lang: funcs: Add runner pure func execution

This adds a function runner that runs pure functions. It will hopefully
be useful for speculative execution of functions for compile time
determination of types.

lang/funcs/funcs.go

@@ -21,8 +21,14 @@ package funcs
 import (
 	"fmt"
 	"strings"
+	"sync"
 
 	"github.com/purpleidea/mgmt/lang/interfaces"
+	"github.com/purpleidea/mgmt/lang/types"
+	"github.com/purpleidea/mgmt/util"
+
+	multierr "github.com/hashicorp/go-multierror"
+	errwrap "github.com/pkg/errors"
 )
 
 const (
@@ -128,3 +134,162 @@ func Map() map[string]func() interfaces.Func {
 	}
 	return m
 }
+
+// PureFuncExec is usually used to provisionally speculate about the result of a
+// pure function, by running it once, and returning the result. Pure functions
+// are expected to only produce one value that depends only on the input values.
+// This won't run any slow functions either.
+func PureFuncExec(handle interfaces.Func, args []types.Value) (types.Value, error) {
+	hostname := ""                                   // XXX: add to interface
+	debug := false                                   // XXX: add to interface
+	logf := func(format string, v ...interface{}) {} // XXX: add to interface
+
+	info := handle.Info()
+	if !info.Pure {
+		return nil, fmt.Errorf("func is not pure")
+	}
+	// if function is expensive to run, we won't run it provisionally
+	if info.Slow {
+		return nil, fmt.Errorf("func is slow")
+	}
+
+	if err := handle.Validate(); err != nil {
+		return nil, errwrap.Wrapf(err, "could not validate func")
+	}
+
+	sig := handle.Info().Sig
+	if sig.Kind != types.KindFunc {
+		return nil, fmt.Errorf("must be kind func")
+	}
+
+	wg := &sync.WaitGroup{}
+	defer wg.Wait()
+
+	errch := make(chan error)
+	input := make(chan types.Value)  // we close this when we're done
+	output := make(chan types.Value) // we create it, func closes it
+
+	init := &interfaces.Init{
+		Hostname: hostname,
+		Input:    input,
+		Output:   output,
+		World:    nil, // should not be used for pure functions
+		Debug:    debug,
+		Logf: func(format string, v ...interface{}) {
+			logf("func: "+format, v...)
+		},
+	}
+
+	if err := handle.Init(init); err != nil {
+		return nil, errwrap.Wrapf(err, "could not init func")
+	}
+
+	close1 := make(chan struct{})
+	close2 := make(chan struct{})
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		defer close(errch) // last one turns out the lights
+		select {
+		case <-close1:
+		}
+		select {
+		case <-close2:
+		}
+	}()
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		defer close(close1)
+		if debug {
+			logf("Running func")
+		}
+		err := handle.Stream() // sends to output chan
+		if debug {
+			logf("Exiting func")
+		}
+		if err == nil {
+			return
+		}
+		// we closed with an error...
+		select {
+		case errch <- errwrap.Wrapf(err, "problem streaming func"):
+		}
+	}()
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		defer close(close2)
+		defer close(input) // we only send one value
+		if len(args) == 0 {
+			return
+		}
+		si := &types.Type{
+			// input to functions are structs
+			Kind: types.KindStruct,
+			Map:  handle.Info().Sig.Map,
+			Ord:  handle.Info().Sig.Ord,
+		}
+		st := types.NewStruct(si)
+
+		for i, arg := range args {
+			name := util.NumToAlpha(i)                // assume (incorrectly) for now...
+			if err := st.Set(name, arg); err != nil { // populate struct
+				select {
+				case errch <- errwrap.Wrapf(err, "struct set failure"):
+				}
+				return
+			}
+		}
+
+		select {
+		case input <- st: // send to function (must not block)
+		case <-close1: // unblock the input send in case stream closed
+			select {
+			case errch <- fmt.Errorf("stream closed early"):
+			}
+		}
+	}()
+
+	once := false
+	var result types.Value
+	var reterr error
+Loop:
+	for {
+		select {
+		case value, ok := <-output: // read from channel
+			if !ok {
+				output = nil
+				continue Loop // only exit via errch closing!
+			}
+			if once {
+				reterr = fmt.Errorf("got more than one value")
+				continue // only exit via errch closing!
+			}
+			once = true
+			result = value // save value
+
+		case err, ok := <-errch: // handle possible errors
+			if !ok {
+				break Loop
+			}
+			e := errwrap.Wrapf(err, "problem streaming func")
+			if reterr != nil {
+				reterr = multierr.Append(reterr, e)
+			} else {
+				reterr = e
+			}
+		}
+	}
+
+	if err := handle.Close(); err != nil {
+		if reterr != nil {
+			err = multierr.Append(err, reterr)
+		}
+		return nil, errwrap.Wrapf(err, "problem closing func")
+	}
+
+	return result, reterr
+}