vpython: Add Python interpreter handling package.

vpython/python/find.go

Index: vpython/python/find.go
diff --git a/vpython/python/find.go b/vpython/python/find.go
new file mode 100644
index 0000000000000000000000000000000000000000..65c62ede0fba69bf302191a76ce57bf9dd8c0c2e
--- /dev/null
+++ b/vpython/python/find.go
@@ -0,0 +1,60 @@
+// 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")
+}