From e28f7b749e2e8895be9c177c633e4c1ac7067c7f Mon Sep 17 00:00:00 2001
From: chz <chz@bingularity.org>
Date: Mon, 21 Nov 2016 23:21:08 -0800
Subject: [PATCH] use read() for sequential reads on fd

Some special files will complain when accessed with pread (e.g.,
relayfs will give "illegal seek"). To avoid these errors, ioctx
uses read() instead of pread() when sequentially reading from a file.
---
 diod/ioctx.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/diod/ioctx.c b/diod/ioctx.c
index aabf3677..3b8d1971 100644
--- a/diod/ioctx.c
+++ b/diod/ioctx.c
@@ -66,6 +66,7 @@ struct ioctx_struct {
     pthread_mutex_t lock;
     int             refcount;
     int             fd;
+    off_t           off;
     DIR             *dir;
     int             lock_type;
     Npqid           qid;
@@ -181,6 +182,7 @@ _ioctx_create_open (Npuser *user, Path path, int flags, u32 mode)
     ioctx->refcount = 1;
     ioctx->lock_type = LOCK_UN;
     ioctx->dir = NULL;
+    ioctx->off = 0;
     ioctx->open_flags = flags;
     ioctx->user = user;
     np_user_incref (user);
@@ -266,7 +268,27 @@ ioctx_open (Npfid *fid, u32 flags, u32 mode)
 int
 ioctx_pread (IOCtx ioctx, void *buf, size_t count, off_t offset)
 {
-    return pread (ioctx->fd, buf, count, offset);
+    ssize_t ret;
+    int has_read = 0;
+
+    // claim read() if fd is at this offset
+    xpthread_mutex_lock (&ioctx->lock);
+    if (ioctx->off != -1 && ioctx->off == offset) {
+        ioctx->off = -1;
+        has_read = 1;
+    }
+    xpthread_mutex_unlock (&ioctx->lock);
+
+    if (!has_read) {
+        return pread (ioctx->fd, buf, count, offset);
+    }
+    ret = read (ioctx->fd, buf, count);
+
+    // next read() operation will follow the end of this one
+    xpthread_mutex_lock (&ioctx->lock);
+    ioctx->off = (ret < 0) ? offset : offset + ret;
+    xpthread_mutex_unlock (&ioctx->lock);
+    return ret;
 }
 
 int