parse-options: add support for parsing subcommands

Several Git commands have subcommands to implement mutually exclusive
"operation modes", and they usually parse their subcommand argument
with a bunch of if-else if statements.

Teach parse-options to handle subcommands as well, which will result
in shorter and simpler code with consistent error handling and error
messages on unknown or missing subcommand, and it will also make
possible for our Bash completion script to handle subcommands
programmatically.

The approach is guided by the following observations:

  - Most subcommands [1] are implemented in dedicated functions, and
    most of those functions [2] either have a signature matching the
    'int cmd_foo(int argc, const char **argc, const char *prefix)'
    signature of builtin commands or can be trivially converted to
    that signature, because they miss only that last prefix parameter
    or have no parameters at all.

  - Subcommand arguments only have long form, and they have no double
    dash prefix, no negated form, and no description, and they don't
    take any arguments, and can't be abbreviated.

  - There must be exactly one subcommand among the arguments, or zero
    if the command has a default operation mode.

  - All arguments following the subcommand are considered to be
    arguments of the subcommand, and, conversely, arguments meant for
    the subcommand may not preceed the subcommand.

So in the end subcommand declaration and parsing would look something
like this:

    parse_opt_subcommand_fn *fn = NULL;
    struct option builtin_commit_graph_options[] = {
        OPT_STRING(0, "object-dir", &opts.obj_dir, N_("dir"),
                   N_("the object directory to store the graph")),
        OPT_SUBCOMMAND("verify", &fn, graph_verify),
        OPT_SUBCOMMAND("write", &fn, graph_write),
        OPT_END(),
    };
    argc = parse_options(argc, argv, prefix, options,
                         builtin_commit_graph_usage, 0);
    return fn(argc, argv, prefix);

Here each OPT_SUBCOMMAND specifies the name of the subcommand and the
function implementing it, and the address of the same 'fn' subcommand
function pointer.  parse_options() then processes the arguments until
it finds the first argument matching one of the subcommands, sets 'fn'
to the function associated with that subcommand, and returns, leaving
the rest of the arguments unprocessed.  If none of the listed
subcommands is found among the arguments, parse_options() will show
usage and abort.

If a command has a default operation mode, 'fn' should be initialized
to the function implementing that mode, and parse_options() should be
invoked with the PARSE_OPT_SUBCOMMAND_OPTIONAL flag.  In this case
parse_options() won't error out when not finding any subcommands, but
will return leaving 'fn' unchanged.  Note that if that default
operation mode has any --options, then the PARSE_OPT_KEEP_UNKNOWN_OPT
flag is necessary as well (otherwise parse_options() would error out
upon seeing the unknown option meant to the default operation mode).

Some thoughts about the implementation:

  - The same pointer to 'fn' must be specified as 'value' for each
    OPT_SUBCOMMAND, because there can be only one set of mutually
    exclusive subcommands; parse_options() will BUG() otherwise.

    There are other ways to tell parse_options() where to put the
    function associated with the subcommand given on the command line,
    but I didn't like them:

      - Change parse_options()'s signature by adding a pointer to
        subcommand function to be set to the function associated with
        the given subcommand, affecting all callsites, even those that
        don't have subcommands.

      - Introduce a specific parse_options_and_subcommand() variant
        with that extra funcion parameter.

  - I decided against automatically calling the subcommand function
    from within parse_options(), because:

      - There are commands that have to perform additional actions
        after option parsing but before calling the function
        implementing the specified subcommand.

      - The return code of the subcommand is usually the return code
        of the git command, but preserving the return code of the
        automatically called subcommand function would have made the
        API awkward.

  - Also add a OPT_SUBCOMMAND_F() variant to allow specifying an
    option flag: we have two subcommands that are purposefully
    excluded from completion ('git remote rm' and 'git stash save'),
    so they'll have to be specified with the PARSE_OPT_NOCOMPLETE
    flag.

  - Some of the 'parse_opt_flags' don't make sense with subcommands,
    and using them is probably just an oversight or misunderstanding.
    Therefore parse_options() will BUG() when invoked with any of the
    following flags while the options array contains at least one
    OPT_SUBCOMMAND:

      - PARSE_OPT_KEEP_DASHDASH: parse_options() stops parsing
        arguments when encountering a "--" argument, so it doesn't
        make sense to expect and keep one before a subcommand, because
        it would prevent the parsing of the subcommand.

        However, this flag is allowed in combination with the
        PARSE_OPT_SUBCOMMAND_OPTIONAL flag, because the double dash
        might be meaningful for the command's default operation mode,
        e.g. to disambiguate refs and pathspecs.

      - PARSE_OPT_STOP_AT_NON_OPTION: As its name suggests, this flag
        tells parse_options() to stop as soon as it encouners a
        non-option argument, but subcommands are by definition not
        options...  so how could they be parsed, then?!

      - PARSE_OPT_KEEP_UNKNOWN: This flag can be used to collect any
        unknown --options and then pass them to a different command or
        subsystem.  Surely if a command has subcommands, then this
        functionality should rather be delegated to one of those
        subcommands, and not performed by the command itself.

        However, this flag is allowed in combination with the
        PARSE_OPT_SUBCOMMAND_OPTIONAL flag, making possible to pass
        --options to the default operation mode.

  - If the command with subcommands has a default operation mode, then
    all arguments to the command must preceed the arguments of the
    subcommand.

    AFAICT we don't have any commands where this makes a difference,
    because in those commands either only the command accepts any
    arguments ('notes' and 'remote'), or only the default subcommand
    ('reflog' and 'stash'), but never both.

