[go] Support "nested" Go main builds

Author: csweichel

README.md

@@ -132,6 +132,8 @@ config:
   buildFlags: []
   # Command that's executed to lint the code
   lintCommand: ["golangci-lint", "run"]
+  # GoMod can point to a go.mod file outside the component root. Leeway expects a go.sum alongside the go.mod.
+  goMod: "../go.mod"
 ```
 
 ### Yarn packages
@@ -173,6 +175,14 @@ config:
   - gitpod/leeway:${__pkg_version}
 ```
 
+The first image name of each Docker dependency which pushed an image will result in a build argument. This mechanism enables a package to build the base image for another one, by using the build argument as `FROM` value.
+The name of this build argument is the package name of the dependency, transformed as follows:
+- `/` is replaced with `_`
+- `:` is replaced with `__`
+- all uppercase.
+
+E.g. `component/nested:docker` becomes `COMPONENT_NESTED__DOCKER`.
+
 ### Generic packages
 ```YAML
 config:

pkg/leeway/build.go

@@ -1078,7 +1078,7 @@ func (p *Package) buildGo(buildctx *buildContext, wd, result string) (res *packa
 		return nil, xerrors.Errorf("package should have Go config")
 	}
 
-	if _, err := os.Stat(filepath.Join(p.C.Origin, "go.mod")); os.IsNotExist(err) {
+	if _, err := os.Stat(filepath.Join(wd, "go.mod")); os.IsNotExist(err) {
 		return nil, xerrors.Errorf("can only build Go modules (missing go.mod file)")
 	}
 

pkg/leeway/package.go

@@ -475,6 +475,7 @@ type GoPkgConfig struct {
 	BuildCommand   []string    `yaml:"buildCommand,omitempty"`
 	LintCommand    []string    `yaml:"lintCommand,omitempty"`
 	GoVersion      string      `yaml:"goVersion,omitempty"`
+	GoMod          string      `yaml:"goMod,omitempty"`
 }
 
 // Validate ensures this config can be acted upon/is valid
@@ -495,6 +496,15 @@ func (cfg GoPkgConfig) Validate() error {
 		}
 	}
 
+	if cfg.GoMod != "" {
+		if filepath.IsAbs(cfg.GoMod) {
+			return xerrors.Errorf("goMod must be relative to the component root")
+		}
+		if !strings.HasSuffix(cfg.GoMod, ".mod") {
+			return xerrors.Errorf("goMod must end with .mod")
+		}
+	}
+
 	return nil
 }
 
@@ -510,12 +520,21 @@ const (
 
 // AdditionalSources returns a list of unresolved sources coming in through this configuration
 func (cfg GoPkgConfig) AdditionalSources(workspaceOrigin string) []string {
+	var res []string
+
 	fn := filepath.Join(workspaceOrigin, "go.work")
 	if _, err := os.Stat(fn); err == nil {
-		return []string{fn}
+		res = append(res, fn)
 	}
 
-	return []string{}
+	if cfg.GoMod != "" {
+		res = append(res,
+			cfg.GoMod,
+			strings.TrimSuffix(cfg.GoMod, ".mod")+".sum",
+		)
+	}
+
+	return res
 }
 
 // DockerPkgConfig configures a Docker package

pkg/leeway/workspace.go

@@ -613,7 +613,7 @@ func loadComponent(ctx context.Context, workspace *Workspace, path string, args
 			}
 
 			if _, err := os.Stat(fn); os.IsNotExist(err) {
-				return comp, xerrors.Errorf("%s: %w", comp.Name, err)
+				return comp, xerrors.Errorf("cannot find additional source for %s: %w", comp.Name, err)
 			}
 			if _, found := completeSources[fn]; found {
 				continue