Add --replace for dependency replacement (#186)

* add --replace to change dependency version

* deduplicate codes

* update README.md

README.md

@@ -62,6 +62,7 @@ Syntax:
 $ xcaddy build [<caddy_version>]
     [--output <file>]
     [--with <module[@version][=replacement]>...]
+    [--replace <module[@version]=replacement>...]
     [--embed <[alias]:path/to/dir>...]
 ```
 
@@ -73,6 +74,7 @@ $ xcaddy build [<caddy_version>]
 
 - `--output` changes the output file.
 - `--with` can be used multiple times to add plugins by specifying the Go module name and optionally its version, similar to `go get`. Module name is required, but specific version and/or local replacement are optional.
+- `--replace` can be used multiple times to replace dependencies to specific forks or local replacements. Useful in development environment to fix bugs in dependencies.
 - `--embed` can be used multiple times to embed directories into the built Caddy executable. The directory can be prefixed with a custom alias and a colon `:` to use it with the `root` directive and sub-directive. 
 
 Examples:
@@ -97,14 +99,14 @@ $ xcaddy build \
     --with github.com/caddyserver/[email protected]=../../my-fork
 ```
 
-You can even replace Caddy core using the `--with` flag:
+You can even replace Caddy core using the `--replace` flag:
 
 ```
 $ xcaddy build \
-    --with github.com/caddyserver/caddy/v2=../../my-caddy-fork
+    --replace github.com/caddyserver/caddy/v2=../../my-caddy-fork
     
 $ xcaddy build \
-    --with github.com/caddyserver/caddy/v2=github.com/my-user/caddy/v2@some-branch
+    --replace github.com/caddyserver/caddy/v2=github.com/my-user/caddy/v2@some-branch
 ```
 
 This allows you to hack on Caddy core (and optionally plug in extra modules at the same time!) with relative ease.

builder.go

@@ -199,6 +199,13 @@ type Dependency struct {
 	Version string `json:"version,omitempty"`
 }
 
+func (d Dependency) String() string {
+	if d.Version != "" {
+		return d.PackagePath + "@" + d.Version
+	}
+	return d.PackagePath
+}
+
 // ReplacementPath represents an old or new path component
 // within a Go module replacement directive.
 type ReplacementPath string

cmd/main.go

@@ -74,20 +74,27 @@ func runBuild(ctx context.Context, args []string) error {
 	var embedDir []string
 	for i := 0; i < len(args); i++ {
 		switch args[i] {
-		case "--with":
+		case "--with", "--replace":
+			arg := args[i]
 			if i == len(args)-1 {
-				return fmt.Errorf("expected value after --with flag")
+				return fmt.Errorf("expected value after %s flag", arg)
 			}
 			i++
 			mod, ver, repl, err := splitWith(args[i])
 			if err != nil {
 				return err
 			}
 			mod = strings.TrimSuffix(mod, "/") // easy to accidentally leave a trailing slash if pasting from a URL, but is invalid for Go modules
-			plugins = append(plugins, xcaddy.Dependency{
-				PackagePath: mod,
-				Version:     ver,
-			})
+			if arg == "--with" {
+				plugins = append(plugins, xcaddy.Dependency{
+					PackagePath: mod,
+					Version:     ver,
+				})
+			}
+
+			if arg != "--with" && repl == "" {
+				return fmt.Errorf("expected value after --replace flag")
+			}
 			if repl != "" {
 				// adjust relative replacements in current working directory since our temporary module is in a different directory
 				if strings.HasPrefix(repl, ".") {
@@ -97,9 +104,8 @@ func runBuild(ctx context.Context, args []string) error {
 					}
 					log.Printf("[INFO] Resolved relative replacement %s to %s", args[i], repl)
 				}
-				replacements = append(replacements, xcaddy.NewReplace(mod, repl))
+				replacements = append(replacements, xcaddy.NewReplace(xcaddy.Dependency{PackagePath: mod, Version: ver}.String(), repl))
 			}
-
 		case "--output":
 			if i == len(args)-1 {
 				return fmt.Errorf("expected value after --output flag")