vpython: Add Python interpreter handling package.

vpython/python/find.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 python
+
+import (
+        "os/exec"
+
+        "github.com/luci/luci-go/common/errors"
+
+        "golang.org/x/net/context"
+)
+
+// Find attempts to find a Python interpreter matching the supplied version
+// using PATH.
+//
+// In order to accommodate multiple configurations on operating systems, Find
+// will attempt to identify versions that appear on the path
+func Find(c context.Context, vers Version) (*Interpreter, error) {
+        // pythonM.m, pythonM, python
+        searches := make([]string, 0, 3)
+        pv := vers
+        pv.Patch = 0
+        if pv.Minor > 0 {
+                searches = append(searches, pv.PythonBase())
+                pv.Minor = 0
+        }
+        if pv.Major > 0 {
+                searches = append(searches, pv.PythonBase())
+                pv.Major = 0
+        }
+        searches = append(searches, pv.PythonBase())
+
+        for _, s := range searches {
+                p, err := exec.LookPath(s)
+                if err != nil {
+                        if e, ok := err.(*exec.Error); ok && e.Err == exec.ErrNotFound {
+                                // Not found is okay.
+                                continue
+                        }
+                        return nil, errors.Annotate(err).Reason("failed to search PATH for: %(interp)q").
+                                D("interp", s).
+                                Err()
+                }
+
+                i := Interpreter{Python: p}
+                iv, err := i.GetVersion(c)
+                if err != nil {
+                        return nil, errors.Annotate(err).Reason("failed to get version for: %(interp)q").
+                                D("interp", p).
+                                Err()
+                }
+                if vers.IsSatisfiedBy(iv) {
+                        return &i, nil
+                }
+        }
+
+        return nil, errors.New("no Python found")
+}