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

Test: actually build both device and client adb this time

This reverts commit 2a34e58d3d64ab183228a51596d6e196b4d373b8.

Change-Id: I8c2b8f24884ff36ff2460dab2500bad777100dfd
diff --git a/socket_spec.cpp b/socket_spec.cpp
index 27e8c46..9ce443e 100644
--- a/socket_spec.cpp
+++ b/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 @@
             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 @@
         *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 @@
         *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) {