[RFC Patch v3 8/8] introduce a new API to aync read data from fd

[Wen Congyang <wency@cn.fujitsu.com>]

In colo mode, we will read some data from an fd.
Introduce a new API to avoid redundant codes.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
---
 tools/libxl/libxl_aoutils.c  | 73 ++++++++++++++++++++++++++++++++++++++++++++
 tools/libxl/libxl_internal.h | 30 ++++++++++++++++++
 2 files changed, 103 insertions(+)

diff --git a/tools/libxl/libxl_aoutils.c b/tools/libxl/libxl_aoutils.c
index 3e0c0ae..2d36403 100644
--- a/tools/libxl/libxl_aoutils.c
+++ b/tools/libxl/libxl_aoutils.c
@@ -542,3 +542,76 @@ bool libxl__async_exec_inuse(const libxl__async_exec_state *aes)
     assert(time_inuse == child_inuse);
     return child_inuse;
 }
+
+
+/*----- data reader -----*/
+
+static void libxl__datareader_init(libxl__datareader_state *drs)
+{
+    assert(drs->ao);
+    libxl__ev_fd_init(&drs->toread);
+    drs->used = 0;
+}
+
+static void libxl__datareader_kill(libxl__datareader_state *drs)
+{
+    STATE_AO_GC(drs->ao);
+
+    libxl__ev_fd_deregister(gc, &drs->toread);
+}
+
+static void datareader_callback(libxl__egc *egc, libxl__datareader_state *drs,
+                                ssize_t size, int errnoval)
+{
+    libxl__datareader_kill(drs);
+    drs->callback(egc, drs, size, errnoval);
+}
+
+static void datareader_readable(libxl__egc *egc, libxl__ev_fd *ev,
+                                int fd, short events, short revents)
+{
+    libxl__datareader_state *drs = CONTAINER_OF(ev, *drs, toread);
+    STATE_AO_GC(drs->ao);
+    int r;
+
+    if (revents & ~POLLIN) {
+        LOG(ERROR, "unexpected poll event 0x%x (should be POLLIN) on %s",
+            revents, drs->readwhat);
+        datareader_callback(egc, drs, -1, 0);
+        return;
+    }
+
+    assert(revents & POLLIN);
+    while (1) {
+        r = read(ev->fd, drs->buf + drs->used, drs->readsize - drs->used);
+        if (r < 0) {
+            if (errno == EINTR)
+                continue;
+            if (errno == EWOULDBLOCK)
+                break;
+            LOGE(ERROR, "error reading %s",
+                 drs->readwhat);
+            datareader_callback(egc, drs, 0, errno);
+            return;
+        }
+        if (r == 0) {
+            datareader_callback(egc, drs, drs->used, 0);
+            break;
+        }
+
+        drs->used += r;
+    }
+}
+
+int libxl__datareader_start(libxl__datareader_state *drs)
+{
+    int rc;
+    STATE_AO_GC(drs->ao);
+
+    libxl__datareader_init(drs);
+
+    rc = libxl__ev_fd_register(gc, &drs->toread, datareader_readable,
+                               drs->readfd, POLLIN);
+
+    return rc;
+}
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 313ad1c..893c208 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -2093,6 +2093,36 @@ void libxl__async_exec_init(libxl__async_exec_state *aes);
 int libxl__async_exec_start(libxl__gc *gc, libxl__async_exec_state *aes);
 bool libxl__async_exec_inuse(const libxl__async_exec_state *aes);
 
+/*----- datareader: read data from one fd to buffer -----*/
+
+typedef struct libxl__datareader_state libxl__datareader_state;
+
+/*
+ * real_size>=1 means all data was read
+ * real_size==0 means failure happened when reading, errnoval is valid, logged
+ * real_size==-1 means some other internal failure, errnoval not valid, logged
+ * In all cases reader is killed before calling this callback
+ */
+typedef void libxl__datareader_callback(libxl__egc *egc,
+     libxl__datareader_state *drs, ssize_t real_size, int errnoval);
+
+struct libxl__datareader_state {
+    /* caller must fill these in, and they must all remain valid */
+    libxl__ao *ao;
+    int readfd;
+    ssize_t readsize;
+    /* for error msgs */
+    const char *readwhat;
+    libxl__datareader_callback *callback;
+    /* It must contain enough space to store readsize bytes */
+    void *buf;
+    /* remaining fields are private to datareader */
+    libxl__ev_fd toread;
+    ssize_t used;
+};
+
+_hidden int libxl__datareader_start(libxl__datareader_state *drs);
+
 /*----- device addition/removal -----*/
 
 typedef struct libxl__ao_device libxl__ao_device;
-- 
1.9.3