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

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(-)