sl0ppty: expand ~ in a path that a config file wrote

`cwd="~/dev/api"` is in the README, in config/layout.example.kdl, and did
nothing. `~` is a shell thing, and no shell has touched a path that arrives
from a layout file -- so it went to chdir() literally, failed, and left the
pane in whatever directory the session happened to be started from. The pane
still ran, so nothing looked wrong until you typed `ls`.

path_expand() handles `~` and `~/...` against $HOME and returns everything
else untouched, including `~user`, which needs the password database and is
not worth linking it in for -- returned unchanged rather than half-expanded
into something wrong.

Applied where a layout's cwd is read, which is one place because a pane
inherits its tab's. Found while documenting a shader directory that was
about to have exactly the same bug.

1 files changed, 25 insertions, 0 deletions

diff --git a/src/path.c b/src/path.c
new file mode 100644
index 0000000..adf824b
--- /dev/null
+++ b/src/path.c
@@ -0,0 +1,25 @@
+/* Paths as people write them.
+ *
+ * `~` is a shell thing: by the time a path reaches us from a config or layout
+ * file no shell has touched it, so `cwd="~/dev/api"` was being handed to
+ * chdir() literally, failing, and leaving the pane in whatever directory the
+ * session happened to be in. Silently \u2014 which is the worst way for a path to
+ * be wrong, because the pane still starts and still works.
+ */
+#include "sl0ppty.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+const char *path_expand(const char *path, char *buf, size_t cap) {
+  if (!path) return NULL;
+  /* `~` and `~/...` only. `~user` needs the password database and is not
+   * worth linking it in for; it is returned unchanged, which is exactly what
+   * it did before, rather than being half-expanded into something wrong. */
+  if (path[0] != '~' || (path[1] && path[1] != '/')) return path;
+  const char *home = getenv("HOME");
+  if (!home || !*home) return path;
+  snprintf(buf, cap, "%s%s", home, path + 1);
+  return buf;
+}