builder/symlink: Factor out into separate functions to make indents more readable

Implement suggestion from https://github.com/NixOS/nixpkgs/pull/188272#discussion_r958608686.

builder/symlink/symlink.go

@@ -15,33 +15,33 @@ type Package struct {
 	ReplacedPath  string `json:"replaced,omitempty"`
 }
 
+type Sources map[string]string
+
+func populateStruct(path string, data interface{}) {
+	pathVal := os.Getenv(path)
+	if len(path) == 0 {
+		panic(fmt.Sprintf("env var '%s' was unset", path))
+	}
+	path = pathVal
+
+	b, err := os.ReadFile(path)
+	if err != nil {
+		panic(err)
+	}
+
+	err = json.Unmarshal(b, &data)
+	if err != nil {
+		panic(err)
+	}
+}
+
 func main() {
-	sources := make(map[string]string)
+	sources := make(Sources)
 	pkgs := make(map[string]*Package)
 
-	{
-		b, err := os.ReadFile(os.Getenv("sourcesPath"))
-		if err != nil {
-			panic(err)
-		}
+	populateStruct("sourcesPath", &sources)
 
-		err = json.Unmarshal(b, &sources)
-		if err != nil {
-			panic(err)
-		}
-	}
-
-	{
-		b, err := os.ReadFile(os.Getenv("jsonPath"))
-		if err != nil {
-			panic(err)
-		}
-
-		err = json.Unmarshal(b, &pkgs)
-		if err != nil {
-			panic(err)
-		}
-	}
+	populateStruct("jsonPath", &pkgs)
 
 	keys := make([]string, 0, len(pkgs))
 	for key := range pkgs {
@@ -49,6 +49,10 @@ func main() {
 	}
 	sort.Strings(keys)
 
+	makeSymlinks(keys, sources)
+}
+
+func makeSymlinks(keys []string, sources Sources) {
 	// Iterate, in reverse order
 	for i := len(keys) - 1; i >= 0; i-- {
 		key := keys[i]
@@ -57,13 +61,27 @@ func main() {
 		paths := []string{key}
 
 		for _, path := range paths {
-
 			vendorDir := filepath.Join("vendor", filepath.Dir(path))
-			if err := os.MkdirAll(vendorDir, 0755); err != nil {
+			if err := os.MkdirAll(vendorDir, 0o755); err != nil {
 				panic(err)
 			}
 
-			if _, err := os.Stat(filepath.Join("vendor", path)); err == nil {
+			vendorPath := filepath.Join("vendor", path)
+			if _, err := os.Stat(vendorPath); err == nil {
+				populateVendorPath(vendorPath, src)
+				continue
+			}
+
+			// If the file doesn't already exist, just create a simple symlink
+			err := os.Symlink(src, vendorPath)
+			if err != nil {
+				panic(err)
+			}
+		}
+	}
+}
+
+func populateVendorPath(vendorPath string, src string) {
 	files, err := os.ReadDir(src)
 	if err != nil {
 		panic(err)
@@ -71,32 +89,21 @@ func main() {
 
 	for _, f := range files {
 		innerSrc := filepath.Join(src, f.Name())
-					dst := filepath.Join("vendor", path, f.Name())
+		dst := filepath.Join(vendorPath, f.Name())
 		if err := os.Symlink(innerSrc, dst); err != nil {
 			// assume it's an existing directory, try to link the directory content instead.
-						// TODO should we do this recursively
+			// TODO should we do this recursively?
 			files, err := os.ReadDir(innerSrc)
 			if err != nil {
 				panic(err)
 			}
 			for _, f := range files {
-							if err := os.Symlink(filepath.Join(innerSrc, f.Name()), filepath.Join(dst, f.Name())); err != nil {
-								fmt.Println("ignore symlink error", filepath.Join(innerSrc, f.Name()), filepath.Join(dst, f.Name()))
+				srcFile := filepath.Join(innerSrc, f.Name())
+				dstFile := filepath.Join(dst, f.Name())
+				if err := os.Symlink(srcFile, dstFile); err != nil {
+					fmt.Println("ignoring symlink error", srcFile, dstFile)
 				}
 			}
 		}
 	}
-
-				continue
-			}
-
-			// If the file doesn't already exist, just create a simple symlink
-			err := os.Symlink(src, filepath.Join("vendor", path))
-			if err != nil {
-				panic(err)
-			}
-
-		}
-	}
-
 }