adb: support forwarding TCP port 0.

This CL adds support to forward or reverse TCP port 0 to allow the
system to automatically select an open port. The resolved port number
will be printed to stdout:
  $ adb forward tcp:0 tcp:8000
  12345
  $ adb reverse tcp:0 tcp:9000
  23456
This allows testing to be more robust by not hardcoding TCP ports which
may already be in use.

Forwarding port 0 is a host-only change and will work with any device,
but reversing port 0 requires the device to be updated with a new adbd
binary.

This CL also does a little bit of cleanup such as moving the alistener
class out of adb.h, and adds some error checking and additional tests.

Bug: 28051746
Test: python -m unittest discover
Test: adb_test
Test: `adb forward` and `adb reverse` with tcp:0
Change-Id: Icaa87346685b403ab5da7f0e6aa186aa091da572

15 files changed, 440 insertions, 172 deletions

diff --git a/Android.mk b/Android.mk
index 6693619..71d5aaf 100644
--- a/Android.mk
+++ b/Android.mk
@@ -58,6 +58,7 @@ LIBADB_SRC_FILES := \
 
 LIBADB_TEST_SRCS := \
     adb_io_test.cpp \
+    adb_listeners_test.cpp \
     adb_utils_test.cpp \
     fdevent_test.cpp \
     socket_test.cpp \
diff --git a/adb.cpp b/adb.cpp
index 49d2936..11e9c68 100644
--- a/adb.cpp
+++ b/adb.cpp
@@ -955,18 +955,25 @@ int handle_forward_request(const char* service, TransportType type, const char*
 
         std::string error;
         InstallStatus r;
+        int resolved_tcp_port = 0;
         if (kill_forward) {
             r = remove_listener(pieces[0].c_str(), transport);
         } else {
-            r = install_listener(pieces[0], pieces[1].c_str(), transport,
-                                 no_rebind, &error);
+            r = install_listener(pieces[0], pieces[1].c_str(), transport, no_rebind,
+                                 &resolved_tcp_port, &error);
         }
         if (r == INSTALL_STATUS_OK) {
 #if ADB_HOST
-            /* On the host: 1st OKAY is connect, 2nd OKAY is status */
+            // On the host: 1st OKAY is connect, 2nd OKAY is status.
             SendOkay(reply_fd);
 #endif
             SendOkay(reply_fd);
+
+            // If a TCP port was resolved, send the actual port number back.
+            if (resolved_tcp_port != 0) {
+                SendProtocolString(reply_fd, android::base::StringPrintf("%d", resolved_tcp_port));
+            }
+
             return 1;
         }
 
diff --git a/adb.h b/adb.h
index ea20800..cb38e61 100644
--- a/adb.h
+++ b/adb.h
@@ -50,7 +50,7 @@ constexpr size_t MAX_PAYLOAD = MAX_PAYLOAD_V2;
 std::string adb_version();
 
 // Increment this when we want to force users to start a new adb server.
-#define ADB_SERVER_VERSION 36
+#define ADB_SERVER_VERSION 37
 
 class atransport;
 struct usb_handle;
@@ -116,29 +116,6 @@ enum ConnectionState {
     kCsUnauthorized,
 };
 
-/* A listener is an entity which binds to a local port
-** and, upon receiving a connection on that port, creates
-** an asocket to connect the new local connection to a
-** specific remote service.
-**
-** TODO: some listeners read from the new connection to
-** determine what exact service to connect to on the far
-** side.
-*/
-struct alistener
-{
-    alistener *next;
-    alistener *prev;
-
-    fdevent fde;
-    int fd;
-
-    char *local_name;
-    char *connect_to;
-    atransport *transport;
-    adisconnect  disconnect;
-};
-
 
 void print_packet(const char *label, apacket *p);
 
diff --git a/adb_listeners.cpp b/adb_listeners.cpp
index e8c2338..f54603c 100644
--- a/adb_listeners.cpp
+++ b/adb_listeners.cpp
@@ -20,18 +20,55 @@
 #include <stdlib.h>
 
 #include <android-base/stringprintf.h>
+#include <android-base/strings.h>
 #include <cutils/sockets.h>
 
 #include "sysdeps.h"
 #include "transport.h"
 
-int gListenAll = 0; /* Not static because it is used in commandline.c. */
-
-static alistener listener_list = {
-    .next = &listener_list,
-    .prev = &listener_list,
+// Not static because it is used in commandline.c.
+int gListenAll = 0;
+
+// A listener is an entity which binds to a local port and, upon receiving a connection on that
+// port, creates an asocket to connect the new local connection to a specific remote service.
+//
+// TODO: some listeners read from the new connection to determine what exact service to connect to
+// on the far side.
+class alistener {
+  public:
+    alistener(const std::string& _local_name, const std::string& _connect_to);
+    ~alistener();
+
+    fdevent fde;
+    int fd = -1;
+
+    std::string local_name;
+    std::string connect_to;
+    atransport* transport = nullptr;
+    adisconnect disconnect;
+
+  private:
+    DISALLOW_COPY_AND_ASSIGN(alistener);
 };
 
