Re: [PATCH] Fix git_mkstemp to return an error when path is too long.

Carlos Rica [off-list ref] writes:

Now the function returns -2 to the caller if the given buffer
is too short to save the entire path for the temporary file.

Signed-off-by: Carlos Rica <redacted>

Trying not to overstep the static buffer is of course a good
thing, and I think it is probably Ok to error out on a TMPDIR
environment variable that is insanely long, instead of using an
extra malloc/free, as underlying mkstemp() would error out if it
is given an insanely long template buffer anyway.

However, I think it is not so useful to return -2, even if that
is done so that you can differentiate the case where the TMPDIR
and/or the template were too long and the case mkstemp() errored
out.

Stop and think for a minute: what does the underlying mkstemp()
do, if the given template is too long?

That's right.  You would get ENAMETOOLONG.  So why don't we do
this instead?

---
 diff.c |    2 +-
 path.c |   25 ++++++++++---------------
 2 files changed, 11 insertions(+), 16 deletions(-)

diff --git a/diff.c b/diff.c
index cd6b0c4..a5fc56b 100644
--- a/diff.c
+++ b/diff.c

@@ -1695,7 +1695,7 @@ static void prep_temp_blob(struct diff_tempfile *temp,
 
 	fd = git_mkstemp(temp->tmp_path, PATH_MAX, ".diff_XXXXXX");
 	if (fd < 0)
-		die("unable to create temp-file");
+		die("unable to create temp-file: %s", strerror(errno));
 	if (write_in_full(fd, blob, size) != size)
 		die("unable to write temp-file");
 	close(fd);

diff --git a/path.c b/path.c
index c4ce962..dc7ded9 100644
--- a/path.c
+++ b/path.c

@@ -71,25 +71,20 @@ char *git_path(const char *fmt, ...)
 /* git_mkstemp() - create tmp file honoring TMPDIR variable */
 int git_mkstemp(char *path, size_t len, const char *template)
 {
-	char *env, *pch = path;
-
-	if ((env = getenv("TMPDIR")) == NULL) {
-		strcpy(pch, "/tmp/");
-		len -= 5;
-		pch += 5;
-	} else {
-		size_t n = snprintf(pch, len, "%s/", env);
-
-		len -= n;
-		pch += n;
+	const char *tmp;
+	size_t n;
+
+	tmp = getenv("TMPDIR");
+	if (!tmp)
+		tmp = "/tmp";
+	n = snprintf(path, len, "%s/%s", tmp, template);
+	if (len <= n) {
+		errno = ENAMETOOLONG;
+		return -1;
 	}
-
-	strlcpy(pch, template, len);
-
 	return mkstemp(path);
 }
 
-
 int validate_headref(const char *path)
 {
 	struct stat st;

I like that new solution. ENAMETOOLONG is fantastic!

Usually, the function is called using the long PATH_MAX constant,
however, the function doesn't force you to supply an insanely long
buffer like that, and therefore is nice to say when the limit is exceeded.