1 files changed, 205 insertions, 0 deletions
diff --git a/src/cmd/go/internal/script/conds.go b/src/cmd/go/internal/script/conds.go
new file mode 100644
index 0000000..d70f274
--- /dev/null
+++ b/src/cmd/go/internal/script/conds.go
@@ -0,0 +1,205 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package script
+
+import (
+	"cmd/go/internal/imports"
+	"fmt"
+	"os"
+	"runtime"
+	"sync"
+)
+
+// DefaultConds returns a set of broadly useful script conditions.
+//
+// Run the 'help' command within a script engine to view a list of the available
+// conditions.
+func DefaultConds() map[string]Cond {
+	conds := make(map[string]Cond)
+
+	conds["GOOS"] = PrefixCondition(
+		"runtime.GOOS == <suffix>",
+		func(_ *State, suffix string) (bool, error) {
+			if suffix == runtime.GOOS {
+				return true, nil
+			}
+			if _, ok := imports.KnownOS[suffix]; !ok {
+				return false, fmt.Errorf("unrecognized GOOS %q", suffix)
+			}
+			return false, nil
+		})
+
+	conds["GOARCH"] = PrefixCondition(
+		"runtime.GOARCH == <suffix>",
+		func(_ *State, suffix string) (bool, error) {
+			if suffix == runtime.GOARCH {
+				return true, nil
+			}
+			if _, ok := imports.KnownArch[suffix]; !ok {
+				return false, fmt.Errorf("unrecognized GOOS %q", suffix)
+			}
+			return false, nil
+		})
+
+	conds["compiler"] = PrefixCondition(
+		"runtime.Compiler == <suffix>",
+		func(_ *State, suffix string) (bool, error) {
+			if suffix == runtime.Compiler {
+				return true, nil
+			}
+			switch suffix {
+			case "gc", "gccgo":
+				return false, nil
+			default:
+				return false, fmt.Errorf("unrecognized compiler %q", suffix)
+			}
+		})
+
+	conds["root"] = BoolCondition("os.Geteuid() == 0", os.Geteuid() == 0)
+
+	return conds
+}
+
+// Condition returns a Cond with the given summary and evaluation function.
+func Condition(summary string, eval func(*State) (bool, error)) Cond {
+	return &funcCond{eval: eval, usage: CondUsage{Summary: summary}}
+}
+
+type funcCond struct {
+	eval  func(*State) (bool, error)
+	usage CondUsage
+}
+
+func (c *funcCond) Usage() *CondUsage { return &c.usage }
+
+func (c *funcCond) Eval(s *State, suffix string) (bool, error) {
+	if suffix != "" {
+		return false, ErrUsage
+	}
+	return c.eval(s)
+}
+
+// PrefixCondition returns a Cond with the given summary and evaluation function.
+func PrefixCondition(summary string, eval func(*State, string) (bool, error)) Cond {
+	return &prefixCond{eval: eval, usage: CondUsage{Summary: summary, Prefix: true}}
+}
+
+type prefixCond struct {
+	eval  func(*State, string) (bool, error)
+	usage CondUsage
+}
+
+func (c *prefixCond) Usage() *CondUsage { return &c.usage }
+
+func (c *prefixCond) Eval(s *State, suffix string) (bool, error) {
+	return c.eval(s, suffix)
+}
+
+// BoolCondition returns a Cond with the given truth value and summary.
+// The Cond rejects the use of condition suffixes.
+func BoolCondition(summary string, v bool) Cond {
+	return &boolCond{v: v, usage: CondUsage{Summary: summary}}
+}
+
+type boolCond struct {
+	v     bool
+	usage CondUsage
+}
+
+func (b *boolCond) Usage() *CondUsage { return &b.usage }
+
+func (b *boolCond) Eval(s *State, suffix string) (bool, error) {
+	if suffix != "" {
+		return false, ErrUsage
+	}
+	return b.v, nil
+}
+
+// OnceCondition returns a Cond that calls eval the first time the condition is
+// evaluated. Future calls reuse the same result.
+//
+// The eval function is not passed a *State because the condition is cached
+// across all execution states and must not vary by state.
+func OnceCondition(summary string, eval func() (bool, error)) Cond {
+	return &onceCond{eval: eval, usage: CondUsage{Summary: summary}}
+}
+
+type onceCond struct {
+	once  sync.Once
+	v     bool
+	err   error
+	eval  func() (bool, error)
+	usage CondUsage
+}
+
+func (l *onceCond) Usage() *CondUsage { return &l.usage }
+
+func (l *onceCond) Eval(s *State, suffix string) (bool, error) {
+	if suffix != "" {
+		return false, ErrUsage
+	}
+	l.once.Do(func() { l.v, l.err = l.eval() })
+	return l.v, l.err
+}
+
+// CachedCondition is like Condition but only calls eval the first time the
+// condition is evaluated for a given suffix.
+// Future calls with the same suffix reuse the earlier result.
+//
+// The eval function is not passed a *State because the condition is cached
+// across all execution states and must not vary by state.
+func CachedCondition(summary string, eval func(string) (bool, error)) Cond {
+	return &cachedCond{eval: eval, usage: CondUsage{Summary: summary, Prefix: true}}
+}
+
+type cachedCond struct {
+	m     sync.Map
+	eval  func(string) (bool, error)
+	usage CondUsage
+}
+
+func (c *cachedCond) Usage() *CondUsage { return &c.usage }
+
+func (c *cachedCond) Eval(_ *State, suffix string) (bool, error) {
+	for {
+		var ready chan struct{}
+
+		v, loaded := c.m.Load(suffix)
+		if !loaded {
+			ready = make(chan struct{})
+			v, loaded = c.m.LoadOrStore(suffix, (<-chan struct{})(ready))
+
+			if !loaded {
+				inPanic := true
+				defer func() {
+					if inPanic {
+						c.m.Delete(suffix)
+					}
+					close(ready)
+				}()
+
+				b, err := c.eval(suffix)
+				inPanic = false
+
+				if err == nil {
+					c.m.Store(suffix, b)
+					return b, nil
+				} else {
+					c.m.Store(suffix, err)
+					return false, err
+				}
+			}
+		}
+
+		switch v := v.(type) {
+		case bool:
+			return v, nil
+		case error:
+			return false, v
+		case <-chan struct{}:
+			<-v
+		}
+	}
+}