interpolated.  The following "fieldname" are understood:
> +
> +objectmode::
> +	The mode of the file which is recorded in the index.
> +objectname::
> +	The name of the file which is recorded in the index.
> +stage::
> +	The stage of the file which is recorded in the index.
> +eolinfo:index::
> +eolinfo:worktree::
> +	The <eolinfo> (see the description of the `--eol` option) of
> +	the contents in the index or in the worktree for the path.
> +eolattr::
> +	The <eolattr> (see the description of the `--eol` option)
> +	that applies to the path.
> +path::
> +	The pathname of the file which is recorded in the index.
>  
>  EXCLUDE PATTERNS
>  ----------------
> diff --git a/builtin/ls-files.c b/builtin/ls-files.c
> index e791b65e7e9..6f3ebcaaff7 100644
> --- a/builtin/ls-files.c
> +++ b/builtin/ls-files.c
> @@ -11,6 +11,7 @@
>  #include "quote.h"
>  #include "dir.h"
>  #include "builtin.h"
> +#include "strbuf.h"
>  #include "tree.h"
>  #include "cache-tree.h"
>  #include "parse-options.h"
> @@ -48,6 +49,7 @@ static char *ps_matched;
>  static const char *with_tree;
>  static int exc_given;
>  static int exclude_args;
> +static const char *format;
>  
>  static const char *tag_cached = "";
>  static const char *tag_unmerged = "";
> @@ -85,6 +87,16 @@ static void write_name(const char *name)
>  				   stdout, line_terminator);
>  }
>  
> +static void write_name_to_buf(struct strbuf *sb, const char *name)
> +{
> +	const char *rel = relative_path(name, prefix_len ? prefix : NULL, sb);
> +
> +	if (line_terminator)
> +		quote_c_style(rel, sb, NULL, 0);
> +	else
> +		strbuf_addstr(sb, rel);
> +}
> +
>  static const char *get_tag(const struct cache_entry *ce, const char *tag)
>  {
>  	static char alttag[4];
> @@ -222,6 +234,73 @@ static void show_submodule(struct repository *superproject,
>  	repo_clear(&subrepo);
>  }
>  
> +struct show_index_data {
> +	const char *pathname;
> +	struct index_state *istate;
> +	const struct cache_entry *ce;
> +};
> +
> +static size_t expand_show_index(struct strbuf *sb, const char *start,
> +				void *context)
> +{
> +	struct show_index_data *data = context;
> +	const char *end;
> +	const char *p;
> +	size_t len = strbuf_expand_literal_cb(sb, start, NULL);
> +	struct stat st;
> +
> +	if (len)
> +		return len;
> +	if (*start != '(')
> +		die(_("bad ls-files format: element '%s' "
> +		      "does not start with '('"), start);
> +
> +	end = strchr(start + 1, ')');
> +	if (!end)
> +		die(_("bad ls-files format: element '%s'"
> +		      "does not end in ')'"), start);
> +
> +	len = end - start + 1;
> +	if (skip_prefix(start, "(objectmode)", &p))
> +		strbuf_addf(sb, "%06o", data->ce->ce_mode);
> +	else if (skip_prefix(start, "(objectname)", &p))
> +		strbuf_add_unique_abbrev(sb, &data->ce->oid, abbrev);
> +	else if (skip_prefix(start, "(stage)", &p))
> +		strbuf_addf(sb, "%d", ce_stage(data->ce));
> +	else if (skip_prefix(start, "(eolinfo:index)", &p))
> +		strbuf_addstr(sb, S_ISREG(data->ce->ce_mode) ?
> +			      get_cached_convert_stats_ascii(data->istate,
> +			      data->ce->name) : "");
> +	else if (skip_prefix(start, "(eolinfo:worktree)", &p))
> +		strbuf_addstr(sb, !lstat(data->pathname, &st) &&
> +			      S_ISREG(st.st_mode) ?
> +			      get_wt_convert_stats_ascii(data->pathname) : "");
> +	else if (skip_prefix(start, "(eolattr)", &p))
> +		strbuf_addstr(sb, get_convert_attr_ascii(data->istate,
> +			      data->pathname));
> +	else if (skip_prefix(start, "(path)", &p))
> +		write_name_to_buf(sb, data->pathname);
> +	else
> +		die(_("bad ls-files format: %%%.*s"), (int)len, start);
> +
> +	return len;
> +}
> +
> +static void show_ce_fmt(struct repository *repo, const struct cache_entry *ce,
> +			const char *format, const char *fullname) {
> +	struct show_index_data data = {
> +		.pathname = fullname,
> +		.istate = repo->index,
> +		.ce = ce,
> +	};
> +	struct strbuf sb = STRBUF_INIT;
> +
> +	strbuf_expand(&sb, format, expand_show_index, &data);
> +	strbuf_addch(&sb, line_terminator);
> +	fwrite(sb.buf, sb.len, 1, stdout);
> +	strbuf_release(&sb);
> +}
> +
>  static void show_ce(struct repository *repo, struct dir_struct *dir,
>  		    const struct cache_entry *ce, const char *fullname,
>  		    const char *tag)
> @@ -236,6 +315,12 @@ static void show_ce(struct repository *repo, struct dir_struct *dir,
>  				  max_prefix_len, ps_matched,
>  				  S_ISDIR(ce->ce_mode) ||
>  				  S_ISGITLINK(ce->ce_mode))) {
> +		if (format) {
> +			show_ce_fmt(repo, ce, format, fullname);
> +			print_debug(ce);
> +			return;
> +		}
> +
>  		tag = get_tag(ce, tag);
>  
>  		if (!show_stage) {
> @@ -675,6 +760,9 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
>  			 N_("suppress duplicate entries")),
>  		OPT_BOOL(0, "sparse", &show_sparse_dirs,
>  			 N_("show sparse directories in the presence of a sparse index")),
> +		OPT_STRING_F(0, "format", &format, N_("format"),
> +			     N_("format to use for the output"),
> +			     PARSE_OPT_NONEG),
>  		OPT_END()
>  	};
>  	int ret = 0;
> @@ -699,6 +787,13 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
>  	for (i = 0; i < exclude_list.nr; i++) {
>  		add_pattern(exclude_list.items[i].string, "", 0, pl, --exclude_args);
>  	}
> +
> +	if (format && (show_stage || show_others || show_killed ||
> +		show_resolve_undo || skipping_duplicates || show_eol || show_tag))
> +			usage_msg_opt("--format cannot used with -s, -o, -k, -t"
> +				      "--resolve-undo, --deduplicate, --eol",
> +				      ls_files_usage, builtin_ls_files_options);