+alistener::alistener(const std::string& _local_name, const std::string& _connect_to)
+    : local_name(_local_name), connect_to(_connect_to) {
+}
+
+alistener::~alistener() {
+    // Closes the corresponding fd.
+    fdevent_remove(&fde);
+
+    if (transport) {
+        transport->RemoveDisconnect(&disconnect);
+    }
+}
+
+// listener_list retains ownership of all created alistener objects. Removing an alistener from
+// this list will cause it to be deleted.
+typedef std::list<std::unique_ptr<alistener>> ListenerList;
+static ListenerList& listener_list = *new ListenerList();
+
 static void ss_listener_event_func(int _fd, unsigned ev, void *_l) {
     if (ev & FDE_READ) {
         sockaddr_storage ss;
@@ -73,7 +110,7 @@ static void listener_event_func(int _fd, unsigned ev, void* _l)
         s = create_local_socket(fd);
         if (s) {
             s->transport = listener->transport;
-            connect_to_remote(s, listener->connect_to);
+            connect_to_remote(s, listener->connect_to.c_str());
             return;
         }
 
@@ -81,66 +118,63 @@ static void listener_event_func(int _fd, unsigned ev, void* _l)
     }
 }
 
-static void free_listener(alistener*  l)
-{
-    if (l->next) {
-        l->next->prev = l->prev;
-        l->prev->next = l->next;
-        l->next = l->prev = l;
-    }
-
-    // closes the corresponding fd
-    fdevent_remove(&l->fde);
-
-    if (l->local_name)
-        free((char*)l->local_name);
-
-    if (l->connect_to)
-        free((char*)l->connect_to);
-
-    if (l->transport) {
-        l->transport->RemoveDisconnect(&l->disconnect);
-    }
-    free(l);
-}
-
+// Called as a transport disconnect function. |arg| is the raw alistener*.
 static void listener_disconnect(void* arg, atransport*) {
-    alistener* listener = reinterpret_cast<alistener*>(arg);
-    listener->transport = nullptr;
-    free_listener(listener);
+    for (auto iter = listener_list.begin(); iter != listener_list.end(); ++iter) {
+        if (iter->get() == arg) {
+            (*iter)->transport = nullptr;
+            listener_list.erase(iter);
+            return;
+        }
+    }
 }
 
-static int local_name_to_fd(const char* name, std::string* error) {
-    if (!strncmp("tcp:", name, 4)) {
-        int port = atoi(name + 4);
+int local_name_to_fd(alistener* listener, int* resolved_tcp_port, std::string* error) {
+    if (android::base::StartsWith(listener->local_name, "tcp:")) {
+        int requested_port = atoi(&listener->local_name[4]);
+        int sock = -1;
         if (gListenAll > 0) {
-            return network_inaddr_any_server(port, SOCK_STREAM, error);
+            sock = network_inaddr_any_server(requested_port, SOCK_STREAM, error);
         } else {
-            return network_loopback_server(port, SOCK_STREAM, error);
+            sock = network_loopback_server(requested_port, SOCK_STREAM, error);
         }
+
+        // If the caller requested port 0, update the listener name with the resolved port.
+        if (sock >= 0 && requested_port == 0) {
+            int local_port = adb_socket_get_local_port(sock);
+            if (local_port > 0) {
+                listener->local_name = android::base::StringPrintf("tcp:%d", local_port);
+                if (resolved_tcp_port != nullptr) {
+                    *resolved_tcp_port = local_port;
+                }
+            }
+        }
+
+        return sock;
     }
 #if !defined(_WIN32)  // No Unix-domain sockets on Windows.
-    // It's nonsensical to support the "reserved" space on the adb host side
-    if (!strncmp(name, "local:", 6)) {
-        return network_local_server(name + 6,
-                ANDROID_SOCKET_NAMESPACE_ABSTRACT, SOCK_STREAM, error);
-    } else if (!strncmp(name, "localabstract:", 14)) {
-        return network_local_server(name + 14,
-                ANDROID_SOCKET_NAMESPACE_ABSTRACT, SOCK_STREAM, error);
-    } else if (!strncmp(name, "localfilesystem:", 16)) {
-        return network_local_server(name + 16,
-                ANDROID_SOCKET_NAMESPACE_FILESYSTEM, SOCK_STREAM, error);
+    // It's nonsensical to support the "reserved" space on the adb host side.
+    if (android::base::StartsWith(listener->local_name, "local:")) {
+        return network_local_server(&listener->local_name[6], ANDROID_SOCKET_NAMESPACE_ABSTRACT,
+                                    SOCK_STREAM, error);
+    } else if (android::base::StartsWith(listener->local_name, "localabstract:")) {
+        return network_local_server(&listener->local_name[14], ANDROID_SOCKET_NAMESPACE_ABSTRACT,
+                                    SOCK_STREAM, error);
+    } else if (android::base::StartsWith(listener->local_name, "localfilesystem:")) {
+        return network_local_server(&listener->local_name[16], ANDROID_SOCKET_NAMESPACE_FILESYSTEM,
+                                    SOCK_STREAM, error);
     }
 
