jobs: background execution (&), non-blocking reap, spin demo

- sys_reap: nohang reap of a finished zombie child (-> pid or 0).
- shell: 'cmd &' launches in the background (prints [pid], no wait); reaps
  finished jobs at the prompt ([pid done]). Foreground now waits for its own
  child specifically (loops wait(), reporting bg completions meanwhile) so kill
  reports the right pid.
- spin: a process that yields forever - a target for ps/kill.
- verified: spin &; ps shows it; kill <pid> removes just that one; immortal
  init; self-finishing bg worker reaped with [pid done].

4 files changed, 38 insertions, 7 deletions

diff --git a/c/gbos.h b/c/gbos.h
index 5f22a5a..f76d1e0 100644
--- a/c/gbos.h
+++ b/c/gbos.h
@@ -41,4 +41,6 @@ unsigned char lookup(const char *name);     /* command name -> program id    */
 unsigned char kill(unsigned char pid);      /* terminate a pid (1 is immortal)*/
 unsigned char psget(unsigned char slot, unsigned char *buf); /* {pid,state,prog}*/
 void          progname(unsigned char id, char *buf);         /* id -> name     */
+void          yield(void);
+unsigned char reap(void);                   /* reap a finished bg job, or 0   */
 #endif
diff --git a/c/libc.s b/c/libc.s
index bcc8199..2e2516b 100644
--- a/c/libc.s
+++ b/c/libc.s
@@ -187,3 +187,15 @@ _progname:
 	ld	c, #21			; SYS_PROGNAME
 	rst	#0x30
 	ret
+
+	.globl _yield, _reap
+	;; void yield(void)
+_yield:
+	ld	c, #11			; SYS_YIELD
+	rst	#0x30
+	ret
+	;; unsigned char reap(void) -> A (reaped pid, or 0)
+_reap:
+	ld	c, #22			; SYS_REAP
+	rst	#0x30
+	ret
diff --git a/c/sh.c b/c/sh.c
index c24ed9f..8aa619e 100644
--- a/c/sh.c
+++ b/c/sh.c
@@ -34,9 +34,10 @@ static void set_cmdline(char **tok, unsigned char start, unsigned char end) {
     *p = 0;
 }
 
-/* run tok[start..end): parse its own >/< redirects, honor forced in/out. */
+/* run tok[start..end): parse its own >/< redirects, honor forced in/out.
+   If bg, launch in the background (don't wait; leave fds to the child). */
 static void run(char **tok, unsigned char start, unsigned char end,
-                unsigned char infd, unsigned char outfd) {
+                unsigned char infd, unsigned char outfd, unsigned char bg) {
     unsigned char i, cmdend = end, myin = NOFD, myout = NOFD, id, pid;
     for (i = start; i < end; i++) {
         if (isop(tok[i], '>')) {
@@ -58,7 +59,13 @@ static void run(char **tok, unsigned char start, unsigned char end,
         else if (outfd != NOFD) setout(outfd);
         exec(id);
     }
-    wait();
+    if (bg) { putc('['); putu(pid); puts("]"); nl(); return; }  /* background */
+    /* wait for THIS child; report any background jobs that finish meanwhile */
+    for (;;) {
+        unsigned char r = wait();
+        if (r == pid || r == NOFD) break;
+        putc('['); putu(r); puts(" done]"); nl();
+    }
 done:
     if (myin != NOFD) close(myin);
     if (myout != NOFD) close(myout);
@@ -68,7 +75,9 @@ void main(void) {
     char line[80];
     char *tok[16];
     for (;;) {
-        unsigned char nt, i, pipepos, c;
+        unsigned char nt, i, pipepos, c, bg;
+        /* reap finished background jobs */
+        { unsigned char p; while ((p = reap())) { putc('['); putu(p); puts(" done]"); nl(); } }
         puts("$ ");
         /* read a line (echoing), stop at newline; exit on EOF */
         i = 0;
@@ -81,19 +90,21 @@ void main(void) {
         line[i] = 0;
         nt = tokenize(line, tok, 16);
         if (nt == 0) continue;
+        bg = 0;
+        if (isop(tok[nt - 1], '&')) { bg = 1; nt--; if (nt == 0) continue; }
         pipepos = 0;
         for (i = 1; i < nt; i++)
             if (isop(tok[i], '|')) { pipepos = i; break; }
         if (pipepos) {
             unsigned char pw = open("__pipe", O_WRITE);
-            run(tok, 0, pipepos, NOFD, pw);
+            run(tok, 0, pipepos, NOFD, pw, 0);
             close(pw);
             { unsigned char pr = open("__pipe", O_READ);
-              run(tok, pipepos + 1, nt, pr, NOFD);
+              run(tok, pipepos + 1, nt, pr, NOFD, 0);
               close(pr); }
             fremove("__pipe");
         } else {
-            run(tok, 0, nt, NOFD, NOFD);
+            run(tok, 0, nt, NOFD, NOFD, bg);
         }
     }
 }
diff --git a/c/spin.c b/c/spin.c
new file mode 100644
index 0000000..2a3d4e8
--- /dev/null
+++ b/c/spin.c
@@ -0,0 +1,6 @@
+#include "gbos.h"
+/* spin: a harmless long-running process that just yields forever.
+   Useful as a target for `ps` and `kill`. */
+void main(void) {
+    for (;;) yield();
+}