There's a whitespace issue here, you need to add a space after "-t",
otherwise we emit:

	fatal: --format cannot used with -s, -o, -k, -t--resolve-undo, --deduplicate, --eol
> +
>  	if (show_tag || show_valid_bit || show_fsmonitor_bit) {
>  		tag_cached = "H ";
>  		tag_unmerged = "M ";
> diff --git a/t/t3013-ls-files-format.sh b/t/t3013-ls-files-format.sh
> new file mode 100755
> index 00000000000..e62bce70f3b
> --- /dev/null
> +++ b/t/t3013-ls-files-format.sh
> @@ -0,0 +1,87 @@
> +#!/bin/sh
> +
> +test_description='git ls-files --format test'
> +
> +TEST_PASSES_SANITIZE_LEAK=true
> +. ./test-lib.sh
> +
> +for flag in -s -o -k -t --resolve-undo --deduplicate --eol
> +do
> +	test_expect_success "usage: --format is incompatible with $flag" '
> +		test_expect_code 129 git ls-files --format="%(objectname)" $flag
> +	'
> +done
> +
> +test_expect_success 'setup' '
> +	printf "LINEONE\nLINETWO\nLINETHREE\n" >o1.txt &&
> +	printf "LINEONE\r\nLINETWO\r\nLINETHREE\r\n" >o2.txt &&
> +	printf "LINEONE\r\nLINETWO\nLINETHREE\n" >o3.txt &&

If you want to do this sort of thing in general this pattern is better:

	x="a b c" &&
	printf "%s\n" $x
        printf "%s\r\n" $x

I.e. you can use printf's auto-repeating, or test_write_lines[1]. But in
this case I tried:

	diff --git a/t/t3013-ls-files-format.sh b/t/t3013-ls-files-format.sh
	index e62bce70f3b..e4c3a788acb 100755
	--- a/t/t3013-ls-files-format.sh
	+++ b/t/t3013-ls-files-format.sh
	@@ -13,9 +13,11 @@ do
	 done
	 
	 test_expect_success 'setup' '
	-	printf "LINEONE\nLINETWO\nLINETHREE\n" >o1.txt &&
	-	printf "LINEONE\r\nLINETWO\r\nLINETHREE\r\n" >o2.txt &&
	-	printf "LINEONE\r\nLINETWO\nLINETHREE\n" >o3.txt &&
	+	lines="LO LINETWO LINETHREE" &&
	+	test_write_lines $lines >o1.txt &&
	+	# Even this passes!
	+	#>o1.txt &&
	+
	 	ln -s o3.txt o4.txt &&
	 	git add "*.txt" &&
	 	git add --chmod +x o1.txt &&

I.e. all tests pass if we don't write o2.txt and o3.txt, and continue to
pass if you uncomment that and make o1.txt an empty file.