 #endif
-    *error = android::base::StringPrintf("unknown local portname '%s'", name);
+    *error = android::base::StringPrintf("unknown local portname '%s'",
+                                         listener->local_name.c_str());
     return -1;
 }
 
 // Write the list of current listeners (network redirections) into a string.
 std::string format_listeners() {
     std::string result;
-    for (alistener* l = listener_list.next; l != &listener_list; l = l->next) {
+    for (auto& l : listener_list) {
         // Ignore special listeners like those for *smartsocket*
         if (l->connect_to[0] == '*') {
             continue;
@@ -149,65 +183,51 @@ std::string format_listeners() {
         // Entries from "adb reverse" have no serial.
         android::base::StringAppendF(&result, "%s %s %s\n",
                                      l->transport->serial ? l->transport->serial : "(reverse)",
-                                     l->local_name, l->connect_to);
+                                     l->local_name.c_str(), l->connect_to.c_str());
     }
     return result;
 }
 
-InstallStatus remove_listener(const char *local_name, atransport* transport) {
-    alistener *l;
-
-    for (l = listener_list.next; l != &listener_list; l = l->next) {
-        if (!strcmp(local_name, l->local_name)) {
-            free_listener(l);
+InstallStatus remove_listener(const char* local_name, atransport* transport) {
+    for (auto iter = listener_list.begin(); iter != listener_list.end(); ++iter) {
+        if (local_name == (*iter)->local_name) {
+            listener_list.erase(iter);
             return INSTALL_STATUS_OK;
         }
     }
     return INSTALL_STATUS_LISTENER_NOT_FOUND;
 }
 
-void remove_all_listeners(void)
-{
-    alistener *l, *l_next;
-    for (l = listener_list.next; l != &listener_list; l = l_next) {
-        l_next = l->next;
+void remove_all_listeners() {
+    auto iter = listener_list.begin();
+    while (iter != listener_list.end()) {
         // Never remove smart sockets.
-        if (l->connect_to[0] == '*')
-            continue;
-        free_listener(l);
+        if ((*iter)->connect_to[0] == '*') {
+            ++iter;
+        } else {
+            iter = listener_list.erase(iter);
+        }
     }
 }
 
-InstallStatus install_listener(const std::string& local_name,
-                                  const char *connect_to,
-                                  atransport* transport,
-                                  int no_rebind,
-                                  std::string* error)
-{
-    for (alistener* l = listener_list.next; l != &listener_list; l = l->next) {
+InstallStatus install_listener(const std::string& local_name, const char* connect_to,
+                               atransport* transport, int no_rebind, int* resolved_tcp_port,
+                               std::string* error) {
+    for (auto& l : listener_list) {
         if (local_name == l->local_name) {
-            char* cto;
-
-            /* can't repurpose a smartsocket */
+            // Can't repurpose a smartsocket.
             if(l->connect_to[0] == '*') {
                 *error = "cannot repurpose smartsocket";
                 return INSTALL_STATUS_INTERNAL_ERROR;
             }
 
-            /* can't repurpose a listener if 'no_rebind' is true */
+            // Can't repurpose a listener if 'no_rebind' is true.
             if (no_rebind) {
                 *error = "cannot rebind";
                 return INSTALL_STATUS_CANNOT_REBIND;
             }
 
-            cto = strdup(connect_to);
-            if(cto == 0) {
-                *error = "cannot duplicate string";
-                return INSTALL_STATUS_INTERNAL_ERROR;
-            }
-
-            free((void*) l->connect_to);
-            l->connect_to = cto;
+            l->connect_to = connect_to;
             if (l->transport != transport) {
                 l->transport->RemoveDisconnect(&l->disconnect);
                 l->transport = transport;
@@ -217,54 +237,29 @@ InstallStatus install_listener(const std::string& local_name,
         }
     }
 
