Initial commit: Schunix as it stands before the SchOS rename

An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.

No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.

kernel/fs/pipe.c

@@ -0,0 +1,166 @@
+#include "types.h"
+#include "defs.h"
+#include "param.h"
+#include "mmu.h"
+#include "proc.h"
+#include "fs.h"
+#include "spinlock.h"
+#include "sleeplock.h"
+#include "file.h"
+#include "posix.h"
+#include "errno.h"
+
+#define PIPESIZE 512
+
+struct pipe {
+  struct spinlock lock;
+  char data[PIPESIZE];
+  uint nread;     // number of bytes read
+  uint nwrite;    // number of bytes written
+  int readopen;   // read fd is still open
+  int writeopen;  // write fd is still open
+};
+
+int
+pipealloc(struct file **f0, struct file **f1)
+{
+  struct pipe *p;
+
+  p = 0;
+  *f0 = *f1 = 0;
+  if((*f0 = filealloc()) == 0 || (*f1 = filealloc()) == 0)
+    goto bad;
+  if((p = (struct pipe*)kalloc()) == 0)
+    goto bad;
+  p->readopen = 1;
+  p->writeopen = 1;
+  p->nwrite = 0;
+  p->nread = 0;
+  initlock(&p->lock, "pipe");
+  (*f0)->type = FD_PIPE;
+  (*f0)->readable = 1;
+  (*f0)->writable = 0;
+  (*f0)->pipe = p;
+  (*f1)->type = FD_PIPE;
+  (*f1)->readable = 0;
+  (*f1)->writable = 1;
+  (*f1)->pipe = p;
+  return 0;
+
+//PAGEBREAK: 20
+ bad:
+  if(p)
+    kfree((char*)p);
+  if(*f0)
+    fileclose(*f0);
+  if(*f1)
+    fileclose(*f1);
+  return -1;
+}
+
+void
+pipeclose(struct pipe *p, int writable)
+{
+  acquire(&p->lock);
+  if(writable){
+    p->writeopen = 0;
+    wakeup(&p->nread);
+  } else {
+    p->readopen = 0;
+    wakeup(&p->nwrite);
+  }
+  if(p->readopen == 0 && p->writeopen == 0){
+    release(&p->lock);
+    kfree((char*)p);
+  } else
+    release(&p->lock);
+}
+
+//PAGEBREAK: 40
+int
+pipewrite(struct pipe *p, char *addr, int n)
+{
+  int i;
+
+  acquire(&p->lock);
+
+  // Writing to a pipe nobody is reading is SIGPIPE and EPIPE, in that order.
+  //
+  // This returned a bare -1 with no signal and no errno, so `yes | head -2`
+  // came back with whatever errno happened to be lying around -- "Invalid
+  // argument" -- instead of quietly ending the way every shell pipeline is
+  // built to. Checked up front as well as in the blocking loop below: the
+  // common case is that head has already exited by the time the next write
+  // arrives, and the pipe is nowhere near full.
+  if(p->readopen == 0){
+    release(&p->lock);
+    sigsend(myproc()->pid, SIGPIPE);
+    return -EPIPE;
+  }
+
+  for(i = 0; i < n; i++){
+    while(p->nwrite == p->nread + PIPESIZE){  //DOC: pipewrite-full
+      if(p->readopen == 0){
+        release(&p->lock);
+        sigsend(myproc()->pid, SIGPIPE);
+        return i > 0 ? i : -EPIPE;    // a short write is still a write
+      }
+      if(myproc()->killed){
+        release(&p->lock);
+        return -1;
+      }
+      wakeup(&p->nread);
+      sleep(&p->nwrite, &p->lock);  //DOC: pipewrite-sleep
+    }
+    p->data[p->nwrite++ % PIPESIZE] = addr[i];
+  }
+  wakeup(&p->nread);  //DOC: pipewrite-wakeup1
+  release(&p->lock);
+  return n;
+}
+
+int
+piperead(struct pipe *p, char *addr, int n)
+{
+  int i;
+
+  acquire(&p->lock);
+  while(p->nread == p->nwrite && p->writeopen){  //DOC: pipe-empty
+    if(myproc()->killed){
+      release(&p->lock);
+      return -1;
+    }
+    sleep(&p->nread, &p->lock); //DOC: piperead-sleep
+  }
+  for(i = 0; i < n; i++){  //DOC: piperead-copy
+    if(p->nread == p->nwrite)
+      break;
+    addr[i] = p->data[p->nread++ % PIPESIZE];
+  }
+  wakeup(&p->nwrite);  //DOC: piperead-wakeup
+  release(&p->lock);
+  return i;
+}
+
+// Readiness for poll(2)/select(2). Reports state, doesn't block.
+int
+pipe_poll(struct pipe *p, int readable, int writable)
+{
+  int rev = 0;
+
+  acquire(&p->lock);
+  if(readable){
+    if(p->nread != p->nwrite)
+      rev |= POLLIN;
+    if(p->writeopen == 0)
+      rev |= POLLHUP;
+  }
+  if(writable){
+    if(p->readopen == 0)
+      rev |= POLLERR;
+    else if(p->nwrite < p->nread + PIPESIZE)
+      rev |= POLLOUT;
+  }
+  release(&p->lock);
+  return rev;
+}