syscalls/pipe: turn into a POSIX-style api with separate rw ends

Without separate read/write ends you can't tell when there are no more writers left if you have multiple readers. Consider this piece of code:
	int fd = pipe();
	fork(); // execution continues in 2 processes
	while (read(fd, &some_buf, sizeof somebuf) >= 0) {
		...
	}

Once both processes call `read()`, it's obvious that no writes are possible - all the processes that hold a reference to the pipe are currently stuck on a `read()` call, so the kernel could just make it return an error in both. But, what then? It's still possible to write to the pipe, and you can't know if the other process will do that. Thus, if you don't want to miss any output, you have to keep reading the pipe. Forever. Both processes end up stuck.

Having separate read/write ends prevents that.

1 files changed, 30 insertions, 9 deletions

diff --git a/src/kernel/pipe.c b/src/kernel/pipe.c
index 8afd359..34cb263 100644
--- a/src/kernel/pipe.c
+++ b/src/kernel/pipe.c
@@ -3,29 +3,41 @@
 #include <kernel/pipe.h>
 #include <kernel/util.h>
 
-void pipe_joinqueue(struct handle *h, bool wants_write,
+bool pipe_joinqueue(struct handle *h, bool wants_write,
 		struct process *proc, void __user *pbuf, size_t pbuflen)
 {
-	struct process **slot = wants_write ? &h->pipe.reader : &h->pipe.writer;
-	if (*slot) {
-		assert((*slot)->state == PS_WAITS4PIPE);
+	assert(h && h->type == HANDLE_PIPE);
+	if (wants_write == h->pipe.write_end) return false;
+	if (!h->pipe.sister) return false;
+	if (h->pipe.queued) {
+		assert(h->pipe.queued->state == PS_WAITS4PIPE);
 		panic_unimplemented();
 	}
 
 	process_transition(proc, PS_WAITS4PIPE);
-	*slot = proc;
+	h->pipe.queued = proc;
 	proc->waits4pipe.pipe = h;
 	proc->waits4pipe.buf = pbuf;
 	proc->waits4pipe.len = pbuflen;
+	return true;
 }
 
 void pipe_trytransfer(struct handle *h) {
-	struct process *rdr = h->pipe.reader, *wtr = h->pipe.writer;
+	struct process *rdr, *wtr;
 	int len;
+	assert(h);
+	if (!h->pipe.sister) {
+		assert(!h->pipe.queued);
+		return;
+	}
+
+	rdr = h->pipe.write_end ? h->pipe.sister->pipe.queued : h->pipe.queued;
+	wtr = h->pipe.write_end ? h->pipe.queued : h->pipe.sister->pipe.queued;
+
 	if (!(rdr && wtr)) return;
 	assert(rdr->state == PS_WAITS4PIPE);
 	assert(wtr->state == PS_WAITS4PIPE);
-	
+
 	len = min(rdr->waits4pipe.len, wtr->waits4pipe.len);
 
 	if (!virt_cpy(
@@ -36,8 +48,17 @@ void pipe_trytransfer(struct handle *h) {
 	}
 	process_transition(rdr, PS_RUNNING);
 	process_transition(wtr, PS_RUNNING);
-	h->pipe.reader = NULL;
-	h->pipe.writer = NULL;
+	h->pipe.queued = NULL;
+	h->pipe.sister->pipe.queued = NULL;
 	regs_savereturn(&rdr->regs, len);
 	regs_savereturn(&wtr->regs, len);
 }
+
+void pipe_invalidate_end(struct handle *h) {
+	struct process *p = h->pipe.queued;
+	if (p) {
+		process_transition(p, PS_RUNNING);
+		regs_savereturn(&p->regs, -1);
+	}
+	h->pipe.queued = NULL;
+}