-    alistener* listener = reinterpret_cast<alistener*>(
-        calloc(1, sizeof(alistener)));
-    if (listener == nullptr) {
-        goto nomem;
-    }
-
-    listener->local_name = strdup(local_name.c_str());
-    if (listener->local_name == nullptr) {
-        goto nomem;
-    }
-
-    listener->connect_to = strdup(connect_to);
-    if (listener->connect_to == nullptr) {
-        goto nomem;
-    }
+    std::unique_ptr<alistener> listener(new alistener(local_name, connect_to));
 
-    listener->fd = local_name_to_fd(listener->local_name, error);
+    listener->fd = local_name_to_fd(listener.get(), resolved_tcp_port, error);
     if (listener->fd < 0) {
-        free(listener->local_name);
-        free(listener->connect_to);
-        free(listener);
         return INSTALL_STATUS_CANNOT_BIND;
     }
 
     close_on_exec(listener->fd);
-    if (!strcmp(listener->connect_to, "*smartsocket*")) {
-        fdevent_install(&listener->fde, listener->fd, ss_listener_event_func,
-                        listener);
+    if (listener->connect_to == "*smartsocket*") {
+        fdevent_install(&listener->fde, listener->fd, ss_listener_event_func, listener.get());
     } else {
-        fdevent_install(&listener->fde, listener->fd, listener_event_func,
-                        listener);
+        fdevent_install(&listener->fde, listener->fd, listener_event_func, listener.get());
     }
     fdevent_set(&listener->fde, FDE_READ);
 
-    listener->next = &listener_list;
-    listener->prev = listener_list.prev;
-    listener->next->prev = listener;
-    listener->prev->next = listener;
     listener->transport = transport;
 
     if (transport) {
-        listener->disconnect.opaque = listener;
+        listener->disconnect.opaque = listener.get();
         listener->disconnect.func   = listener_disconnect;
         transport->AddDisconnect(&listener->disconnect);
     }
-    return INSTALL_STATUS_OK;
 
-nomem:
-    fatal("cannot allocate listener");
-    return INSTALL_STATUS_INTERNAL_ERROR;
+    listener_list.push_back(std::move(listener));
+    return INSTALL_STATUS_OK;
 }
diff --git a/adb_listeners.h b/adb_listeners.h
index fa98eed..8eba00a 100644
--- a/adb_listeners.h
+++ b/adb_listeners.h
@@ -21,6 +21,8 @@
 
 #include <string>
 
+#include <android-base/macros.h>
+
 // error/status codes for install_listener.
 enum InstallStatus {
   INSTALL_STATUS_OK = 0,
@@ -30,10 +32,8 @@ enum InstallStatus {
   INSTALL_STATUS_LISTENER_NOT_FOUND = -4,
 };
 
-InstallStatus install_listener(const std::string& local_name,
-                               const char* connect_to,
-                               atransport* transport,
-                               int no_rebind,
+InstallStatus install_listener(const std::string& local_name, const char* connect_to,
+                               atransport* transport, int no_rebind, int* resolved_tcp_port,
                                std::string* error);
 
