[Xen-devel] [PATCH v3 4/6] tools/libxl: Allow limiting amount copied by datacopier

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

Currently, a datacopier will unconditionally read until EOF on its read fd.

For migration v2, libxl needs to read records of a specific length out of the
migration stream, without reading any further data.

Introduce a parameter, bytes_to_read, which may be used to stop the datacopier
ahead of reaching EOF. If bytes_to_read is set to -1, then the datacopier will
read until EOF.

Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
[Rewrite commit message]
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
---
 tools/libxl/libxl_aoutils.c    | 13 +++++++++----
 tools/libxl/libxl_bootloader.c |  2 ++
 tools/libxl/libxl_dom.c        |  1 +
 tools/libxl/libxl_internal.h   |  1 +
 4 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/tools/libxl/libxl_aoutils.c b/tools/libxl/libxl_aoutils.c
index 6882ca3..dff5d71 100644
--- a/tools/libxl/libxl_aoutils.c
+++ b/tools/libxl/libxl_aoutils.c
@@ -145,7 +145,8 @@ static void datacopier_check_state(libxl__egc *egc, 
libxl__datacopier_state *dc)
                 return;
             }
         }
-    } else if (!libxl__ev_fd_isregistered(&dc->toread)) {
+    } else if (!libxl__ev_fd_isregistered(&dc->toread) ||
+               dc->bytes_to_read == 0) {
         /* we have had eof */
         datacopier_callback(egc, dc, 0, 0);
         return;
@@ -231,9 +232,9 @@ static void datacopier_readable(libxl__egc *egc, 
libxl__ev_fd *ev,
             buf->used = 0;
             LIBXL_TAILQ_INSERT_TAIL(&dc->bufs, buf, entry);
         }
-        int r = read(ev->fd,
-                     buf->buf + buf->used,
-                     sizeof(buf->buf) - buf->used);
+        int 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;
@@ -259,6 +260,10 @@ static void datacopier_readable(libxl__egc *egc, 
libxl__ev_fd *ev,
         buf->used += r;
         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)
+            break;
     }
     datacopier_check_state(egc, dc);
 }
diff --git a/tools/libxl/libxl_bootloader.c b/tools/libxl/libxl_bootloader.c
index 79947d4..0263ef9 100644
--- a/tools/libxl/libxl_bootloader.c
+++ b/tools/libxl/libxl_bootloader.c
@@ -516,6 +516,7 @@ static void bootloader_gotptys(libxl__egc *egc, 
libxl__openpty_state *op)
 
     bl->keystrokes.ao = ao;
     bl->keystrokes.maxsz = BOOTLOADER_BUF_OUT;
+    bl->keystrokes.bytes_to_read = -1;
     bl->keystrokes.copywhat =
         GCSPRINTF("bootloader input for domain %"PRIu32, bl->domid);
     bl->keystrokes.callback =         bootloader_keystrokes_copyfail;
@@ -527,6 +528,7 @@ static void bootloader_gotptys(libxl__egc *egc, 
libxl__openpty_state *op)
 
     bl->display.ao = ao;
     bl->display.maxsz = BOOTLOADER_BUF_IN;
+    bl->display.bytes_to_read = -1;
     bl->display.copywhat =
         GCSPRINTF("bootloader output for domain %"PRIu32, bl->domid);
     bl->display.callback =         bootloader_display_copyfail;
diff --git a/tools/libxl/libxl_dom.c b/tools/libxl/libxl_dom.c
index a16d4a1..108676f 100644
--- a/tools/libxl/libxl_dom.c
+++ b/tools/libxl/libxl_dom.c
@@ -1922,6 +1922,7 @@ void libxl__domain_save_device_model(libxl__egc *egc,
     dc->readfd = -1;
     dc->writefd = fd;
     dc->maxsz = INT_MAX;
+    dc->bytes_to_read = -1;
     dc->copywhat = GCSPRINTF("qemu save file for domain %"PRIu32, dss->domid);
     dc->writewhat = "save/migration stream";
     dc->callback = save_device_model_datacopier_done;
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index fcbec7f..58ac658 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -2545,6 +2545,7 @@ struct libxl__datacopier_state {
     libxl__ao *ao;
     int readfd, writefd;
     ssize_t maxsz;
+    ssize_t bytes_to_read; /* set to -1 to read until EOF */
     const char *copywhat, *readwhat, *writewhat; /* for error msgs */
     FILE *log; /* gets a copy of everything */
     libxl__datacopier_callback *callback;
-- 
2.1.0


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