vpython: Print Python command when run.

vpython/run.go

 // Copyright 2017 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package vpython
 
 import (
         "os"
         "os/exec"
         "os/signal"
@@ skipping 77 matching lines @@
                 prefixPATH(e, ve.BinDir)
 
                 // Run our bootstrapped Python command.
                 cmd := ve.Interpreter().IsolatedCommand(c, opts.Args...)
                 cmd.Dir = opts.WorkDir
                 cmd.Env = e.Sorted()
                 cmd.Stdin = os.Stdin
                 cmd.Stdout = os.Stdout
                 cmd.Stderr = os.Stderr
 
+                logging.Debugf(c, "Running Python command: %s\nWorkDir: %s\nEnv: %s", cmd.Args, cmd.Dir, cmd.Env)
+
+                // Output the Python command being executed.
                 if err := runAndForwardSignals(c, cmd, cancelFunc); err != nil {
                         return errors.Annotate(err).Reason("failed to execute bootstrapped Python").Err()
                 }
                 return nil
         })
         if err != nil {
                 return errors.Annotate(err).Err()
         }
         return nil
 }
@@ skipping 52 matching lines @@
         components = append([]string(nil), components...)
 
         // If there is a current PATH (likely), add that to the end.
         cur, _ := env.Get("PATH")
         if len(cur) > 0 {
                 components = append(components, cur)
         }
 
         env.Set("PATH", strings.Join(components, string(os.PathListSeparator)))
 }