Centralize Go validation and build environments

internal/process/process.go

@@ -9,16 +9,18 @@ import (
 	"os"
 	"os/exec"
 	"sort"
+	"strings"
 )
 
-// Invocation describes one child process. Environment values overlay the
-// current process environment for this child only.
+// Invocation describes one child process. UnsetEnv removes inherited values
+// before Env overlays values for this child only.
 type Invocation struct {
-	Args   []string
-	Dir    string
-	Env    map[string]string
-	Stdout io.Writer
-	Stderr io.Writer
+	Args     []string
+	Dir      string
+	Env      map[string]string
+	UnsetEnv []string
+	Stdout   io.Writer
+	Stderr   io.Writer
 }
 
 // Run starts the command in the requested directory and waits for it to exit.
@@ -30,8 +32,8 @@ func Run(ctx context.Context, invocation Invocation) error {
 
 	command := exec.CommandContext(ctx, invocation.Args[0], invocation.Args[1:]...)
 	command.Dir = invocation.Dir
-	if len(invocation.Env) != 0 {
-		command.Env = append(os.Environ(), environmentEntries(invocation.Env)...)
+	if len(invocation.Env) != 0 || len(invocation.UnsetEnv) != 0 {
+		command.Env = environmentEntries(invocation.Env, invocation.UnsetEnv)
 	}
 	command.Stdout = invocation.Stdout
 	command.Stderr = invocation.Stderr
@@ -45,7 +47,21 @@ func Run(ctx context.Context, invocation Invocation) error {
 	return nil
 }
 
-func environmentEntries(environment map[string]string) []string {
+func environmentEntries(overlay map[string]string, unset []string) []string {
+	environment := make(map[string]string, len(os.Environ())+len(overlay))
+	for _, entry := range os.Environ() {
+		key, value, ok := strings.Cut(entry, "=")
+		if ok {
+			environment[key] = value
+		}
+	}
+	for _, key := range unset {
+		delete(environment, key)
+	}
+	for key, value := range overlay {
+		environment[key] = value
+	}
+
 	keys := make([]string, 0, len(environment))
 	for key := range environment {
 		keys = append(keys, key)