[PATCH/RFC v3 1/4] setup: add gentle version of read_gitfile

["Erik Elfström" <erik.elfstrom@gmail.com>]

read_gitfile will die on most error cases. This makes it unsuitable
for speculative calls. Extract the core logic and provide a gentle
version that returns NULL on failure.

The first usecase of the new gentle version will be to probe for
submodules during git clean.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Erik Elfström <erik.elfstrom@gmail.com>
---
If this is going to be used for speculative probing should there
be a sanity check before:
	buf = xmalloc(st.st_size + 1);
	len = read_in_full(fd, buf, st.st_size);
Something like:
	if (st.st_size > PATH_MAX*2) {
		error = N;
		goto cleanup_return;
	{
What do you think?

 cache.h |  1 +
 setup.c | 94 ++++++++++++++++++++++++++++++++++++++++++++++++++---------------
 2 files changed, 74 insertions(+), 21 deletions(-)

diff --git a/cache.h b/cache.h
index 3d3244b..9d9199d 100644
--- a/cache.h
+++ b/cache.h
@@ -432,6 +432,7 @@ extern const char *get_git_namespace(void);
 extern const char *strip_namespace(const char *namespaced_ref);
 extern const char *get_git_work_tree(void);
 extern const char *read_gitfile(const char *path);
+extern const char *read_gitfile_gently(const char *path);
 extern const char *resolve_gitdir(const char *suspect);
 extern void set_git_work_tree(const char *tree);
 
diff --git a/setup.c b/setup.c
index 979b13f..a33b293 100644
--- a/setup.c
+++ b/setup.c
@@ -332,38 +332,46 @@ static int check_repository_format_gently(const char *gitdir, int *nongit_ok)
 	return 0;
 }
 
-/*
- * Try to read the location of the git directory from the .git file,
- * return path to git directory if found.
- */
-const char *read_gitfile(const char *path)
+static const char *read_gitfile_gently_or_non_gently(const char *path, int gently)
 {
-	char *buf;
+	int error = 0;
+	char *buf = NULL;
 	char *dir;
 	const char *slash;
 	struct stat st;
 	int fd;
 	ssize_t len;
-
-	if (stat(path, &st))
-		return NULL;
-	if (!S_ISREG(st.st_mode))
-		return NULL;
+	if (stat(path, &st)) {
+		error = 1;
+		goto cleanup_return;
+	}
+	if (!S_ISREG(st.st_mode)) {
+		error = 2;
+		goto cleanup_return;
+	}
 	fd = open(path, O_RDONLY);
-	if (fd < 0)
-		die_errno("Error opening '%s'", path);
+	if (fd < 0) {
+		error = 3;
+		goto cleanup_return;
+	}
 	buf = xmalloc(st.st_size + 1);
 	len = read_in_full(fd, buf, st.st_size);
 	close(fd);
-	if (len != st.st_size)
-		die("Error reading %s", path);
+	if (len != st.st_size) {
+		error = 4;
+		goto cleanup_return;
+	}
 	buf[len] = '\0';
-	if (!starts_with(buf, "gitdir: "))
-		die("Invalid gitfile format: %s", path);
+	if (!starts_with(buf, "gitdir: ")) {
+		error = 5;
+		goto cleanup_return;
+	}
 	while (buf[len - 1] == '\n' || buf[len - 1] == '\r')
 		len--;
-	if (len < 9)
-		die("No path in gitfile: %s", path);
+	if (len < 9) {
+		error = 6;
+		goto cleanup_return;
+	}
 	buf[len] = '\0';
 	dir = buf + 8;
 
@@ -378,14 +386,58 @@ const char *read_gitfile(const char *path)
 		buf = dir;
 	}
 
-	if (!is_git_directory(dir))
-		die("Not a git repository: %s", dir);
+	if (!is_git_directory(dir)) {
+		error = 7;
+		goto cleanup_return;
+	}
 	path = real_path(dir);
 
+cleanup_return:
 	free(buf);
+
+	if (error) {
+		if (gently)
+			return NULL;
+
+		switch (error) {
+		case 1: // failed to stat
+		case 2: // not regular file
+			return NULL;
+		case 3:
+			die_errno("Error opening '%s'", path);
+		case 4:
+			die("Error reading %s", path);
+		case 5:
+			die("Invalid gitfile format: %s", path);
+		case 6:
+			die("No path in gitfile: %s", path);
+		case 7:
+			die("Not a git repository: %s", dir);
+		default:
+			assert(0);
+		}
+	}
+
 	return path;
 }
 
+/*
+ * Try to read the location of the git directory from the .git file,
+ * return path to git directory if found, die on (most) failures.
+ */
+const char *read_gitfile(const char *path)
+{
+	return read_gitfile_gently_or_non_gently(path, 0);
+}
+
+/*
+ * Same as read_gitfile but return NULL on failure.
+ */
+const char *read_gitfile_gently(const char *path)
+{
+	return read_gitfile_gently_or_non_gently(path, 1);
+}
+
 static const char *setup_explicit_git_dir(const char *gitdirenv,
 					  struct strbuf *cwd,
 					  int *nongit_ok)
-- 
2.4.0.rc2.5.g2871d5e