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 `>>`.

3 files changed, 144 insertions, 1 deletions

diff --git a/src/fs.asm b/src/fs.asm
index 9aeaeb7..30dfdd6 100644
--- a/src/fs.asm
+++ b/src/fs.asm
@@ -698,7 +698,7 @@ sys_open::
 .write
     ld a, [wFsInode]
     or a
-    jr nz, .trunc
+    jr nz, .exists
     ; --- create ---
     call alloc_inode
     or a
@@ -724,6 +724,10 @@ sys_open::
     or a
     jr z, .fail
     jr .allocof
+.exists
+    ld a, [wFsMode]
+    cp O_APPEND
+    jr z, .allocof              ; append: keep the data, seek to the end below
 .trunc
     ld a, [wFsInode]
     call file_truncate
@@ -755,6 +759,34 @@ sys_open::
     xor a
     ld [hl+], a                 ; pos lo
     ld [hl], a                  ; pos hi
+    ld a, [wFsMode]
+    cp O_APPEND
+    jr nz, .ofdone
+    ; append: start writing at the end. With no seek syscall this is the only
+    ; way to add to a file - and without it nothing on the Game Boy can build
+    ; a multi-line file, shell scripts included.
+    push bc
+    dec hl                      ; HL = &pos lo
+    push hl
+    ld a, [wFsInode]
+    call inode_ptr
+    ld a, l
+    add I_SIZE
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a
+    ld a, [hl+]
+    ld c, a
+    ld a, [hl]
+    ld b, a                     ; BC = size
+    pop hl
+    ld a, c
+    ld [hl+], a
+    ld a, b
+    ld [hl], a                  ; pos = size
+    pop bc
+.ofdone
     ld a, c
     ret
 
@@ -1162,6 +1194,100 @@ sys_chdir::
     ret
 
 ; =============================================================================
+; sys_stat(DE = &statreq) -> A = 0 ok / $FF no such path
+;
+; req[SR_PATH] is a pointer to the path; the type, link count, size and inode
+; come back in the same block. (The syscall trap needs HL for its dispatch
+; table, so a call with both a path and an output buffer uses a request block -
+; the shape SYS_NET already established.)
+; =============================================================================
+sys_stat::
+    ld a, e
+    ld [wFsStatReq], a
+    ld a, d
+    ld [wFsStatReq+1], a
+    ld h, d
+    ld l, e
+    ld a, [hl+]                 ; DE = req.path
+    ld e, a
+    ld a, [hl]
+    ld d, a
+    call copy_path
+    call resolve                ; A = inode or 0 (handles "/", "." and paths)
+    or a
+    jr z, .fail
+    ld [wFsInode], a
+    call inode_ptr              ; HL = &inode: type, nlink, size lo, size hi
+    ld a, [wFsStatReq]
+    add SR_TYPE
+    ld e, a
+    ld a, [wFsStatReq+1]
+    adc 0
+    ld d, a                     ; DE = &req[SR_TYPE]
+    ld b, 4
+.cpst
+    ld a, [hl+]
+    ld [de], a
+    inc de
+    dec b
+    jr nz, .cpst
+    ld a, [wFsInode]
+    ld [de], a                  ; SR_INO follows SR_SIZE
+    xor a
+    ret
+.fail
+    ld a, $FF
+    ret
+
+; =============================================================================
+; sys_fsstat(DE = buf[4]) -> buf = { blocks total, blocks free,
+;                                    inodes total, inodes free }
+; The bitmaps are already cached in fs_bmbuf (write-through), so this is just
+; a popcount - the same one boot_fs prints at startup.
+; =============================================================================
+sys_fsstat::
+    ld a, FS_NBLOCKS
+    ld [de], a
+    inc de
+    ld hl, fs_bmbuf + BM_BLOCKS
+    ld b, FS_NBLOCKS / 8
+    call bm_free
+    ld a, c
+    ld [de], a
+    inc de
+    ld a, NINODES
+    ld [de], a
+    inc de
+    ld hl, fs_bmbuf + BM_INODES
+    ld b, NINODES / 8
+    call bm_free
+    ld a, c
+    ld [de], a
+    xor a
+    ret
+
+; bm_free - HL = bitmap, B = bytes -> C = count of CLEAR bits (= free units).
+; Preserves DE (the caller is walking its output buffer with it).
+bm_free:
+    ld c, 0
+.byte
+    ld a, [hl+]
+    cpl                         ; set bits = free
+    push de
+    ld e, 8
+.bit
+    rra
+    jr nc, .used
+    inc c
+.used
+    dec e
+    jr nz, .bit
+    pop de
+    dec b
+    jr nz, .byte
+    ret
+
+; =============================================================================
 ; sys_opendir(DE = path) -> A = 0 ok / $FF ; sets the directory ls enumerates.
 ; Empty path or "." means the current directory.
 ; =============================================================================
diff --git a/src/kdata.asm b/src/kdata.asm
index 7a6aab9..b6c2b68 100644
--- a/src/kdata.asm
+++ b/src/kdata.asm
@@ -53,6 +53,7 @@ wFsDir::     DS 1       ; directory inode for dir_* ops
 wFsDirBlk::  DS 1       ; that directory's data block (for write-back)
 wFsCur::     DS 1       ; current inode during path resolution
 wFsPath::    DS 40      ; kernel-side copy of a path argument
+wFsStatReq:: DS 2       ; sys_stat: the caller's request block
 wFsSlashPtr::DS 2       ; last '/' seen while splitting a path
 wListDir::   DS 1       ; directory sys_list enumerates
 wKChar::     DS 1       ; scratch for KPutc
diff --git a/src/syscall.asm b/src/syscall.asm
index 0ed9f85..9e5ca33 100644
--- a/src/syscall.asm
+++ b/src/syscall.asm
@@ -71,6 +71,9 @@ SyscallTable:
     dw sys_sleep                ; 34 SLEEP
     dw sys_pollcon              ; 35 POLLCON
     dw sys_uptime               ; 36 UPTIME
+    dw sys_stat                 ; 37 STAT
+    dw sys_fsstat               ; 38 FSSTAT
+    dw sys_isatty               ; 39 ISATTY
 
 ; -----------------------------------------------------------------------------
 sys_nosys:
@@ -332,6 +335,19 @@ sys_sleep:
     ret
 
 ; -----------------------------------------------------------------------------
+; sys_isatty() -> A = 1 if this process's stdin is the console, 0 if it is a
+; file or a pipe. The shell uses it to tell an interactive session from a
+; script (`sh < file`): no prompt, no echo, and EOF is just the end.
+; -----------------------------------------------------------------------------
+sys_isatty:
+    call CurStdin
+    cp STD_CONSOLE
+    ld a, 0
+    ret nz
+    inc a
+    ret
+
+; -----------------------------------------------------------------------------
 ; sys_exit(code=B) -- become a zombie awaiting reap; never returns.
 ;   - store exit code, set PS_ZOMBIE
 ;   - reparent any children to init (so they can still be waited on)