tools/battor_agent: Adds a tool to find connected BattOrs

tools/battor_agent/battor_agent_bin.cc

Index: tools/battor_agent/battor_agent_bin.cc
diff --git a/tools/battor_agent/battor_agent_bin.cc b/tools/battor_agent/battor_agent_bin.cc
index 360146dd39d81771ec4d0d8779fa84e99147d973..6f16a989854013ced082a73d496c69e322835dbe 100644
--- a/tools/battor_agent/battor_agent_bin.cc
+++ b/tools/battor_agent/battor_agent_bin.cc
@@ -19,6 +19,7 @@
 #include "base/threading/thread.h"
 #include "tools/battor_agent/battor_agent.h"
 #include "tools/battor_agent/battor_error.h"
+#include "tools/battor_agent/battor_finder.h"
 
 using std::cout;
 using std::endl;
@@ -47,12 +48,11 @@ void PrintSupportsExplicitClockSync() {
   cout << battor::BattOrAgent::SupportsExplicitClockSync() << endl;
 }
 
-// Retrieves argument argnum from the argument list, printing the usage
-// guidelines and exiting with an error code if the argument doesn't exist.
+// Retrieves argument argnum from the argument list, or an empty string if the
+// argument doesn't exist.
 std::string GetArg(int argnum, int argc, char* argv[]) {
   if (argnum >= argc) {
-    PrintUsage();
-    exit(1);
+    return "";

[sky, 2016/01/21 22:59:20]

nit: std::string()

[charliea (OOO until 10-5), 2016/01/22 16:11:03]

Done.

   }
 
   return argv[argnum];
@@ -89,6 +89,10 @@ class BattOrAgentBin : public BattOrAgent::Listener {
   // Runs the BattOr binary and returns the exit code.
   int Run(int argc, char* argv[]) {
     std::string cmd = GetArg(1, argc, argv);
+    if (cmd.empty()) {
+      PrintUsage();
+      exit(1);
+    }
 
     // SupportsExplicitClockSync doesn't need to use the serial connection, so
     // handle it separately.
@@ -98,6 +102,18 @@ class BattOrAgentBin : public BattOrAgent::Listener {
     }
 
     std::string path = GetArg(2, argc, argv);
+    // If no path is specified, see if we can find a BattOr and use that.
+    if (path.empty())
+      path = BattOrFinder::FindBattOr();
+
+    // If we don't have any BattOr to use, exit.
+    if (path.empty()) {
+      cout << "Unable to find a BattOr, and no explicit BattOr path was "
+              "specified."
+           << endl;
+      exit(1);
+    }
+
     SetUp(path);
 
     if (cmd == "StartTracing") {