C implementation of the 'git' program, take two.

This patch provides a C implementation of the 'git' program and
introduces support for putting the git-* commands in a directory
of their own. It also saves some time on executing those commands
in a tight loop and it prints the currently available git commands
in a nicely formatted list.

The location of the GIT_EXEC_PATH (name discussion's closed, thank gods)
can be obtained by running

	git --exec-path

which will hopefully give porcelainistas ample time to adapt their
heavy-duty loops to call the core programs directly and thus save
the extra fork() / execve() overhead, although that's not really
necessary any more.

The --exec-path value is prepended to $PATH, so the git-* programs
should Just Work without ever requiring any changes to how they call
other programs in the suite.

Some timing values for 10000 invocations of git-var >&/dev/null:
	git.sh: 24.194s
	git.c:   9.044s
	git-var: 7.377s

The git-<tab><tab> behaviour can, along with the someday-to-be-deprecated
git-<command> form of invocation, be indefinitely retained by adding
the following line to one's .bash_profile or equivalent:

	PATH=$PATH:$(git --exec-path)

Experimental libraries can be used by either setting the environment variable
GIT_EXEC_PATH, or by using

	git --exec-path=/some/experimental/exec-path

Relative paths are properly grok'ed as exec-path values.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 229 insertions, 0 deletions