 std::string format_listeners();
diff --git a/adb_listeners_test.cpp b/adb_listeners_test.cpp
new file mode 100644
index 0000000..b697769
--- /dev/null
+++ b/adb_listeners_test.cpp
@@ -0,0 +1,166 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "adb_listeners.h"
+
+#include <gtest/gtest.h>
+
+#include <android-base/stringprintf.h>
+#include <android-base/strings.h>
+
+#include "fdevent.h"
+#include "sysdeps.h"
+#include "transport.h"
+
+// Returns true if the given listener is present in format_listeners(). Empty parameters will
+// be ignored.
+static bool listener_is_installed(const std::string& serial, const std::string& source,
+                                  const std::string& dest) {
+    // format_listeners() gives lines of "<serial> <source> <dest>\n".
+    for (const std::string& line : android::base::Split(format_listeners(), "\n")) {
+        std::vector<std::string> info = android::base::Split(line, " ");
+        if (info.size() == 3 &&
+                (serial.empty() || info[0] == serial) &&
+                (source.empty() || info[1] == source) &&
+                (dest.empty() || info[2] == dest)) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
+class AdbListenersTest : public ::testing::Test {
+  public:
+    void SetUp() override {
+        // We don't need an fdevent loop, but adding/removing listeners must be done from the
+        // fdevent thread if one exists. Since previously run tests may have created an fdevent
+        // thread, we need to reset to prevent the thread check.
+        fdevent_reset();
+    }
+
+    void TearDown() override {
+        // Clean up any listeners that may have been installed.
+        remove_all_listeners();
+
+        // Make sure we didn't leave any dangling events.
+        ASSERT_EQ(0u, fdevent_installed_count());
+    }
+
+  protected:
+    atransport transport_;
+};
+
+TEST_F(AdbListenersTest, test_install_listener) {
+    std::string error;
+
+    ASSERT_EQ(INSTALL_STATUS_OK,
+              install_listener("tcp:9000", "tcp:9000", &transport_, false, nullptr, &error));
+    ASSERT_TRUE(error.empty());
+
+    ASSERT_TRUE(listener_is_installed("", "tcp:9000", "tcp:9000"));
+}
+
+TEST_F(AdbListenersTest, test_install_listener_rebind) {
+    std::string error;
+
+    ASSERT_EQ(INSTALL_STATUS_OK,
+              install_listener("tcp:9000", "tcp:9000", &transport_, false, nullptr, &error));
+    ASSERT_TRUE(error.empty());
+
+    ASSERT_EQ(INSTALL_STATUS_OK,
+              install_listener("tcp:9000", "tcp:9001", &transport_, false, nullptr, &error));
+    ASSERT_TRUE(error.empty());
+
+    ASSERT_TRUE(listener_is_installed("", "tcp:9000", "tcp:9001"));
+}
+
+TEST_F(AdbListenersTest, test_install_listener_no_rebind) {
+    std::string error;
+
+    ASSERT_EQ(INSTALL_STATUS_OK,
+              install_listener("tcp:9000", "tcp:9000", &transport_, true, nullptr, &error));
+    ASSERT_TRUE(error.empty());
+
+    ASSERT_EQ(INSTALL_STATUS_CANNOT_REBIND,
+              install_listener("tcp:9000", "tcp:9001", &transport_, true, nullptr, &error));
+    ASSERT_FALSE(error.empty());
+
+    ASSERT_TRUE(listener_is_installed("", "tcp:9000", "tcp:9000"));
+}
+
+TEST_F(AdbListenersTest, test_install_listener_tcp_port_0) {
+    int port = 0;
+    std::string error;
+
+    ASSERT_EQ(INSTALL_STATUS_OK,
+              install_listener("tcp:0", "tcp:9000", &transport_, true, &port, &error));
+    ASSERT_TRUE(error.empty());
+
+    ASSERT_TRUE(listener_is_installed("", android::base::StringPrintf("tcp:%d", port), "tcp:9000"));
+}
+
+TEST_F(AdbListenersTest, test_remove_listener) {
+    std::string error;
+
+    ASSERT_EQ(INSTALL_STATUS_OK,
+              install_listener("tcp:9000", "tcp:9000", &transport_, false, nullptr, &error));
+    ASSERT_TRUE(error.empty());
+
+    ASSERT_EQ(INSTALL_STATUS_OK, remove_listener("tcp:9000", &transport_));
+    ASSERT_TRUE(format_listeners().empty());
+}
+
+TEST_F(AdbListenersTest, test_remove_nonexistent_listener) {
+    std::string error;
+
+    ASSERT_EQ(INSTALL_STATUS_OK,
+              install_listener("tcp:9000", "tcp:9000", &transport_, false, nullptr, &error));
+    ASSERT_TRUE(error.empty());
+
+    ASSERT_EQ(INSTALL_STATUS_LISTENER_NOT_FOUND, remove_listener("tcp:1", &transport_));
+    ASSERT_TRUE(listener_is_installed("", "tcp:9000", "tcp:9000"));
+}
+
+TEST_F(AdbListenersTest, test_remove_all_listeners) {
+    std::string error;
+
+    ASSERT_EQ(INSTALL_STATUS_OK,
+              install_listener("tcp:9000", "tcp:9000", &transport_, false, nullptr, &error));
+    ASSERT_TRUE(error.empty());
+
+    ASSERT_EQ(INSTALL_STATUS_OK,
+              install_listener("tcp:9001", "tcp:9001", &transport_, false, nullptr, &error));
+    ASSERT_TRUE(error.empty());
+
+    remove_all_listeners();
+    ASSERT_TRUE(format_listeners().empty());
+}
+
+TEST_F(AdbListenersTest, test_transport_disconnect) {
+    std::string error;
+
+    ASSERT_EQ(INSTALL_STATUS_OK,
+              install_listener("tcp:9000", "tcp:9000", &transport_, false, nullptr, &error));
+    ASSERT_TRUE(error.empty());
+
+    ASSERT_EQ(INSTALL_STATUS_OK,
+              install_listener("tcp:9001", "tcp:9001", &transport_, false, nullptr, &error));
+    ASSERT_TRUE(error.empty());
+
+    transport_.RunDisconnects();
+    ASSERT_TRUE(format_listeners().empty());
+}
diff --git a/adb_utils.cpp b/adb_utils.cpp
index 3333fc6..5d4755f 100644
--- a/adb_utils.cpp
+++ b/adb_utils.cpp
@@ -27,6 +27,7 @@
 #include <algorithm>
 
 #include <android-base/logging.h>
+#include <android-base/parseint.h>
 #include <android-base/stringprintf.h>
 #include <android-base/strings.h>
 
