kernel: stat/fsstat/isatty syscalls, and O_APPEND

Four things userland had no way to ask for:

- SYS_STAT: type, link count, size and inode for a path. The trap needs HL
  for its dispatch table, so a call taking both a path and an output buffer
  passes a request block - the shape SYS_NET already established. usr/stat.h
  wraps it, header-only like sock.h, because the block must live in the
  caller's RAM.
- SYS_FSSTAT: blocks/inodes, total and free. The bitmaps are already cached
  write-through in fs_bmbuf, so this is the popcount boot_fs already prints.
- SYS_ISATTY: is this process's stdin the console? The shell needs it to tell
  an interactive session from a script.
- O_APPEND: open a file positioned at EOF. There is no seek syscall, so this
  is the only way to add to a file - without it nothing running ON the Game
  Boy can build a multi-line file, shell scripts included.

Together these back ls -l, df, and `>>`.

1 files changed, 36 insertions, 0 deletions

diff --git a/usr/stat.h b/usr/stat.h
new file mode 100644
index 0000000..10acc54
--- /dev/null
+++ b/usr/stat.h
@@ -0,0 +1,36 @@
+#ifndef STAT_H
+#define STAT_H
+/* stat.h - file metadata and filesystem totals. Header-only (one copy per
+   program), like sock.h: the request block has to live in the caller's RAM.
+   The kernel trap needs HL for its dispatch table, so a call with both a path
+   and an output buffer passes a request block rather than two pointers. */
+#include "gbos.h"
+
+#define ST_FILE 1
+#define ST_DIR  2
+
+struct gstat {
+    unsigned char type; /* ST_FILE / ST_DIR */
+    unsigned char nlink;
+    unsigned int size; /* bytes */
+    unsigned char ino;
+};
+
+static unsigned char _sreq[7]; /* path ptr, then type/nlink/size/ino */
+
+/* -> 1 on success, 0 if the path does not exist */
+static unsigned char gstat(const char *path, struct gstat *st) {
+    _sreq[0] = (unsigned char)(unsigned int)path;
+    _sreq[1] = (unsigned char)(((unsigned int)path) >> 8);
+    if (sys_stat(_sreq) != 0) return 0;
+    st->type = _sreq[2];
+    st->nlink = _sreq[3];
+    st->size = (unsigned int)_sreq[4] | ((unsigned int)_sreq[5] << 8);
+    st->ino = _sreq[6];
+    return 1;
+}
+
+/* out[] = { blocks total, blocks free, inodes total, inodes free } */
+static void gfsstat(unsigned char *out) { sys_fsstat(out); }
+
+#endif