Re: [RFC PATCH v2 3/4] setup: introduce new helper 'is_git_directory_verbose'

diff --git a/setup.c b/setup.c
index e9a9ecda19..a0fb68f7f6 100644
--- a/setup.c
+++ b/setup.c

@@ -347,7 +347,7 @@ int get_common_dir_noenv(struct strbuf *sb, const char *gitdir)
 	return ret;
 }
 
-static int validate_headref(const char *path)
+static int validate_headref(const char *path, struct strbuf *err)
 {
 	struct stat st;
 	char buffer[256];

@@ -356,14 +356,23 @@ static int validate_headref(const char *path)
 	int fd;
 	ssize_t len;
 
-	if (lstat(path, &st) < 0)
+	if (lstat(path, &st) < 0) {
+		if (err)
+			strbuf_addf(err, _("could not stat HEAD at '%s'"), path);

 		return -1;
+	}
 
 	/* Make sure it is a "refs/.." symlink */
 	if (S_ISLNK(st.st_mode)) {
 		len = readlink(path, buffer, sizeof(buffer)-1);
 		if (len >= 5 && !memcmp("refs/", buffer, 5))
 			return 0;
+		if (len == -1 && err)
+			strbuf_addf(err, _("could not read the symlink HEAD at '%s'"),
+				    path);

@@ -396,9 +411,71 @@ static int validate_headref(const char *path)
 	if (get_oid_hex_any(buffer, &oid) != GIT_HASH_UNKNOWN)
 		return 0;
 
+	if (err)
+		strbuf_addf(err, _("HEAD at '%s' does not point to a valid symbolic"
+				   " link or an object ID"), path);
+
 	return -1;
 }
 
+/*
+ * A variant of is_git_directory that gives additional
+ * context via 'err' about why a given suspect is not
+ * a valid git repository.
+ */
+static int is_git_directory_verbose(const char *suspect, struct strbuf *err)
+{
+	struct strbuf path = STRBUF_INIT;
+	char *objdir;
+	int ret = 0;
+	size_t len;
+
+	/* Check worktree-related signatures */
+	strbuf_addstr(&path, suspect);
+	strbuf_complete(&path, '/');
+	strbuf_addstr(&path, "HEAD");
+	if (validate_headref(path.buf, err))
+		goto done;
+
+	strbuf_reset(&path);
+	get_common_dir(&path, suspect);
+	len = path.len;
+
+	/* Check non-worktree-related signatures */
+	objdir = getenv(DB_ENVIRONMENT);
+	if (objdir) {
+		if (access(objdir, X_OK)) {
+			if (err)
+				strbuf_addf(err, _("cannot access object directory '%s'"
+						   " set via $%s\n"), objdir, DB_ENVIRONMENT);
+			goto done;
+		}
+	} else {
+		strbuf_setlen(&path, len);
+		strbuf_addstr(&path, "/objects");
+		if (access(path.buf, X_OK)) {
+			if (err)
+				strbuf_addf(err, _("cannot access object directory '%s'"),
+					    path.buf);
+			goto done;
+		}
+	}
+
+	strbuf_setlen(&path, len);
+	strbuf_addstr(&path, "/refs");
+	if (access(path.buf, X_OK)) {
+		if (err)
+			strbuf_addf(err, _("cannot access refs directory '%s'"), path.buf);
+		goto done;
+	}
+
+	ret = 1;
+done:
+	strbuf_release(&path);
+	return ret;
+
+}
+
 /*
  * Test if it looks like we're at a git directory.
  * We want to see: