config: allow tweaking whitespace between value and comment

Extending the previous step, this allows the whitespace placed after
the value before the "# comment message" to be tweaked by tweaking
the preprocessing rule to:

 * If the given comment string begins with one or more whitespace
   characters followed by '#', it is passed intact.

 * If the given comment string begins with '#', a Space is
   prepended.

 * Otherwise, " # " (Space, '#', Space) is prefixed.

 * A string with LF in it cannot be used as a comment string.

Unlike the previous step, which unconditionally added a space after
the value before writing the "# comment string", because the above
preprocessing already gives a whitespace before the '#', the
resulting string is written immediately after copying the value.

And the sanity checking rule becomes

 * comment string after the above massaging that comes into
   git_config_set_multivar_in_file_gently() must

   - begin with zero or more whitespace characters followed by '#'.
   - not have a LF in it.

I personally think this is over-engineered, but since I thought
things through anyway, here it is in the patch form.  The logic to
tweak end-user supplied comment string is encapsulated in a new
helper function, git_config_prepare_comment_string(), so if new
front-end callers would want to use the same massaging rules, it is
easily reused.

Unfortunately I do not think of a way to tweak the preprocessing
rules further to optionally allow having no blank after the value,
i.e. to produce

	[section]
		variable = value#comment

(which is a valid way to say section.variable=value, by the way)
without sacrificing the ergonomics for the more usual case, so this
time I really stop here.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-config.txt

@@ -89,9 +89,15 @@ OPTIONS
 
 --comment <message>::
 	Append a comment at the end of new or modified lines.
-	Unless _<message>_ begins with "#", a string "# " (hash
-	followed by a space) is prepended to it. The _<message>_ must not
-	contain linefeed characters (no multi-line comments are permitted).
+
+	If _<message>_ begins with one or more whitespaces followed
+	by "#", it is used as-is.  If it begins with "#", a space is
+	prepended before it is used.  Otherwise, a string " # " (a
+	space followed by a hash followed by a space) is prepended
+	to it.  And the resulting string is placed immediately after
+	the value defined for the variable.  The _<message>_ must
+	not contain linefeed characters (no multi-line comments are
+	permitted).
 
 --get::
 	Get the value for a given key (optionally filtered by a regex

builtin/config.c

@@ -841,12 +841,7 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 		flags |= CONFIG_FLAGS_FIXED_VALUE;
 	}
 
-	if (comment) {
-		if (strchr(comment, '\n'))
-			die(_("no multi-line comment allowed: '%s'"), comment);
-		if (comment[0] != '#')
-			comment = xstrfmt("# %s", comment);
-	}
+	comment = git_config_prepare_comment_string(comment);
 
 	if (actions & PAGING_ACTIONS)
 		setup_auto_pager("config", 1);

config.c

@@ -3044,7 +3044,7 @@ static ssize_t write_pair(int fd, const char *key, const char *value,
 		}
 
 	if (comment)
-		strbuf_addf(&sb, "%s %s\n", quote, comment);
+		strbuf_addf(&sb, "%s%s\n", quote, comment);
 	else
 		strbuf_addf(&sb, "%s\n", quote);
 
@@ -3172,6 +3172,62 @@ void git_config_set(const char *key, const char *value)
 	trace2_cmd_set_config(key, value);
 }
 
+/*
+ * The ownership rule is that the caller will own the string
+ * if it receives a piece of memory different from what it passed
+ * as the parameter.
+ */
+const char *git_config_prepare_comment_string(const char *comment)
+{
+	size_t leading_blanks;
+
+	if (!comment)
+		return NULL;
+
+	if (strchr(comment, '\n'))
+		die(_("no multi-line comment allowed: '%s'"), comment);
+
+	/*
+	 * If it begins with one or more leading whitespace characters
+	 * followed by '#", the comment string is used as-is.
+	 *
+	 * If it begins with '#', a SP is inserted between the comment
+	 * and the value the comment is about.
+	 *
+	 * Otherwise, the value is followed by a SP followed by '#'
+	 * followed by SP and then the comment string comes.
+	 */
+
+	leading_blanks = strspn(comment, " \t");
+	if (leading_blanks && comment[leading_blanks] == '#')
+		; /* use it as-is */
+	else if (comment[0] == '#')
+		comment = xstrfmt(" %s", comment);
+	else
+		comment = xstrfmt(" # %s", comment);
+
+	return comment;
+}
+
+static void validate_comment_string(const char *comment)
+{
+	size_t leading_blanks;
+
+	if (!comment)
+		return;
+	/*
+	 * The front-end must have massaged the comment string
+	 * properly before calling us.
+	 */
+	if (strchr(comment, '\n'))
+		BUG("multi-line comments are not permitted: '%s'", comment);
+
+	leading_blanks = strspn(comment, " \t");
+	if (!leading_blanks || comment[leading_blanks] != '#')
+		BUG("comment must begin with one or more SP followed by '#': '%s'",
+		    comment);
+}
+
 /*
  * If value==NULL, unset in (remove from) config,
  * if value_pattern!=NULL, disregard key/value pairs where value does not match.
@@ -3211,16 +3267,7 @@ int git_config_set_multivar_in_file_gently(const char *config_filename,
 	size_t contents_sz;
 	struct config_store_data store = CONFIG_STORE_INIT;
 
-	if (comment) {
-		/*
-		 * The front-end must have massaged the comment string
-		 * properly before calling us.
-		 */
-		if (strchr(comment, '\n'))
-			BUG("multi-line comments are not permitted: '%s'", comment);
-		if (comment[0] != '#')
-			BUG("comment should begin with '#': '%s'", comment);
-	}
+	validate_comment_string(comment);
 
 	/* parse-key returns negative; flip the sign to feed exit(3) */
 	ret = 0 - git_config_parse_key(key, &store.key, &store.baselen);

config.h

@@ -338,6 +338,8 @@ void git_config_set_multivar(const char *, const char *, const char *, unsigned)
 int repo_config_set_multivar_gently(struct repository *, const char *, const char *, const char *, unsigned);
 int git_config_set_multivar_in_file_gently(const char *, const char *, const char *, const char *, const char *, unsigned);
 
+const char *git_config_prepare_comment_string(const char *);
+
 /**
  * takes four parameters:
  *

t/t1300-config.sh

@@ -74,6 +74,8 @@ cat > expect << EOF
 	penguin = gentoo # Pygoscelis papua
 	disposition = peckish # find fish
 	foo = bar #abc
+	spsp = value # and comment
+	htsp = value	# and comment
 [Sections]
 	WhatEver = Second
 EOF
@@ -82,6 +84,10 @@ test_expect_success 'append comments' '
 	git config --replace-all --comment="Pygoscelis papua" section.penguin gentoo &&
 	git config --comment="find fish" section.disposition peckish &&
 	git config --comment="#abc" section.foo bar &&
+
+	git config --comment="and comment" section.spsp value &&
+	git config --comment="	# and comment" section.htsp value &&
+
 	test_cmp expect .git/config
 '