Reject the -toolexec flag (ko-build#752)

pkg/build/gobuild.go

@@ -690,6 +690,16 @@ func createBuildArgs(buildCfg Config) ([]string, error) {
 		args = append(args, fmt.Sprintf("-ldflags=%s", strings.Join(buildCfg.Ldflags, " ")))
 	}
 
+	// Reject any flags that attempt to set --toolexec (with or
+	// without =, with one or two -s)
+	for _, a := range args {
+		for _, d := range []string{"-", "--"} {
+			if a == d+"toolexec" || strings.HasPrefix(a, d+"toolexec=") {
+				return nil, fmt.Errorf("cannot set %s", a)
+			}
+		}
+	}
+
 	return args, nil
 }
 

test/build-configs/.ko.yaml

@@ -19,3 +19,9 @@ builds:
 - id: bar-app
   dir: ./bar
   main: ./cmd
+- id: toolexec
+  dir: ./toolexec
+  main: ./cmd
+  flags:
+  - -toolexec
+  - go

test/build-configs/toolexec/cmd/main.go

@@ -0,0 +1,21 @@
+// Copyright 2022 Google LLC All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("toolexec")
+}

test/build-configs/toolexec/go.mod

@@ -0,0 +1,17 @@
+// Copyright 2022 Google LLC All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+module example.com/toolexec
+
+go 1.16