Improve build config issue error scenario ko-build#483 (ko-build#487)

* Add build config usage log statement

There is currently no indication whether `ko` picks one of the configured
build configurations from the `.ko.yaml` configuration file for a build.

Add log statement to print the build config being picked for the build.

Introduce default entry for build config `ID` in case it is not specified.

* Add path check for build configuration settings

Add `os.Stat` to verify that the path that is configured in the build
configuration entry is valid. As a side effect, this will print out an error
message in case someone sets an import path like `github.com/google/ko` in
the `main` field of the build config.

* Fix trimpath command line flag in README

Fixed wrong command line flag `--trimpath` to `-trimpath`.

README.md

@@ -127,7 +127,7 @@ baseImageOverrides:
 ### Overriding Go build settings
 
 By default, `ko` builds the binary with no additional build flags other than
-`--trimpath` (depending on the Go version). You can replace the default build
+`-trimpath`. You can replace the default build
 arguments by providing build flags and ldflags using a
 [GoReleaser](https://github.com/goreleaser/goreleaser) influenced `builds`
 configuration section in your `.ko.yaml`.

pkg/build/gobuild.go

@@ -695,6 +695,10 @@ func (g *gobuild) configForImportPath(ip string) Config {
 		config.Flags = append(config.Flags, "-gcflags", "all=-N -l")
 	}
 
+	if config.ID != "" {
+		log.Printf("Using build config %s for %s", config.ID, ip)
+	}
+
 	return config
 }
 

pkg/commands/options/build.go

@@ -134,6 +134,12 @@ func (bo *BuildOptions) LoadConfig() error {
 func createBuildConfigMap(workingDirectory string, configs []build.Config) (map[string]build.Config, error) {
 	buildConfigsByImportPath := make(map[string]build.Config)
 	for i, config := range configs {
+		// In case no ID is specified, use the index of the build config in
+		// the ko YAML file as a reference (debug help).
+		if config.ID == "" {
+			config.ID = fmt.Sprintf("#%d", i)
+		}
+
 		// Make sure to behave like GoReleaser by defaulting to the current
 		// directory in case the build or main field is not set, check
 		// https://goreleaser.com/customization/build/ for details
@@ -154,6 +160,11 @@ func createBuildConfigMap(workingDirectory string, configs []build.Config) (map[
 			path = filepath.Dir(config.Main)
 		}
 
+		// Verify that the path actually leads to a local file (https://github.com/google/ko/issues/483)
+		if _, err := os.Stat(filepath.Join(baseDir, path)); err != nil {
+			return nil, err
+		}
+
 		// By default, paths configured in the builds section are considered
 		// local import paths, therefore add a "./" equivalent as a prefix to
 		// the constructured import path