Allow python.bat files for finding Python in GN.

tools/gn/setup.cc

Index: tools/gn/setup.cc
diff --git a/tools/gn/setup.cc b/tools/gn/setup.cc
index 774d877691e58ba8cfac76df38e2a4ceae92b9c3..459b87bc5d6e3da36a1856dc89b192131d33f513 100644
--- a/tools/gn/setup.cc
+++ b/tools/gn/setup.cc
@@ -16,6 +16,7 @@
 #include "base/process/launch.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
+#include "base/strings/sys_string_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #include "build/build_config.h"
 #include "tools/gn/commands.h"
@@ -149,7 +150,38 @@ void DecrementWorkCount() {
 }
 
 #if defined(OS_WIN)
+
+// Given the path to a batch file that runs Python, extracts the name of the
+// executable actually implementing Python. Generally people write a batch file
+// to put something named "python" on the path, which then just redirects to
+// a python.exe somewhere else. This step decodes that setup. On failure,
+// returns empty path.
+base::FilePath PythonBatToExe(const base::FilePath& bat_path) {
+  // Note exciting double-quoting to allow spaces. The /c switch seems to check
+  // for quotes around the whole thing and then deletes them. If you want to
+  // quote the first argument in addition (to allow for spaces in the Python
+  // path, you need *another* set of quotes around that, likewise, we need
+  // two quotes at the end.
+  base::string16 command = L"cmd.exe /c \"\"";
+  command.append(bat_path.value());
+  command.append(L"\" -c \"import sys; print sys.executable\"\"");
+
+  std::string python_path;
+  if (base::GetAppOutput(command, &python_path)) {
+    base::TrimWhitespaceASCII(python_path, base::TRIM_ALL, &python_path);
+
+    // Python uses the system multibyte code page for sys.executable.
+    base::FilePath exe_path(base::SysNativeMBToWide(python_path));
+
+    // Check for reasonable output, cmd may have output an error message.
+    if (base::PathExists(exe_path))
+      return exe_path;
+  }
+  return base::FilePath();
+}
+
 const base::char16 kPythonExeName[] = L"python.exe";
+const base::char16 kPythonBatName[] = L"python.bat";
 
 base::FilePath FindWindowsPython() {
   base::char16 current_directory[MAX_PATH];
@@ -179,6 +211,15 @@ base::FilePath FindWindowsPython() {
         base::FilePath(component).Append(kPythonExeName);
     if (base::PathExists(candidate_exe))
       return candidate_exe;
+
+    // Also allow python.bat, but convert into the .exe.
+    base::FilePath candidate_bat =
+        base::FilePath(component).Append(kPythonBatName);
+    if (base::PathExists(candidate_bat)) {
+      base::FilePath python_exe = PythonBatToExe(candidate_bat);
+      if (!python_exe.empty())
+        return python_exe;
+    }
   }
   return base::FilePath();
 }