Revert "Revert "Support socket activation of adb server""

Test: actually build both device and client adb this time

This reverts commit c77904e3c5.

Change-Id: I8c2b8f24884ff36ff2460dab2500bad777100dfd

adb/SOCKET-ACTIVATION.txt

@@ -0,0 +1,42 @@
+adb can be configured to work with systemd-style socket activation,
+allowing the daemon to start automatically when the adb control port
+is forwarded across a network. You need two files, placed in the usual
+systemd service directories (e.g., ~/.config/systemd/user for a user
+service).
+
+adb.service:
+
+--- START adb.service CUT HERE ---
+[Unit]
+Description=adb
+After=adb.socket
+Requires=adb.socket
+[Service]
+Type=simple
+# FD 3 is part of the systemd interface
+ExecStart=/path/to/adb server nodaemon -L acceptfd:3
+--- END adb.service CUT HERE ---
+
+--- START adb.socket CUT HERE ---
+[Unit]
+Description=adb
+PartOf=adb.service
+[Socket]
+ListenStream=127.0.0.1:5037
+Accept=no
+[Install]
+WantedBy=sockets.target
+--- END adb.socket CUT HERE ---
+
+After installing the adb service, the adb server will be started
+automatically on any connection to 127.0.0.1:5037 (the default adb
+control port), even after adb kill-server kills the server.
+
+Other "superserver" launcher systems (like macOS launchd) can be
+configured analogously. The important part is that adb be started with
+"server" and "nodaemon" command line arguments and that the listen
+address (passed to -L) name a file descriptor that's ready to
+accept(2) connections and that's already bound to the desired address
+and listening. inetd-style pre-accepted sockets do _not_ work in this
+configuration: the file descriptor passed to acceptfd must be the
+serve socket, not the accepted connection socket.

adb/client/adb_client.cpp

@@ -222,7 +222,7 @@ std::optional<std::string> adb_get_server_executable_path() {
     int port;
     std::string error;
     if (!parse_tcp_socket_spec(__adb_server_socket_spec, nullptr, &port, nullptr, &error)) {
-        LOG(FATAL) << "failed to parse server socket spec: " << error;
+        return {};
     }
 
     return adb_get_android_dir_path() + OS_PATH_SEPARATOR + "adb." + std::to_string(port);

adb/client/commandline.cpp

@@ -107,6 +107,7 @@ static void help() {
         "       localfilesystem:<unix domain socket name>\n"
         "       dev:<character device name>\n"
         "       jdwp:<process pid> (remote only)\n"
+        "       acceptfd:<fd> (listen only)\n"
         " forward --remove LOCAL   remove specific forward socket connection\n"
         " forward --remove-all     remove all forward socket connections\n"
         " ppp TTY [PARAMETER...]   run PPP over USB\n"

adb/socket_spec.cpp

@@ -16,6 +16,7 @@
 
 #include "socket_spec.h"
 
+#include <limits>
 #include <string>
 #include <string_view>
 #include <unordered_map>
@@ -28,10 +29,12 @@
 #include <cutils/sockets.h>
 
 #include "adb.h"
+#include "adb_utils.h"
 #include "sysdeps.h"
 
 using namespace std::string_literals;
 
+using android::base::ConsumePrefix;
 using android::base::StringPrintf;
 
 #if defined(__linux__)
@@ -131,7 +134,7 @@ bool is_socket_spec(std::string_view spec) {
             return true;
         }
     }
-    return spec.starts_with("tcp:");
+    return spec.starts_with("tcp:") || spec.starts_with("acceptfd:");
 }
 
 bool is_local_socket_spec(std::string_view spec) {
@@ -235,6 +238,9 @@ bool socket_spec_connect(unique_fd* fd, std::string_view address, int* port, std
         *error = "vsock is only supported on linux";
         return false;
 #endif  // ADB_LINUX
+    } else if (address.starts_with("acceptfd:")) {
+        *error = "cannot connect to acceptfd";
+        return false;
     }
 
     for (const auto& it : kLocalSocketTypes) {
@@ -334,6 +340,46 @@ int socket_spec_listen(std::string_view spec, std::string* error, int* resolved_
         *error = "vsock is only supported on linux";
         return -1;
 #endif  // ADB_LINUX
+    } else if (ConsumePrefix(&spec, "acceptfd:")) {
+#if ADB_WINDOWS
+        *error = "socket activation not supported under Windows";
+        return -1;
+#else
+        // We inherited the socket from some kind of launcher. It's already bound and
+        // listening. Return a copy of the FD instead of the FD itself so we implement the
+        // normal "listen" contract and can succeed more than once.
+        unsigned int fd_u;
+        if (!ParseUint(&fd_u, spec) || fd_u > std::numeric_limits<int>::max()) {
+            *error = "invalid fd";
+            return -1;
+        }
+        int fd = static_cast<int>(fd_u);
+        int flags = get_fd_flags(fd);
+        if (flags < 0) {
+            *error = android::base::StringPrintf("could not get flags of inherited fd %d: '%s'", fd,
+                                                 strerror(errno));
+            return -1;
+        }
+        if (flags & FD_CLOEXEC) {
+            *error = android::base::StringPrintf("fd %d was not inherited from parent", fd);
+            return -1;
+        }
+
+        int dummy_sock_type;
+        socklen_t dummy_sock_type_size = sizeof(dummy_sock_type);
+        if (getsockopt(fd, SOL_SOCKET, SO_TYPE, &dummy_sock_type, &dummy_sock_type_size)) {
+            *error = android::base::StringPrintf("fd %d does not refer to a socket", fd);
+            return -1;
+        }
+
+        int new_fd = fcntl(fd, F_DUPFD_CLOEXEC, 0);
+        if (new_fd < 0) {
+            *error = android::base::StringPrintf("could not dup inherited fd %d: '%s'", fd,
+                                                 strerror(errno));
+            return -1;
+        }
+        return new_fd;
+#endif
     }
 
     for (const auto& it : kLocalSocketTypes) {

adb/sysdeps.h

@@ -349,8 +349,15 @@ static __inline__ bool adb_is_separator(char c) {
     return c == '/';
 }
 
+static __inline__ int get_fd_flags(borrowed_fd fd) {
+    return fcntl(fd.get(), F_GETFD);
+}
+
 static __inline__ void close_on_exec(borrowed_fd fd) {
-    fcntl(fd.get(), F_SETFD, FD_CLOEXEC);
+    int flags = get_fd_flags(fd);
+    if (flags >= 0 && (flags & FD_CLOEXEC) == 0) {
+        fcntl(fd.get(), F_SETFD, flags | FD_CLOEXEC);
+    }
 }
 
 // Open a file and return a file descriptor that may be used with unix_read(),