diff --git a/git.c b/git.c
new file mode 100644
index 000000000..d18980135
--- /dev/null
+++ b/git.c
@@ -0,0 +1,229 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <limits.h>
+#include <stdarg.h>
+#include <glob.h>
+
+#ifndef PATH_MAX
+# define PATH_MAX 4096
+#endif
+
+static const char git_usage[] =
+	"Usage: git [--version] [--exec-path[=GIT_EXEC_PATH]] [--help] COMMAND [ ARGS ]";
+
+struct string_list {
+	size_t len;
+	char *str;
+	struct string_list *next;
+};
+
+/* most gui terms set COLUMNS (although some don't export it) */
+static int term_columns(void)
+{
+	char *col_string = getenv("COLUMNS");
+	int n_cols = 0;
+
+	if (col_string && (n_cols = atoi(col_string)) > 0)
+		return n_cols;
+
+	return 80;
+}
+
+static inline void mput_char(char c, unsigned int num)
+{
+	while(num--)
+		putchar(c);
+}
+
+static void pretty_print_string_list(struct string_list *list, int longest)
+{
+	int cols = 1;
+	int space = longest + 1; /* min 1 SP between words */
+	int max_cols = term_columns() - 1; /* don't print *on* the edge */
+
+	if (space < max_cols)
+		cols = max_cols / space;
+
+	while (list) {
+		int c;
+		printf("  ");
+
+		for (c = cols; c && list; list = list->next) {
+			printf("%s", list->str);
+
+			if (--c)
+				mput_char(' ', space - list->len);
+		}
+		putchar('\n');
+	}
+}
+
+static void list_commands(const char *exec_path, const char *pattern)
+{
+	struct string_list *list = NULL, *tail = NULL;
+	unsigned int longest = 0, i;
+	glob_t gl;
+
+	if (chdir(exec_path) < 0) {
+		printf("git: '%s': %s\n", exec_path, strerror(errno));
+		exit(1);
+	}
+
+	i = glob(pattern, 0, NULL, &gl);
+	switch(i) {
+	case GLOB_NOSPACE:
+		puts("Out of memory when running glob()");
+		exit(2);
+	case GLOB_ABORTED:
+		printf("'%s': Read error: %s\n", exec_path, strerror(errno));
+		exit(2);
+	case GLOB_NOMATCH:
+		printf("No git commands available in '%s'.\n", exec_path);
+		printf("Do you need to specify --exec-path or set GIT_EXEC_PATH?\n");
+		exit(1);
+	}
+
+	for (i = 0; i < gl.gl_pathc; i++) {
+		int len = strlen(gl.gl_pathv[i] + 4);
+
+		if (access(gl.gl_pathv[i], X_OK))
+			continue;
+
+		if (longest < len)
+			longest = len;
+
+		if (!tail)
+			tail = list = malloc(sizeof(struct string_list));
+		else {
+			tail->next = malloc(sizeof(struct string_list));
+			tail = tail->next;
+		}
+		tail->len = len;
+		tail->str = gl.gl_pathv[i] + 4;
+		tail->next = NULL;
+	}
+
+	printf("git commands available in '%s'\n", exec_path);
+	printf("----------------------------");
+	mput_char('-', strlen(exec_path));
+	putchar('\n');
+	pretty_print_string_list(list, longest);
+	putchar('\n');
+}
+
+#ifdef __GNUC__
+static void usage(const char *exec_path, const char *fmt, ...)
+	__attribute__((__format__(__printf__, 2, 3), __noreturn__));
+#endif
+static void usage(const char *exec_path, const char *fmt, ...)
+{
+	if (fmt) {
+		va_list ap;
+
+		va_start(ap, fmt);
+		printf("git: ");
+		vprintf(fmt, ap);
+		va_end(ap);
+		putchar('\n');
+	}
+	else
+		puts(git_usage);
+
+	putchar('\n');
+
+	if(exec_path)
+		list_commands(exec_path, "git-*");
+
+	exit(1);
+}
+
+static void prepend_to_path(const char *dir, int len)
+{
+	char *path, *old_path = getenv("PATH");
+	int path_len = len;
+
+	if (!old_path)
+		old_path = "/bin:/usr/bin:.";
+
+	path_len = len + strlen(old_path) + 1;
+
+	path = malloc(path_len + 1);
+	path[path_len + 1] = '\0';
+
+	memcpy(path, dir, len);
+	path[len] = ':';
+	memcpy(path + len + 1, old_path, path_len - len);
+
+	setenv("PATH", path, 1);
+}
+
+int main(int argc, char **argv, char **envp)
+{
+	char git_command[PATH_MAX + 1];
+	char wd[PATH_MAX + 1];
+	int i, len, show_help = 0;
+	char *exec_path = getenv("GIT_EXEC_PATH");
+
+	getcwd(wd, PATH_MAX);
+
+	if (!exec_path)
+		exec_path = GIT_EXEC_PATH;
+
+	for (i = 1; i < argc; i++) {
+		char *arg = argv[i];
+
+		if (strncmp(arg, "--", 2))
+			break;
+
+		arg += 2;
+
+		if (!strncmp(arg, "exec-path", 9)) {
+			arg += 9;
+			if (*arg == '=')
+				exec_path = arg + 1;
+			else {
+				puts(exec_path);
+				exit(0);
+			}
+		}
+		else if (!strcmp(arg, "version")) {
+			printf("git version %s\n", GIT_VERSION);
+			exit(0);
+		}
+		else if (!strcmp(arg, "help"))
+			show_help = 1;
+		else if (!show_help)
+			usage(NULL, NULL);
+	}
+
+	if (i >= argc || show_help)
+		usage(exec_path, NULL);
+
+	/* allow relative paths, but run with exact */
+	if (chdir(exec_path)) {
+		printf("git: '%s': %s\n", exec_path, strerror(errno));
+		exit (1);
+	}
+
+	getcwd(git_command, sizeof(git_command));
+	chdir(wd);
+
+	len = strlen(git_command);
+	prepend_to_path(git_command, len);
+
+	strncat(&git_command[len], "/git-", sizeof(git_command) - len);
+	len += 5;
+	strncat(&git_command[len], argv[i], sizeof(git_command) - len);
+
+	if (access(git_command, X_OK))
+		usage(exec_path, "'%s' is not a git-command", argv[i]);
+
+	/* execve() can only ever return if it fails */
+	execve(git_command, &argv[i], envp);
+	printf("Failed to run command '%s': %s\n", git_command, strerror(errno));
+
+	return 1;
+}