testscript: add build:tag condition

testscript/doc.go

@@ -105,6 +105,8 @@ should only run when the condition is satisfied. The predefined conditions are:
  - [link] for whether the OS has hard link support
  - [symlink] for whether the OS has symbolic link support
  - [exec:prog] for whether prog is available for execution (found by exec.LookPath)
+ - [build:tag] for whether the release build tag is set; user-defined build tags
+   are not currently supported
 
 A condition can be negated: [!short] means to run the rest of the line
 when testing.Short() is false.

testscript/testdata/build.txt

@@ -0,0 +1,8 @@
+[!exec:sh] skip 'sh not found in $PATH'
+
+# test that the build:tag condition works for set build tags
+[build:go1.16] exec sh -c 'exit 0'
+[build:go1.17] exec sh -c 'exit 0'
+
+# test that the build:tag condition works for unset build tags
+[build:go2] exec sh -c 'exit 1'

testscript/testscript.go

@@ -12,6 +12,7 @@ import (
 	"context"
 	"flag"
 	"fmt"
+	"go/build"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -572,6 +573,14 @@ func (ts *TestScript) condition(cond string) (bool, error) {
 			return err == nil
 		}).(bool)
 		return ok, nil
+	case strings.HasPrefix(cond, "build:"):
+		tag := cond[len("build:"):]
+		for _, releaseTag := range build.Default.ReleaseTags {
+			if releaseTag == tag {
+				return true, nil
+			}
+		}
+		return false, nil
 	case ts.params.Condition != nil:
 		return ts.params.Condition(cond)
 	default: