setup: introduce startup_info->original_cwd

Removing the current working directory causes all subsequent git
commands run from that directory to get confused and fail with a message
about being unable to read the current working directory:

    $ git status
    fatal: Unable to read current working directory: No such file or directory

Non-git commands likely have similar warnings or even errors, e.g.

    $ bash -c 'echo hello'
    shell-init: error retrieving current directory: getcwd: cannot access parent directories: No such file or directory
    hello

This confuses end users, particularly since the command they get the
error from is not the one that caused the problem; the problem came from
the side-effect of some previous command.

We would like to avoid removing the current working directory of our
parent process; towards this end, introduce a new variable,
startup_info->original_cwd, that tracks the current working directory
that we inherited from our parent process.  For convenience of later
comparisons, we prefer that this new variable store a path relative to
the toplevel working directory (thus much like 'prefix'), except without
the trailing slash.

Subsequent commits will make use of this new variable.

Acked-by: Derrick Stolee <stolee@gmail.com>
Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1834,8 +1834,10 @@ void overlay_tree_on_index(struct index_state *istate,
 struct startup_info {
 	int have_repository;
 	const char *prefix;
+	const char *original_cwd;
 };
 extern struct startup_info *startup_info;
+extern const char *tmp_original_cwd;
 
 /* merge.c */
 struct commit_list;

common-main.c

@@ -26,6 +26,7 @@ static void restore_sigpipe_to_default(void)
 int main(int argc, const char **argv)
 {
 	int result;
+	struct strbuf tmp = STRBUF_INIT;
 
 	trace2_initialize_clock();
 
@@ -49,6 +50,9 @@ int main(int argc, const char **argv)
 	trace2_cmd_start(argv);
 	trace2_collect_process_info(TRACE2_PROCESS_INFO_STARTUP);
 
+	if (!strbuf_getcwd(&tmp))
+		tmp_original_cwd = strbuf_detach(&tmp, NULL);
+
 	result = cmd_main(argc, argv);
 
 	trace2_cmd_exit(result);

setup.c

@@ -12,6 +12,7 @@ static int work_tree_config_is_bogus;
 
 static struct startup_info the_startup_info;
 struct startup_info *startup_info = &the_startup_info;
+const char *tmp_original_cwd;
 
 /*
  * The input parameter must contain an absolute path, and it must already be
@@ -432,6 +433,69 @@ void setup_work_tree(void)
 	initialized = 1;
 }
 
+static void setup_original_cwd(void)
+{
+	struct strbuf tmp = STRBUF_INIT;
+	const char *worktree = NULL;
+	int offset = -1;
+
+	if (!tmp_original_cwd)
+		return;
+
+	/*
+	 * startup_info->original_cwd points to the current working
+	 * directory we inherited from our parent process, which is a
+	 * directory we want to avoid removing.
+	 *
+	 * For convience, we would like to have the path relative to the
+	 * worktree instead of an absolute path.
+	 *
+	 * Yes, startup_info->original_cwd is usually the same as 'prefix',
+	 * but differs in two ways:
+	 *   - prefix has a trailing '/'
+	 *   - if the user passes '-C' to git, that modifies the prefix but
+	 *     not startup_info->original_cwd.
+	 */
+
+	/* Normalize the directory */
+	strbuf_realpath(&tmp, tmp_original_cwd, 1);
+	free((char*)tmp_original_cwd);
+	tmp_original_cwd = NULL;
+	startup_info->original_cwd = strbuf_detach(&tmp, NULL);
+
+	/*
+	 * Get our worktree; we only protect the current working directory
+	 * if it's in the worktree.
+	 */
+	worktree = get_git_work_tree();
+	if (!worktree)
+		goto no_prevention_needed;
+
+	offset = dir_inside_of(startup_info->original_cwd, worktree);
+	if (offset >= 0) {
+		/*
+		 * If startup_info->original_cwd == worktree, that is already
+		 * protected and we don't need original_cwd as a secondary
+		 * protection measure.
+		 */
+		if (!*(startup_info->original_cwd + offset))
+			goto no_prevention_needed;
+
+		/*
+		 * original_cwd was inside worktree; precompose it just as
+		 * we do prefix so that built up paths will match
+		 */
+		startup_info->original_cwd = \
+			precompose_string_if_needed(startup_info->original_cwd
+						    + offset);
+		return;
+	}
+
+no_prevention_needed:
+	free((char*)startup_info->original_cwd);
+	startup_info->original_cwd = NULL;
+}
+
 static int read_worktree_config(const char *var, const char *value, void *vdata)
 {
 	struct repository_format *data = vdata;
@@ -1330,6 +1394,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		setenv(GIT_PREFIX_ENVIRONMENT, "", 1);
 	}
 
+	setup_original_cwd();
 
 	strbuf_release(&dir);
 	strbuf_release(&gitdir);