@@ -230,3 +231,26 @@ bool set_file_block_mode(int fd, bool block) {
     return true;
 }
 #endif
+
+bool forward_targets_are_valid(const std::string& source, const std::string& dest,
+                               std::string* error) {
+    if (android::base::StartsWith(source, "tcp:")) {
+        // The source port may be 0 to allow the system to select an open port.
+        int port;
+        if (!android::base::ParseInt(&source[4], &port) || port < 0) {
+            *error = android::base::StringPrintf("Invalid source port: '%s'", &source[4]);
+            return false;
+        }
+    }
+
+    if (android::base::StartsWith(dest, "tcp:")) {
+        // The destination port must be > 0.
+        int port;
+        if (!android::base::ParseInt(&dest[4], &port) || port <= 0) {
+            *error = android::base::StringPrintf("Invalid destination port: '%s'", &dest[4]);
+            return false;
+        }
+    }
+
+    return true;
+}
diff --git a/adb_utils.h b/adb_utils.h
index 89fcd66..abf481b 100644
--- a/adb_utils.h
+++ b/adb_utils.h
@@ -43,6 +43,13 @@ bool set_file_block_mode(int fd, bool block);
 
 extern int adb_close(int fd);
 
+// Given forward/reverse targets, returns true if they look sane. If an error is found, fills
+// |error| and returns false.
+// Currently this only checks "tcp:" targets. Additional checking could be added for other targets
+// if needed.
+bool forward_targets_are_valid(const std::string& source, const std::string& dest,
+                               std::string* error);
+
 // Helper to automatically close an FD when it goes out of scope.
 class ScopedFd {
   public:
diff --git a/adb_utils_test.cpp b/adb_utils_test.cpp
index f1ebaa1..aabc5d7 100644
--- a/adb_utils_test.cpp
+++ b/adb_utils_test.cpp
@@ -149,3 +149,24 @@ TEST(adb_utils, set_file_block_mode) {
   ASSERT_EQ(0, adb_close(fd));
 }
 #endif
+
+TEST(adb_utils, test_forward_targets_are_valid) {
+    std::string error;
+
+    // Source port can be >= 0.
+    EXPECT_FALSE(forward_targets_are_valid("tcp:-1", "tcp:9000", &error));
+    EXPECT_TRUE(forward_targets_are_valid("tcp:0", "tcp:9000", &error));
+    EXPECT_TRUE(forward_targets_are_valid("tcp:8000", "tcp:9000", &error));
+
+    // Destination port must be >0.
+    EXPECT_FALSE(forward_targets_are_valid("tcp:8000", "tcp:-1", &error));
+    EXPECT_FALSE(forward_targets_are_valid("tcp:8000", "tcp:0", &error));
+
+    // Port must be a number.
+    EXPECT_FALSE(forward_targets_are_valid("tcp:", "tcp:9000", &error));
+    EXPECT_FALSE(forward_targets_are_valid("tcp:a", "tcp:9000", &error));
+    EXPECT_FALSE(forward_targets_are_valid("tcp:22x", "tcp:9000", &error));
+    EXPECT_FALSE(forward_targets_are_valid("tcp:8000", "tcp:", &error));
+    EXPECT_FALSE(forward_targets_are_valid("tcp:8000", "tcp:a", &error));
+    EXPECT_FALSE(forward_targets_are_valid("tcp:8000", "tcp:22x", &error));
+}
diff --git a/client/main.cpp b/client/main.cpp
index 27b7109..65640ad 100644
--- a/client/main.cpp
+++ b/client/main.cpp
@@ -117,7 +117,7 @@ int adb_server_main(int is_daemon, int server_port, int ack_reply_fd) {
 
     std::string error;
     std::string local_name = android::base::StringPrintf("tcp:%d", server_port);
-    if (install_listener(local_name, "*smartsocket*", nullptr, 0, &error)) {
+    if (install_listener(local_name, "*smartsocket*", nullptr, 0, nullptr, &error)) {
         fatal("could not install *smartsocket* listener: %s", error.c_str());
     }
 
diff --git a/commandline.cpp b/commandline.cpp
index 45c6142..28dbb78 100644
--- a/commandline.cpp
+++ b/commandline.cpp
@@ -126,7 +126,7 @@ static void help() {
         "                                    <serial> \" \" <local> \" \" <remote> \"\\n\"\n"
         "  adb forward <local> <remote> - forward socket connections\n"
         "                                 forward specs are one of: \n"
-        "                                   tcp:<port>\n"
+        "                                   tcp:<port> (<local> may be \"tcp:0\" to pick any open port)\n"
         "                                   localabstract:<unix domain socket name>\n"
         "                                   localreserved:<unix domain socket name>\n"
         "                                   localfilesystem:<unix domain socket name>\n"
@@ -140,7 +140,7 @@ static void help() {
         "  adb reverse --list           - list all reverse socket connections from device\n"
         "  adb reverse <remote> <local> - reverse socket connections\n"
         "                                 reverse specs are one of:\n"
-        "                                   tcp:<port>\n"
+        "                                   tcp:<port> (<remote> may be \"tcp:0\" to pick any open port)\n"
         "                                   localabstract:<unix domain socket name>\n"
         "                                   localreserved:<unix domain socket name>\n"
         "                                   localfilesystem:<unix domain socket name>\n"
@@ -1719,7 +1719,7 @@ int adb_commandline(int argc, const char **argv) {
             }
         }
 
