[PATCH v4] setup: don't die if realpath(3) fails on getcwd(3)

[Kevin Locke <kevin@kevinlocke.name>]

Prior to Git 2.35.0, git could be run from an inaccessible working
directory so long as the git repository specified by options and/or
environment variables was accessible.  For example:

    git init repo
    mkdir -p a/b
    cd a/b
    chmod u-x ..
    git -C "${PWD%/a/b}/repo" status

If this example seems a bit contrived, consider running with the
repository owner as a substitute UID (e.g. with runuser(1) or sudo(8))
without ensuring the working directory is accessible by that user.

The code added by e6f8861bd4 ("setup: introduce
startup_info->original_cwd") to preserve the working directory attempts
to normalize the path using strbuf_realpath().  If that fails, as in the
case above, it is treated as a fatal error.

This commit treats strbuf_realpath() errors as non-fatal.  If an error
occurs, setup_original_cwd() will continue without applying removal
prevention for cwd, resulting in the pre-2.35.0 behavior.  The risk
should be minimal, since git will not operate on a repository with
inaccessible ancestors, this behavior is only known to occur when cwd is
a descendant of the repository, an ancestor of cwd is inaccessible, and
no ancestors of the repository are inaccessible.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
---

Changes since v3:
 * Free tmp_original_cwd in both codepaths.
 * Return after strbuf_realpath() fails, rather than jumping to
   no_prevention_needed, to avoid unnecessary free(NULL) and NULL
   reassignment.
 * Invert the condition and remove the else block to match the
   return-on-error code style for better readability.
 * Stop adding "Try" to comment, since strbuf_realpath() hasn't
   been optional since v1.

Changes since v2:
 * Use trace2_data_string(), rather than trace_printf(), to report
   realpath failure.

Changes since v1:
 * Set startup_info->original_cwd = NULL when strbuf_realpath() fails,
   rather than setting it to the un-normalized path.
 * Add a trace message when realpath fails to aid debugging.
 * Remove potential realpath failure cause from comment before it.
 * Improve format for reference to e6f8861bd4 in commit message.
 * Clarify when the pre-2.35.0 behavior may occur as a result of this
   commit in the commit message.
 * Remove 'Fixes:' tag from commit message.

 setup.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/setup.c b/setup.c
index a7b36f3ffbf..e0a99df512f 100644
--- a/setup.c
+++ b/setup.c
@@ -459,7 +459,16 @@ static void setup_original_cwd(void)
 	 */
 
 	/* Normalize the directory */
-	strbuf_realpath(&tmp, tmp_original_cwd, 1);
+	if (!strbuf_realpath(&tmp, tmp_original_cwd, 0)) {
+		trace2_data_string("setup", the_repository,
+				   "realpath-path", tmp_original_cwd);
+		trace2_data_string("setup", the_repository,
+				   "realpath-failure", strerror(errno));
+		free((char*)tmp_original_cwd);
+		tmp_original_cwd = NULL;
+		return;
+	}
+
 	free((char*)tmp_original_cwd);
 	tmp_original_cwd = NULL;
 	startup_info->original_cwd = strbuf_detach(&tmp, NULL);
-- 
2.35.1