Test Go Library Examples

Add markdown test to run go vet on go library examples. Fix existing go
library examples.

Closes #4703

docs/libraries/go.md

@@ -39,7 +39,7 @@ go get github.com/brimdata/zed
 ### ZSON Reader
 
 Read ZSON from stdin, dereference field `s`, and print results:
-```
+```mdtest-go-example
 package main
 
 import (
@@ -54,7 +54,7 @@ import (
 
 func main() {
 	zctx := zed.NewContext()
-	reader := zsonio.NewReader(os.Stdin, zctx)
+	reader := zsonio.NewReader(zctx, os.Stdin)
 	for {
 		val, err := reader.Read()
 		if err != nil {
@@ -65,7 +65,7 @@ func main() {
 		}
 		s := val.Deref("s")
 		if s == nil {
-			s = zctx.Missing()
+			s = zctx.Missing().Ptr()
 		}
 		fmt.Println(zson.String(s))
 	}
@@ -105,7 +105,7 @@ zed create -lake scratch Demo
 echo '{s:"hello, world"}{x:1}{s:"good bye"}' | zed load -lake scratch -use Demo -
 ```
 Now replace `main.go` with this code:
-```
+```mdtest-go-example
 package main
 
 import (
@@ -129,7 +129,7 @@ func main() {
 		log.Fatalln(err)
 	}
 	ctx := context.TODO()
-	lake, err := api.OpenLake(ctx, uri.String())
+	lake, err := api.OpenLake(ctx, nil, uri.String())
 	if err != nil {
 		log.Fatalln(err)
 	}
@@ -149,7 +149,7 @@ func main() {
 		}
 		s := val.Deref("s")
 		if s == nil {
-			s = zctx.Missing()
+			s = zctx.Missing().Ptr()
 		}
 		fmt.Println(zson.String(s))
 	}

mdtest/mdtest.go

@@ -192,6 +192,11 @@ func parseMarkdown(source []byte) (map[string]string, []*Test, error) {
 				Line:     fcbLineNumber(commandFCB, source),
 			})
 			commandFCB = nil
+		case "mdtest-go-example":
+			tests = append(tests, &Test{
+				GoExample: fcbLines(fcb, source),
+				Line:      fcbLineNumber(fcb, source),
+			})
 		}
 		return ast.WalkContinue, nil
 	})

mdtest/test.go

@@ -3,24 +3,30 @@ package mdtest
 import (
 	"errors"
 	"fmt"
+	"os"
 	"os/exec"
+	"path/filepath"
 	"strings"
 
 	"github.com/pmezard/go-difflib/difflib"
 )
 
 // Test represents a single test in a Markdown file.
 type Test struct {
-	Command  string
-	Dir      string
-	Expected string
-	Fails    bool
-	Head     bool
-	Line     int
+	Command   string
+	Dir       string
+	Expected  string
+	Fails     bool
+	Head      bool
+	Line      int
+	GoExample string
 }
 
 // Run runs the test, returning nil on success.
 func (t *Test) Run() error {
+	if t.GoExample != "" {
+		return t.vetGoExample()
+	}
 	c := exec.Command("bash", "-e", "-o", "pipefail")
 	c.Dir = t.Dir
 	c.Stdin = strings.NewReader(t.Command)
@@ -57,3 +63,21 @@ func (t *Test) Run() error {
 	}
 	return nil
 }
+
+func (t *Test) vetGoExample() error {
+	dir, err := os.MkdirTemp("", "")
+	if err != nil {
+		return err
+	}
+	defer os.RemoveAll(dir)
+	path := filepath.Join(dir, "main.go")
+	if err := os.WriteFile(path, []byte(t.GoExample), 0666); err != nil {
+		return err
+	}
+	_, err = exec.Command("go", "vet", path).Output()
+	var exitErr *exec.ExitError
+	if errors.As(err, &exitErr) {
+		return fmt.Errorf("could not vet go example: %s", string(exitErr.Stderr))
+	}
+	return err
+}