-        std::string cmd;
+        std::string cmd, error;
         if (strcmp(argv[0], "--list") == 0) {
             if (argc != 1) return usage();
             return adb_query_command(host_prefix + ":list-forward");
@@ -1733,14 +1733,37 @@ int adb_commandline(int argc, const char **argv) {
         } else if (strcmp(argv[0], "--no-rebind") == 0) {
             // forward --no-rebind <local> <remote>
             if (argc != 3) return usage();
-            cmd = host_prefix + ":forward:norebind:" + argv[1] + ";" + argv[2];
+            if (forward_targets_are_valid(argv[1], argv[2], &error)) {
+                cmd = host_prefix + ":forward:norebind:" + argv[1] + ";" + argv[2];
+            }
         } else {
             // forward <local> <remote>
             if (argc != 2) return usage();
-            cmd = host_prefix + ":forward:" + argv[0] + ";" + argv[1];
+            if (forward_targets_are_valid(argv[0], argv[1], &error)) {
+                cmd = host_prefix + ":forward:" + argv[0] + ";" + argv[1];
+            }
+        }
+
+        if (!error.empty()) {
+            fprintf(stderr, "error: %s\n", error.c_str());
+            return 1;
         }
 
-        return adb_command(cmd) ? 0 : 1;
+        int fd = adb_connect(cmd, &error);
+        if (fd < 0 || !adb_status(fd, &error)) {
+            adb_close(fd);
+            fprintf(stderr, "error: %s\n", error.c_str());
+            return 1;
+        }
+
+        // Server or device may optionally return a resolved TCP port number.
+        std::string resolved_port;
+        if (ReadProtocolString(fd, &resolved_port, &error) && !resolved_port.empty()) {
+            printf("%s\n", resolved_port.c_str());
+        }
+
+        ReadOrderlyShutdown(fd);
+        return 0;
     }
     /* do_sync_*() commands */
     else if (!strcmp(argv[0], "ls")) {
diff --git a/daemon/main.cpp b/daemon/main.cpp
index 7f40b96..916bedf 100644
--- a/daemon/main.cpp
+++ b/daemon/main.cpp
@@ -142,10 +142,8 @@ static void drop_privileges(int server_port) {
         std::string error;
         std::string local_name =
             android::base::StringPrintf("tcp:%d", server_port);
-        if (install_listener(local_name, "*smartsocket*", nullptr, 0,
-                             &error)) {
-            LOG(FATAL) << "Could not install *smartsocket* listener: "
-                       << error;
+        if (install_listener(local_name, "*smartsocket*", nullptr, 0, nullptr, &error)) {
+            LOG(FATAL) << "Could not install *smartsocket* listener: " << error;
         }
     }
 }
diff --git a/sysdeps.h b/sysdeps.h
index 81d201e..3586da8 100644
--- a/sysdeps.h
+++ b/sysdeps.h
@@ -287,6 +287,9 @@ extern int  adb_socket_accept(int  serverfd, struct sockaddr*  addr, socklen_t
 #undef   accept
 #define  accept  ___xxx_accept
 
+// Returns the local port number of a bound socket, or -1 on failure.
+int adb_socket_get_local_port(int fd);
+
 extern int  adb_setsockopt(int  fd, int  level, int  optname, const void*  optval, socklen_t  optlen);
 
 #undef   setsockopt
@@ -691,6 +694,10 @@ static __inline__ int  adb_socket_accept(int  serverfd, struct sockaddr*  addr,
 #undef   accept
 #define  accept  ___xxx_accept
 
+inline int adb_socket_get_local_port(int fd) {
+    return socket_get_local_port(fd);
+}
+
 // Operate on a file descriptor returned from unix_open() or a well-known file
 // descriptor such as STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO.
 //
diff --git a/sysdeps_win32.cpp b/sysdeps_win32.cpp
index bc09fdc..faf7f3e 100644
--- a/sysdeps_win32.cpp
+++ b/sysdeps_win32.cpp
@@ -1128,6 +1128,24 @@ int adb_getsockname(int fd, struct sockaddr* sockaddr, socklen_t* optlen) {
     return result;
 }
 
