[PATCH v2 2/9] setup: introduce startup_info->original_cwd

["Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com>]

From: Elijah Newren <newren@gmail.com>

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.

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 cache.h |  1 +
 git.c   |  2 ++
 setup.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 52 insertions(+)

diff --git a/cache.h b/cache.h
index eba12487b99..d7903c65b57 100644
--- a/cache.h
+++ b/cache.h
@@ -1834,6 +1834,7 @@ 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;
 
diff --git a/git.c b/git.c
index 5ff21be21f3..2c98ab48936 100644
--- a/git.c
+++ b/git.c
@@ -866,6 +866,8 @@ int cmd_main(int argc, const char **argv)
 
 	trace_command_performance(argv);
 
+	startup_info->original_cwd = xgetcwd();
+
 	/*
 	 * "git-xxxx" is the same as "git xxxx", but we obviously:
 	 *
diff --git a/setup.c b/setup.c
index 347d7181ae9..f30657723ea 100644
--- a/setup.c
+++ b/setup.c
@@ -432,6 +432,54 @@ 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;
+
+	/*
+	 * startup_info->original_cwd wass set early on in cmd_main(), unless
+	 * we're an auxiliary tool like git-remote-http or test-tool.
+	 */
+	if (!startup_info->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 incidentally 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, startup_info->original_cwd, 1);
+	free((char*)startup_info->original_cwd);
+	startup_info->original_cwd = strbuf_detach(&tmp, NULL);
+
+	/* Find out if this is in the worktree */
+	worktree = get_git_work_tree();
+	if (worktree)
+		offset = dir_inside_of(startup_info->original_cwd, worktree);
+	if (offset >= 0) {
+		/*
+		 * 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);
+	}
+}
+
 static int read_worktree_config(const char *var, const char *value, void *vdata)
 {
 	struct repository_format *data = vdata;
@@ -1330,6 +1378,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		setenv(GIT_PREFIX_ENVIRONMENT, "", 1);
 	}
 
+	setup_original_cwd();
 
 	strbuf_release(&dir);
 	strbuf_release(&gitdir);
-- 
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