  - The 'argv' array passed to subcommand functions currently starts
    with the name of the subcommand.  Keep this behavior.  AFAICT no
    subcommand functions depend on the actual content of 'argv[0]',
    but the parse_options() call handling their options expects that
    the options start at argv[1].

  - To support handling subcommands programmatically in our Bash
    completion script, 'git cmd --git-completion-helper' will now list
    both subcommands and regular --options, if any.  This means that
    the completion script will have to separate subcommands (i.e.
    words without a double dash prefix) from --options on its own, but
    that's rather easy to do, and it's not much work either, because
    the number of subcommands a command might have is rather low, and
    those commands accept only a single --option or none at all.  An
    alternative would be to introduce a separate option that lists
    only subcommands, but then the completion script would need not
    one but two git invocations and command substitutions for commands
    with subcommands.

    Note that this change doesn't affect the behavior of our Bash
    completion script, because when completing the --option of a
    command with subcommands, e.g. for 'git notes --<TAB>', then all
    subcommands will be filtered out anyway, as none of them will
    match the word to be completed starting with that double dash
    prefix.

[1] Except 'git rerere', because many of its subcommands are
    implemented in the bodies of the if-else if statements parsing the
    command's subcommand argument.

[2] Except 'credential', 'credential-store' and 'fsmonitor--daemon',
    because some of the functions implementing their subcommands take
    special parameters.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.h

@@ -11,6 +11,7 @@ enum parse_opt_type {
 	OPTION_GROUP,
 	OPTION_NUMBER,
 	OPTION_ALIAS,
+	OPTION_SUBCOMMAND,
 	/* options with no arguments */
 	OPTION_BIT,
 	OPTION_NEGBIT,
@@ -34,6 +35,7 @@ enum parse_opt_flags {
 	PARSE_OPT_NO_INTERNAL_HELP = 1 << 4,
 	PARSE_OPT_ONE_SHOT = 1 << 5,
 	PARSE_OPT_SHELL_EVAL = 1 << 6,
+	PARSE_OPT_SUBCOMMAND_OPTIONAL = 1 << 7,
 };
 
 enum parse_opt_option_flags {
@@ -56,6 +58,7 @@ enum parse_opt_result {
 	PARSE_OPT_ERROR = -1,	/* must be the same as error() */
 	PARSE_OPT_DONE = 0,	/* fixed so that "return 0" works */
 	PARSE_OPT_NON_OPTION,
+	PARSE_OPT_SUBCOMMAND,
 	PARSE_OPT_UNKNOWN
 };
 
@@ -67,6 +70,9 @@ typedef enum parse_opt_result parse_opt_ll_cb(struct parse_opt_ctx_t *ctx,
 					      const struct option *opt,
 					      const char *arg, int unset);
 
+typedef int parse_opt_subcommand_fn(int argc, const char **argv,
+				    const char *prefix);
+
 /*
  * `type`::
  *   holds the type of the option, you must have an OPTION_END last in your
@@ -76,7 +82,8 @@ typedef enum parse_opt_result parse_opt_ll_cb(struct parse_opt_ctx_t *ctx,
  *   the character to use as a short option name, '\0' if none.
  *
  * `long_name`::
- *   the long option name, without the leading dashes, NULL if none.
+ *   the long option (without the leading dashes) or subcommand name,
+ *   NULL if none.
  *
  * `value`::
  *   stores pointers to the values to be filled.
@@ -93,7 +100,7 @@ typedef enum parse_opt_result parse_opt_ll_cb(struct parse_opt_ctx_t *ctx,
  *
  * `help`::
  *   the short help associated to what the option does.
- *   Must never be NULL (except for OPTION_END).
+ *   Must never be NULL (except for OPTION_END and OPTION_SUBCOMMAND).
  *   OPTION_GROUP uses this pointer to store the group header.
  *   Should be wrapped by N_() for translation.
  *
@@ -131,6 +138,9 @@ typedef enum parse_opt_result parse_opt_ll_cb(struct parse_opt_ctx_t *ctx,
  * `ll_callback`::
  *   pointer to the callback to use for OPTION_LOWLEVEL_CALLBACK
  *
+ * `subcommand_fn`::
+ *   pointer to a function to use for OPTION_SUBCOMMAND.
+ *   It will be put in value when the subcommand is given on the command line.
  */
 struct option {
 	enum parse_opt_type type;
@@ -145,6 +155,7 @@ struct option {
 	intptr_t defval;
 	parse_opt_ll_cb *ll_callback;
 	intptr_t extra;
+	parse_opt_subcommand_fn *subcommand_fn;
 };
 
 #define OPT_BIT_F(s, l, v, h, b, f) { OPTION_BIT, (s), (l), (v), NULL, (h), \
@@ -206,6 +217,14 @@ struct option {
 #define OPT_ALIAS(s, l, source_long_name) \
 	{ OPTION_ALIAS, (s), (l), (source_long_name) }
 
+#define OPT_SUBCOMMAND_F(l, v, fn, f) { \
+	.type = OPTION_SUBCOMMAND, \
+	.long_name = (l), \
+	.value = (v), \
+	.flags = (f), \
+	.subcommand_fn = (fn) }
+#define OPT_SUBCOMMAND(l, v, fn)    OPT_SUBCOMMAND_F((l), (v), (fn), 0)
+
 /*
  * parse_options() will filter out the processed options and leave the
  * non-option arguments in argv[]. argv0 is assumed program name and
@@ -295,6 +314,7 @@ struct parse_opt_ctx_t {
 	int argc, cpidx, total;
 	const char *opt;
 	enum parse_opt_flags flags;
+	unsigned has_subcommands;
 	const char *prefix;
 	const char **alias_groups; /* must be in groups of 3 elements! */
 	struct option *updated_options;