+int adb_socket_get_local_port(int fd) {
+    sockaddr_storage addr_storage;
+    socklen_t addr_len = sizeof(addr_storage);
+
+    if (adb_getsockname(fd, reinterpret_cast<sockaddr*>(&addr_storage), &addr_len) < 0) {
+        D("adb_socket_get_local_port: adb_getsockname failed: %s", strerror(errno));
+        return -1;
+    }
+
+    if (!(addr_storage.ss_family == AF_INET || addr_storage.ss_family == AF_INET6)) {
+        D("adb_socket_get_local_port: unknown address family received: %d", addr_storage.ss_family);
+        errno = ECONNABORTED;
+        return -1;
+    }
+
+    return ntohs(reinterpret_cast<sockaddr_in*>(&addr_storage)->sin_port);
+}
+
 int  adb_shutdown(int  fd)
 {
     FH   f = _fh_from_int(fd, __func__);
@@ -1154,9 +1172,7 @@ int adb_socketpair(int sv[2]) {
     int server = -1;
     int client = -1;
     int accepted = -1;
-    sockaddr_storage addr_storage;
-    socklen_t addr_len = sizeof(addr_storage);
-    sockaddr_in* addr = nullptr;
+    int local_port = -1;
     std::string error;
 
     server = network_loopback_server(0, SOCK_STREAM, &error);
@@ -1165,20 +1181,14 @@ int adb_socketpair(int sv[2]) {
         goto fail;
     }
 
-    if (adb_getsockname(server, reinterpret_cast<sockaddr*>(&addr_storage), &addr_len) < 0) {
-        D("adb_socketpair: adb_getsockname failed: %s", strerror(errno));
-        goto fail;
-    }
-
-    if (addr_storage.ss_family != AF_INET) {
-        D("adb_socketpair: unknown address family received: %d", addr_storage.ss_family);
-        errno = ECONNABORTED;
+    local_port = adb_socket_get_local_port(server);
+    if (local_port < 0) {
+        D("adb_socketpair: failed to get server port number: %s", error.c_str());
         goto fail;
     }
+    D("adb_socketpair: bound on port %d", local_port);
 
-    addr = reinterpret_cast<sockaddr_in*>(&addr_storage);
-    D("adb_socketpair: bound on port %d", ntohs(addr->sin_port));
-    client = network_loopback_client(ntohs(addr->sin_port), SOCK_STREAM, &error);
+    client = network_loopback_client(local_port, SOCK_STREAM, &error);
     if (client < 0) {
         D("adb_socketpair: failed to connect client: %s", error.c_str());
         goto fail;
diff --git a/test_device.py b/test_device.py
index 2a3be88..cdc57c6 100644
--- a/test_device.py
+++ b/test_device.py
@@ -191,6 +191,22 @@ class ForwardReverseTest(DeviceTest):
         msg = self.device.forward_list()
         self.assertEqual('', msg.strip())
 
+    def test_forward_tcp_port_0(self):
+        self.assertEqual('', self.device.forward_list().strip(),
+                         'Forwarding list must be empty to run this test.')
+
+        try:
+            # If resolving TCP port 0 is supported, `adb forward` will print
+            # the actual port number.
+            port = self.device.forward('tcp:0', 'tcp:8888').strip()
+            if not port:
+                raise unittest.SkipTest('Forwarding tcp:0 is not available.')
+
+            self.assertTrue(re.search(r'tcp:{}.+tcp:8888'.format(port),
+                                      self.device.forward_list()))
+        finally:
+            self.device.forward_remove_all()
+
     def test_reverse(self):
         msg = self.device.reverse_list()
         self.assertEqual('', msg.strip(),
@@ -210,6 +226,22 @@ class ForwardReverseTest(DeviceTest):
         msg = self.device.reverse_list()
         self.assertEqual('', msg.strip())
 
+    def test_reverse_tcp_port_0(self):
+        self.assertEqual('', self.device.reverse_list().strip(),
+                         'Reverse list must be empty to run this test.')
+
+        try:
+            # If resolving TCP port 0 is supported, `adb reverse` will print
+            # the actual port number.
+            port = self.device.reverse('tcp:0', 'tcp:8888').strip()
+            if not port:
+                raise unittest.SkipTest('Reversing tcp:0 is not available.')
+
+            self.assertTrue(re.search(r'tcp:{}.+tcp:8888'.format(port),
+                                      self.device.reverse_list()))
+        finally:
+            self.device.reverse_remove_all()
+
     # Note: If you run this test when adb connect'd to a physical device over
     # TCP, it will fail in adb reverse due to https://code.google.com/p/android/issues/detail?id=189821
     def test_forward_reverse_echo(self):