config-batch: create parse loop and unknown command

As we build new features in the config-batch command, we define the
plaintext protocol with line-by-line output and responses. To think to the
future, we make sure that the protocol has a clear way to respond to an
unknown command or an unknown version of that command.

Signed-off-by: Derrick Stolee <stolee@gmail.com>

Author: derrickstolee

Documentation/git-config-batch.adoc

@@ -13,7 +13,28 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-TODO
+Tools frequently need to change their behavior based on values stored in
+Git's configuration files. These files may have complicated conditions
+for including extra files, so it is difficult to produce an independent
+parser. To avoid executing multiple processes to discover or modify
+multiple configuration values, the `git config-batch` command allows a
+single process to handle multiple requests using a machine-parseable
+interface across `stdin` and `stdout`.
+
+PROTOCOL
+--------
+By default, the protocol uses line feeds (`LF`) to signal the end of a
+command over `stdin` or a response over `stdout`.
+
+The protocol will be extended in the future, and consumers should be
+resilient to older Git versions not understanding the latest command
+set. Thus, if the Git version includes the `git config-batch` builtin
+but doesn't understand an input command, it will return a single line
+response:
+
+```
+unknown_command LF
+```
 
 SEE ALSO
 --------

builtin/config-batch.c

@@ -3,17 +3,135 @@
 #include "config.h"
 #include "environment.h"
 #include "parse-options.h"
+#include "strbuf.h"
+#include "string-list.h"
 
 static const char *const builtin_config_batch_usage[] = {
 	N_("git config-batch <options>"),
 	NULL
 };
 
+#define UNKNOWN_COMMAND "unknown_command"
+
+static int emit_response(const char *response, ...)
+{
+	va_list params;
+	const char *token;
+
+	printf("%s", response);
+
+	va_start(params, response);
+	while ((token = va_arg(params, const char *)))
+		printf(" %s", token);
+	va_end(params);
+
+	printf("\n");
+	fflush(stdout);
+	return 0;
+}
+
+/**
+ * A function pointer type for defining a command. The function is
+ * responsible for handling different versions of the command name.
+ *
+ * These functions should only return a negative value if they result
+ * in such a catastrophic failure that the process should end.
+ *
+ * Return 0 on success.
+ */
+typedef int (*command_fn)(struct repository *repo, int version,
+			  int argc, const char **argv);
+
+static int unknown_commmand(struct repository *repo UNUSED, int version UNUSED,
+			    int argc UNUSED, const char **argv UNUSED)
+{
+	return emit_response(UNKNOWN_COMMAND, NULL);
+}
+
+struct command {
+	const char *name;
+	command_fn fn;
+};
+
+static struct command commands[] = {
+	/* unknown_command must be last. */
+	{
+		.name = "",
+		.fn   = unknown_commmand,
+	},
+};
+
+#define COMMAND_COUNT ((size_t)(sizeof(commands) / sizeof(*commands)))
+
+/**
+ * Process a single line from stdin and process the command.
+ *
+ * Returns 0 on successful processing of command, including the
+ * unknown_command output.
+ *
+ * Returns 1 on natural exit due to exist signal of empty line.
+ *
+ * Returns negative value on other catastrophic error.
+ */
+static int process_command(struct repository *repo)
+{
+	static struct strbuf line = STRBUF_INIT;
+	struct string_list tokens = STRING_LIST_INIT_NODUP;
+	const char *command;
+	int version;
+	int argc = 0;
+	const char **argv = NULL;
+	int res = 0;
+
+	strbuf_getline(&line, stdin);
+
+	if (!line.len)
+		return 1;
+
+	string_list_split_in_place(&tokens, line.buf, " ", -1);
+
+	if (tokens.nr < 2) {
+		res = error(_("expected at least 2 tokens, got %"PRIuMAX), tokens.nr);
+		goto cleanup;
+	}
+
+	command = tokens.items[0].string;
+
+	if (!git_parse_int(tokens.items[1].string, &version)) {
+		res = error(_("unable to parse '%s' to integer"),
+			    tokens.items[1].string);
+		goto cleanup;
+	}
+
+	argc = tokens.nr - 2;
+	CALLOC_ARRAY(argv, argc + 1);
+
+	for (size_t i = 2; i < tokens.nr; i++)
+		argv[i - 2] = tokens.items[i].string;
+
+	for (size_t i = 0; i < COMMAND_COUNT; i++) {
+		if (!commands[i].name[0] || /* unknown command */
+		    !strcmp(command, commands[i].name)) {
+			res = commands[i].fn(repo, version, argc, argv);
+			goto cleanup;
+		}
+	}
+
+	BUG(_("scanned to end of command list, including 'unknown_command'"));
+
+cleanup:
+	free(argv);
+	strbuf_reset(&line);
+	string_list_clear(&tokens, 0);
+	return res;
+}
+
 int cmd_config_batch(int argc,
 		     const char **argv,
 		     const char *prefix,
 		     struct repository *repo)
 {
+	int res = 0;
 	struct option options[] = {
 		OPT_END(),
 	};
@@ -26,5 +144,9 @@ int cmd_config_batch(int argc,
 
 	repo_config(repo, git_default_config, NULL);
 
-	return 0;
+	while (!(res = process_command(repo)));
+
+	if (res == 1)
+		return 0;
+	die(_("an unrecoverable error occurred during command execution"));
 }

t/t1312-config-batch.sh

@@ -4,9 +4,22 @@ test_description='Test git config-batch'
 
 . ./test-lib.sh
 
-test_expect_success 'help text' '
-	test_must_fail git config-batch -h >out &&
-	grep usage out
+test_expect_success 'no commands' '
+	echo | git config-batch >out &&
+	test_must_be_empty out
+'
+
+test_expect_success 'unknown_command' '
+	echo unknown_command >expect &&
+	echo "bogus 1 line of tokens" >in &&
+	git config-batch >out <in &&
+	test_cmp expect out
+'
+
+test_expect_success 'failed to parse version' '
+	echo "bogus BAD_VERSION line of tokens" >in &&
+	test_must_fail git config-batch 2>err <in &&
+	test_grep BAD_VERSION err
 '
 
 test_done