adb: add IOVector.

An IOVector is a collection of immutable reference counted blocks which
can have its head detached at an arbitrary index. This is extremely
useful for implementing packet-framed protocols like adb on top of a
stream protocol like TCP: a stream reader can read blocks, append them
to the end of the IOVector, and then pull packets off of the front.
This also lends itself naturally towards scatter/gather I/O, which will
enable us to read data from disk and send it across the wire with a
theoretical minimum number of copies in USB, and one extra copy over
TCP.

Since this is basically a generalization of std::deque<Range>, delete
Range and replace its uses with IOVector.

Test: adb_test
Test: wine adb_test.exe
Change-Id: I06561ad0bb25a3a51b378b61d257b5b04b41d9c4
diff --git a/adb/sockets.cpp b/adb/sockets.cpp
index 9a6dcbe..de3215d 100644
--- a/adb/sockets.cpp
+++ b/adb/sockets.cpp
@@ -113,14 +113,14 @@
 };
 
 static SocketFlushResult local_socket_flush_incoming(asocket* s) {
-    while (!s->packet_queue.empty()) {
-        Range& r = s->packet_queue.front();
-
-        int rc = adb_write(s->fd, r.data(), r.size());
-        if (rc == static_cast<int>(r.size())) {
-            s->packet_queue.pop_front();
+    if (!s->packet_queue.empty()) {
+        std::vector<adb_iovec> iov = s->packet_queue.iovecs();
+        ssize_t rc = adb_writev(s->fd, iov.data(), iov.size());
+        if (rc > 0 && static_cast<size_t>(rc) == s->packet_queue.size()) {
+            s->packet_queue.clear();
         } else if (rc > 0) {
-            r.drop_front(rc);
+            // TODO: Implement a faster drop_front?
+            s->packet_queue.take_front(rc);
             fdevent_add(s->fde, FDE_WRITE);
             return SocketFlushResult::TryAgain;
         } else if (rc == -1 && errno == EAGAIN) {
@@ -130,7 +130,6 @@
             // We failed to write, but it's possible that we can still read from the socket.
             // Give that a try before giving up.
             s->has_write_error = true;
-            break;
         }
     }
 
@@ -217,8 +216,7 @@
 static int local_socket_enqueue(asocket* s, apacket::payload_type data) {
     D("LS(%d): enqueue %zu", s->id, data.size());
 
-    Range r(std::move(data));
-    s->packet_queue.push_back(std::move(r));
+    s->packet_queue.append(std::move(data));
     switch (local_socket_flush_incoming(s)) {
         case SocketFlushResult::Destroyed:
             return -1;
@@ -622,7 +620,7 @@
     D("SS(%d): enqueue %zu", s->id, data.size());
 
     if (s->smart_socket_data.empty()) {
-        // TODO: Make this a BlockChain?
+        // TODO: Make this an IOVector?
         s->smart_socket_data.assign(data.begin(), data.end());
     } else {
         std::copy(data.begin(), data.end(), std::back_inserter(s->smart_socket_data));