[atari] Buffer line on minimal getchar

The Atari OS expects that IOCB screen editor inputs are line-atomic; it
corrupts the cursor if writes happen in the meantime. It's generally
expected in C that one can interleave getchar and putchar, and that
putchars only take effect after a line is entered. Accordingly, this
change line-buffers the minimal getchar for Atari targets.

Fixes #378

mos-platform/atari8-common/getchar.c

@@ -1,4 +1,5 @@
 #include <atari.h>
+#include <stdbool.h>
 #include <stdio.h>
 
 __attribute__((always_inline, weak)) int __to_ascii(void *ctx,
@@ -18,12 +19,38 @@ __attribute__((always_inline, weak)) int __to_ascii(void *ctx,
   }
 }
 
-int __getchar() {
-  OS.iocb[0].command = IOCB_GETCHR;
-  OS.iocb[0].buflen = 0;
+// The Atari OS screen editor expects that screen editor reads are atomic with
+// line granularity. Corruption results if characters are output while a read is
+// in progress. Since this is unusual on other C targets (it's usually fine to
+// interleave getchar and putchar), getchar is line buffered, even in the
+// minimal stdio here.
+
+// Provide space for a full logical line (3 screen lines and a newline).
+static char buf[40 * 3 + 1];
+// Index of next character to be read.
+static char buf_idx;
+// Length of the filled region of the buffer.
+static char buf_len;
+
+// Fill the input buffer and return true on error.
+static bool fill_buf(void) {
+  buf_idx = 0;
+  buf_len = 0;
+  OS.iocb[0].command = IOCB_GETREC;
+  OS.iocb[0].buffer = buf;
+  OS.iocb[0].buflen = sizeof(buf) - 1;
   const unsigned char channel = 0;
-  unsigned char c;
   unsigned char status;
-  asm volatile("jsr $e456\n" : "=a"(c), "=y"(status) : "x"(channel) : "p");
-  return status < 0x80 ? c : EOF;
+  asm volatile("jsr $e456\n" : "=y"(status) : "x"(channel) : "p");
+  if (status >= 0x80)
+    return EOF;
+  buf_len = OS.iocb[0].buflen;
+  return status >= 0x80;
+}
+
+int __getchar() {
+  if (buf_idx == buf_len)
+    if (fill_buf())
+      return EOF;
+  return buf[buf_idx++];
 }