So is this some incomplete test setup code that was never used & we
could drop?

> +	ln -s o3.txt o4.txt &&
> +	git add "*.txt" &&
> +	git add --chmod +x o1.txt &&
> +	git update-index --add --cacheinfo 160000 $(git hash-object o1.txt) o5.txt &&

Do:

	oid=$(git hash-object ..) &&
	git update-index ... "$(oid)"

Otherwise we hide the exit code of "git-hash-object", e.g. if it returns
the hash and then segfaults.

> +	git commit -m base
> +'
> +
> +test_expect_success 'git ls-files --format objectmode v.s. -s' '
> +	git ls-files -s | awk "{print \$1}" >expect &&

Same in this case and below, i.e. let's not hide "git" on the lhs of a
pipe. So:

	git ls-files >files &&
	awk ... <files >expect

In this case all your awk-ing can be replaced with (continued)...

> +	git ls-files --format="%(objectmode)" >actual &&
> +	test_cmp expect actual
> +'
> +
> +test_expect_success 'git ls-files --format objectname v.s. -s' '
> +	git ls-files -s | awk "{print \$2}" >expect &&

...

	cut -d" " -f2

...

> +	git ls-files --format="%(objectname)" >actual &&
> +	test_cmp expect actual
> +'
> +
> +test_expect_success 'git ls-files --format v.s. --eol' '
> +	git ls-files --eol >tmp &&
> +	sed -e "s/	/ /g" -e "s/  */ /g" tmp >expect 2>err &&
> +	test_must_be_empty err &&
> +	git ls-files --format="i/%(eolinfo:index) w/%(eolinfo:worktree) attr/%(eolattr) %(path)" >actual 2>err &&
> +	test_must_be_empty err &&
> +	test_cmp expect actual
> +'
> +
> +test_expect_success 'git ls-files --format path v.s. -s' '
> +	git ls-files -s | awk "{print \$4}" >expect &&

...

	cut -f2

I.e. instead of the 4th whitespace field ask for the 2nd \t-delimited
field. There's nothing wrong with using awk per-se, but let's use the
simpler "cut" for such a simple use-case.

> +	git ls-files --format="%(path)" >actual &&
> +	test_cmp expect actual
> +'
> +
> +test_expect_success 'git ls-files --format with -m' '
> +	echo change >o1.txt &&
> +	cat >expect <<-\EOF &&
> +	o1.txt
> +	o5.txt
> +	EOF
> +	git ls-files --format="%(path)" -m >actual &&
> +	test_cmp expect actual
> +'
> +
> +test_expect_success 'git ls-files --format with -d' '
> +	echo o6 >o6.txt &&
> +	git add o6.txt &&
> +	rm o6.txt &&
> +	cat >expect <<-\EOF &&
> +	o5.txt
> +	o6.txt
> +	EOF
> +	git ls-files --format="%(path)" -d >actual &&
> +	test_cmp expect actual
> +'
> +
> +test_expect_success 'git ls-files --format imitate --stage' '
> +	git ls-files --stage >expect &&
> +	git ls-files --format="%(objectmode) %(objectname) %(stage)%x09%(path)" >actual &&
> +	test_cmp expect actual
> +'
> +
> +test_expect_success 'git ls-files --format with --debug' '
> +	git ls-files --debug >expect &&
> +	git ls-files --format="%(path)" --debug >actual &&
> +	test_cmp expect actual
> +'
> +
> +test_done

The rest of this (and especially the C code) all looks good to me at
this point, thanks!

1. Aside: I've found the test_write_lines helper to be rather strange
   for us to carry. I.e. most helpers provide a briefer or less
   buggy/tricky way to do something, but in that case:

	test_write_lines
	printf "%s\n"

   So we have it to write something in a more verbose way than we need,
   as we can see experimentally from all tests passing with:

	perl -pi -e 's[test_write_lines ][printf "%s\\n" ]g' t/t[0-9]*.sh

   It seems to me that per
   https://lore.kernel.org/git/[email-masked]/
   and
   https://lore.kernel.org/git/[email-masked]/
   it was suggested without knowing that we could use printf to do the
   same.

   The implementation that landed in ac9afcc31cd (test: add
   test_write_lines helper, 2014-04-27) was fixed up to use printf,
   without re-visiting why we were carrying a helper when an even
   shorter printf would do...