iov: add qemu_iovec_concat_iov()

The qemu_iovec_concat() function copies a subset of a QEMUIOVector.  The
new qemu_iovec_concat_iov() function does the same for a iov/cnt pair.

It is easy to define qemu_iovec_concat() in terms of
qemu_iovec_concat_iov().  The existing code is mostly unchanged, except
for the assertion src->size >= soffset, which cannot be efficiently
checked upfront on a iov/cnt pair.  Instead we assert upon hitting the
end of src with an unsatisfied soffset.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

include/qemu-common.h

@@ -329,6 +329,9 @@ void qemu_iovec_init_external(QEMUIOVector *qiov, struct iovec *iov, int niov);
 void qemu_iovec_add(QEMUIOVector *qiov, void *base, size_t len);
 void qemu_iovec_concat(QEMUIOVector *dst,
                        QEMUIOVector *src, size_t soffset, size_t sbytes);
+void qemu_iovec_concat_iov(QEMUIOVector *dst,
+                           struct iovec *src_iov, unsigned int src_cnt,
+                           size_t soffset, size_t sbytes);
 void qemu_iovec_destroy(QEMUIOVector *qiov);
 void qemu_iovec_reset(QEMUIOVector *qiov);
 size_t qemu_iovec_to_buf(QEMUIOVector *qiov, size_t offset,

iov.c

@@ -288,6 +288,36 @@ void qemu_iovec_add(QEMUIOVector *qiov, void *base, size_t len)
     ++qiov->niov;
 }
 
+/*
+ * Concatenates (partial) iovecs from src_iov to the end of dst.
+ * It starts copying after skipping `soffset' bytes at the
+ * beginning of src and adds individual vectors from src to
+ * dst copies up to `sbytes' bytes total, or up to the end
+ * of src_iov if it comes first.  This way, it is okay to specify
+ * very large value for `sbytes' to indicate "up to the end
+ * of src".
+ * Only vector pointers are processed, not the actual data buffers.
+ */
+void qemu_iovec_concat_iov(QEMUIOVector *dst,
+                           struct iovec *src_iov, unsigned int src_cnt,
+                           size_t soffset, size_t sbytes)
+{
+    int i;
+    size_t done;
+    assert(dst->nalloc != -1);
+    for (i = 0, done = 0; done < sbytes && i < src_cnt; i++) {
+        if (soffset < src_iov[i].iov_len) {
+            size_t len = MIN(src_iov[i].iov_len - soffset, sbytes - done);
+            qemu_iovec_add(dst, src_iov[i].iov_base + soffset, len);
+            done += len;
+            soffset = 0;
+        } else {
+            soffset -= src_iov[i].iov_len;
+        }
+    }
+    assert(soffset == 0); /* offset beyond end of src */
+}
+
 /*
  * Concatenates (partial) iovecs from src to the end of dst.
  * It starts copying after skipping `soffset' bytes at the
@@ -301,22 +331,7 @@ void qemu_iovec_add(QEMUIOVector *qiov, void *base, size_t len)
 void qemu_iovec_concat(QEMUIOVector *dst,
                        QEMUIOVector *src, size_t soffset, size_t sbytes)
 {
-    int i;
+    qemu_iovec_concat_iov(dst, src->iov, src->niov, soffset, sbytes);
-    size_t done;
-    struct iovec *siov = src->iov;
-    assert(dst->nalloc != -1);
-    assert(src->size >= soffset);
-    for (i = 0, done = 0; done < sbytes && i < src->niov; i++) {
-        if (soffset < siov[i].iov_len) {
-            size_t len = MIN(siov[i].iov_len - soffset, sbytes - done);
-            qemu_iovec_add(dst, siov[i].iov_base + soffset, len);
-            done += len;
-            soffset = 0;
-        } else {
-            soffset -= siov[i].iov_len;
-        }
-    }
-    /* return done; */
 }
 
 void qemu_iovec_destroy(QEMUIOVector *qiov)