Using the standard provider way

Other providers should be used with the SetChallengeProvider function
and should supply a New function. In your case this would be
NewHTTPProviderWebroot taking the path as an argument.

acme/client.go

@@ -12,7 +12,6 @@ import (
 	"io/ioutil"
 	"log"
 	"net"
-	"os"
 	"regexp"
 	"strconv"
 	"strings"
@@ -117,21 +116,6 @@ func (c *Client) SetChallengeProvider(challenge Challenge, p ChallengeProvider)
 	return nil
 }
 
-// SetWebRoot specifies a custom folder path to be used for HTTP based challenges.
-// If this option is used, lego will not bind any port to listen to,
-// instead it will write the challenge in a file into path/.well-known/acme-challenge/
-func (c *Client) SetWebRoot(path string) error {
-	if _, err := os.Stat(path); os.IsNotExist(err) {
-		return err
-	}
-
-	if chlng, ok := c.solvers[HTTP01]; ok {
-		chlng.(*httpChallenge).provider = &httpChallengeWebRoot{path: path}
-	}
-
-	return nil
-}
-
 // SetHTTPAddress specifies a custom interface:port to be used for HTTP based challenges.
 // If this option is not used, the default port 80 and all interfaces will be used.
 // To only specify a port and no interface use the ":port" notation.

acme/http_challenge_test.go

@@ -79,7 +79,7 @@ func TestHTTPChallengeWebRoot(t *testing.T) {
 
 		return nil
 	}
-	solver := &httpChallenge{jws: j, validate: mockValidate, provider: &httpChallengeWebRoot{path: "webroot"}}
+	solver := &httpChallenge{jws: j, validate: mockValidate, provider: &HTTPProviderWebroot{path: "webroot"}}
 
 	os.MkdirAll("webroot/.well-known/acme-challenge", 0777)
 	if err := solver.Solve(clientChallenge, "localhost:23457"); err != nil {
@@ -92,11 +92,11 @@ func TestHTTPChallengeWebRootInvalidPath(t *testing.T) {
 	privKey, _ := generatePrivateKey(rsakey, 128)
 	j := &jws{privKey: privKey.(*rsa.PrivateKey)}
 	clientChallenge := challenge{Type: HTTP01, Token: "http2"}
-	solver := &httpChallenge{jws: j, validate: stubValidate, provider: &httpChallengeWebRoot{path: "/invalid-path-123456"}}
+	solver := &httpChallenge{jws: j, validate: stubValidate, provider: &HTTPProviderWebroot{path: "/invalid-\000-path"}}
 
 	if err := solver.Solve(clientChallenge, "localhost:123456"); err == nil {
 		t.Errorf("Solve error: got %v, want error", err)
-	} else if want := "Could not write file in webroot"; !strings.Contains(err.Error(), want) {
+	} else if want := "Could not create required directories in webroot"; !strings.Contains(err.Error(), want) {
 		t.Errorf("Solve error: got %q, want content %q", err.Error(), want)
 	}
 }

acme/http_challenge_webroot.go

@@ -7,15 +7,35 @@ import (
 	"path"
 )
 
-// httpChallengeWebRoot implements ChallengeProvider for `http-01` challenge
-type httpChallengeWebRoot struct {
+// HTTPProviderWebroot implements ChallengeProvider for `http-01` challenge
+type HTTPProviderWebroot struct {
 	path string
 }
 
-// Present makes the token available at `HTTP01ChallengePath(token)`
-func (w *httpChallengeWebRoot) Present(domain, token, keyAuth string) error {
+// NewHTTPProviderWebroot returns a HTTPProviderWebroot instance with a configured webroot path
+func NewHTTPProviderWebroot(path string) (*HTTPProviderWebroot, error) {
+	if _, err := os.Stat(path); os.IsNotExist(err) {
+		return nil, fmt.Errorf("Webroot path does not exist")
+	}
+
+	c := &HTTPProviderWebroot{
+		path: path,
+	}
+
+	return c, nil
+}
+
+// Present makes the token available at `HTTP01ChallengePath(token)` by creating a file in the given webroot path
+func (w *HTTPProviderWebroot) Present(domain, token, keyAuth string) error {
 	var err error
-	err = ioutil.WriteFile(path.Join(w.path, HTTP01ChallengePath(token)), []byte(keyAuth), 0777)
+
+	challengeFilePath := path.Join(w.path, HTTP01ChallengePath(token))
+	err = os.MkdirAll(path.Dir(challengeFilePath), 0777)
+	if err != nil {
+		return fmt.Errorf("Could not create required directories in webroot for HTTP challenge -> %v", err)
+	}
+
+	err = ioutil.WriteFile(challengeFilePath, []byte(keyAuth), 0777)
 	if err != nil {
 		return fmt.Errorf("Could not write file in webroot for HTTP challenge -> %v", err)
 	}
@@ -23,7 +43,8 @@ func (w *httpChallengeWebRoot) Present(domain, token, keyAuth string) error {
 	return nil
 }
 
-func (w *httpChallengeWebRoot) CleanUp(domain, token, keyAuth string) error {
+// CleanUp removes the file created for the challenge
+func (w *HTTPProviderWebroot) CleanUp(domain, token, keyAuth string) error {
 	var err error
 	err = os.Remove(path.Join(w.path, HTTP01ChallengePath(token)))
 	if err != nil {

cli_handlers.go

@@ -44,7 +44,12 @@ func setup(c *cli.Context) (*Configuration, *Account, *acme.Client) {
 	}
 
 	if c.GlobalIsSet("webroot") {
-		client.SetWebRoot(c.GlobalString("webroot"))
+		provider, err := acme.NewHTTPProviderWebroot(c.GlobalString("webroot"))
+		if err != nil {
+			logger().Fatal(err)
+		}
+
+		client.SetChallengeProvider(HTTP01, provider)
 	}
 	if c.GlobalIsSet("http") {
 		client.SetHTTPAddress(c.GlobalString("http"))