tools/libxl: Extend datacopier to support reading into a buffer

Currently a datacopier may source its data from an fd or local buffer, but its
destination must be an fd.  For migration v2, libxl needs to read from the
migration stream into a local buffer.

Implement a "read into local buffer" mode, invoked when readbuf is set and
writefd is -1.  On success, the callback passes the number of bytes read.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
[Rewrite commit message]
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/libxl/libxl_aoutils.c

@@ -134,7 +134,7 @@ static void datacopier_check_state(libxl__egc *egc, libxl__datacopier_state *dc)
     STATE_AO_GC(dc->ao);
     int rc;
     
-    if (dc->used) {
+    if (dc->used && !dc->readbuf) {
         if (!libxl__ev_fd_isregistered(&dc->towrite)) {
             rc = libxl__ev_fd_register(gc, &dc->towrite, datacopier_writable,
                                        dc->writefd, POLLOUT);
@@ -148,7 +148,7 @@ static void datacopier_check_state(libxl__egc *egc, libxl__datacopier_state *dc)
     } else if (!libxl__ev_fd_isregistered(&dc->toread) ||
                dc->bytes_to_read == 0) {
         /* we have had eof */
-        datacopier_callback(egc, dc, 0, 0);
+        datacopier_callback(egc, dc, 0, dc->readbuf ? dc->used : 0);
         return;
     } else {
         /* nothing buffered, but still reading */
@@ -216,25 +216,31 @@ static void datacopier_readable(libxl__egc *egc, libxl__ev_fd *ev,
     }
     assert(revents & POLLIN);
     for (;;) {
-        while (dc->used >= dc->maxsz) {
-            libxl__datacopier_buf *rm = LIBXL_TAILQ_FIRST(&dc->bufs);
-            dc->used -= rm->used;
-            assert(dc->used >= 0);
-            LIBXL_TAILQ_REMOVE(&dc->bufs, rm, entry);
-            free(rm);
-        }
+        libxl__datacopier_buf *buf = NULL;
+        int r;
 
-        libxl__datacopier_buf *buf =
-            LIBXL_TAILQ_LAST(&dc->bufs, libxl__datacopier_bufs);
-        if (!buf || buf->used >= sizeof(buf->buf)) {
-            buf = malloc(sizeof(*buf));
-            if (!buf) libxl__alloc_failed(CTX, __func__, 1, sizeof(*buf));
-            buf->used = 0;
-            LIBXL_TAILQ_INSERT_TAIL(&dc->bufs, buf, entry);
-        }
-        int r = read(ev->fd, buf->buf + buf->used,
+        if (dc->readbuf) {
+            r = read(ev->fd, dc->readbuf + dc->used, dc->bytes_to_read);
+        } else {
+            while (dc->used >= dc->maxsz) {
+                libxl__datacopier_buf *rm = LIBXL_TAILQ_FIRST(&dc->bufs);
+                dc->used -= rm->used;
+                assert(dc->used >= 0);
+                LIBXL_TAILQ_REMOVE(&dc->bufs, rm, entry);
+                free(rm);
+            }
+
+            buf = LIBXL_TAILQ_LAST(&dc->bufs, libxl__datacopier_bufs);
+            if (!buf || buf->used >= sizeof(buf->buf)) {
+                buf = malloc(sizeof(*buf));
+                if (!buf) libxl__alloc_failed(CTX, __func__, 1, sizeof(*buf));
+                buf->used = 0;
+                LIBXL_TAILQ_INSERT_TAIL(&dc->bufs, buf, entry);
+            }
+            r = read(ev->fd, buf->buf + buf->used,
                      min_t(size_t, sizeof(buf->buf) - buf->used,
                            (dc->bytes_to_read == -1) ? SIZE_MAX : dc->bytes_to_read));
+        }
         if (r < 0) {
             if (errno == EINTR) continue;
             if (errno == EWOULDBLOCK) break;
@@ -257,9 +263,11 @@ static void datacopier_readable(libxl__egc *egc, libxl__ev_fd *ev,
                 return;
             }
         }
-        buf->used += r;
+        if (!dc->readbuf) {
+            buf->used += r;
+            assert(buf->used <= sizeof(buf->buf));
+        }
         dc->used += r;
-        assert(buf->used <= sizeof(buf->buf));
         if (dc->bytes_to_read > 0)
             dc->bytes_to_read -= r;
         if (dc->bytes_to_read == 0)
@@ -318,15 +326,20 @@ int libxl__datacopier_start(libxl__datacopier_state *dc)
 
     libxl__datacopier_init(dc);
 
+    assert(dc->readfd >= 0 || dc->writefd >= 0);
+    assert(!(dc->readbuf && dc->bytes_to_read == -1));
+
     if (dc->readfd >= 0) {
         rc = libxl__ev_fd_register(gc, &dc->toread, datacopier_readable,
                                    dc->readfd, POLLIN);
         if (rc) goto out;
     }
 
-    rc = libxl__ev_fd_register(gc, &dc->towrite, datacopier_writable,
-                               dc->writefd, POLLOUT);
-    if (rc) goto out;
+    if (dc->writefd >= 0) {
+        rc = libxl__ev_fd_register(gc, &dc->towrite, datacopier_writable,
+                                   dc->writefd, POLLOUT);
+        if (rc) goto out;
+    }
 
     return 0;
 

tools/libxl/libxl_internal.h

@@ -2524,8 +2524,9 @@ typedef struct libxl__datacopier_buf libxl__datacopier_buf;
 
 /* onwrite==1 means failure happened when writing, logged, errnoval is valid
  * onwrite==0 means failure happened when reading
- *     errnoval==0 means we got eof and all data was written
- *     errnoval!=0 means we had a read error, logged
+ *     errnoval>=0 means we got eof and all data was written or number of bytes
+ *                 written when in read mode
+ *     errnoval<0 means we had a read error, logged
  * onwrite==-1 means some other internal failure, errnoval not valid, logged
  * If we get POLLHUP, we call callback_pollhup(..., onwrite, -1);
  * or if callback_pollhup==0 this is an internal failure, as above.
@@ -2550,6 +2551,9 @@ struct libxl__datacopier_state {
     FILE *log; /* gets a copy of everything */
     libxl__datacopier_callback *callback;
     libxl__datacopier_callback *callback_pollhup;
+    void *readbuf; /* Set this to read data into it without writing to an
+                      fd. The buffer should be at least as large as the
+                      bytes_to_read parameter, which should not be -1. */
     /* remaining fields are private to datacopier */
     libxl__ev_fd toread, towrite;
     ssize_t used;