Make android_logger_set_prune_list() sane

The current version requires callers to supply a string with 32 extra
bytes for liblog to internally prepend "setPruneList ", and to have
enough space to parse logd's return string.  That is an unacceptable
requirement on callers.

This change removes that requirement by having liblog allocate the
needed std::string in any case.

It also stops writing back the 'success' or 'Invalid' string to the
caller's buffer, since that is redundant as well.

Test: changing prune settings works.
Change-Id: Ic0f03a229f0b9a77d03adcb91288370c3bd42903
diff --git a/liblog/logd_reader.cpp b/liblog/logd_reader.cpp
index 96e7a61..6865c14 100644
--- a/liblog/logd_reader.cpp
+++ b/liblog/logd_reader.cpp
@@ -33,6 +33,8 @@
 #include <time.h>
 #include <unistd.h>
 
+#include <string>
+
 #include <cutils/sockets.h>
 #include <private/android_filesystem_config.h>
 #include <private/android_logger.h>
@@ -249,22 +251,14 @@
   return SendLogdControlMessage(buf, len);
 }
 
-int android_logger_set_prune_list(struct logger_list* logger_list, char* buf, size_t len) {
+int android_logger_set_prune_list(struct logger_list* logger_list, const char* buf, size_t len) {
   if (logger_list->mode & ANDROID_LOG_PSTORE) {
     return -EINVAL;
   }
 
-  const char cmd[] = "setPruneList ";
-  const size_t cmdlen = sizeof(cmd) - 1;
+  std::string cmd = "setPruneList " + std::string{buf, len};
 
-  if (strlen(buf) > (len - cmdlen)) {
-    return -ENOMEM; /* KISS */
-  }
-  memmove(buf + cmdlen, buf, len - cmdlen);
-  buf[len - 1] = '\0';
-  memcpy(buf, cmd, cmdlen);
-
-  return check_log_success(buf, SendLogdControlMessage(buf, len));
+  return check_log_success(cmd.data(), SendLogdControlMessage(cmd.data(), cmd.size()));
 }
 
 static int logdOpen(struct logger_list* logger_list) {