fix: honour paths in Grafana URLs (#95)

Previously we assumed that the Grafana instance lived at the root
of the domain provided in GRAFANA_URL (or the equivalent header).

This commit changes that so we support Grafana instances living at
subpaths, such as http://example.com/grafana.

Fixes #79.

Author: sd2k

mcpgrafana.go

@@ -101,6 +101,10 @@ func GrafanaAPIKeyFromContext(ctx context.Context) string {
 
 type grafanaClientKey struct{}
 
+func makeBasePath(path string) string {
+	return strings.Join([]string{strings.TrimRight(path, "/"), "api"}, "/")
+}
+
 // ExtractGrafanaClientFromEnv is a StdioContextFunc that extracts Grafana configuration
 // from environment variables and injects a configured client into the context.
 var ExtractGrafanaClientFromEnv server.StdioContextFunc = func(ctx context.Context) context.Context {
@@ -116,6 +120,7 @@ var ExtractGrafanaClientFromEnv server.StdioContextFunc = func(ctx context.Conte
 			panic(fmt.Errorf("invalid %s: %w", grafanaURLEnvVar, err))
 		}
 		cfg.Host = parsedURL.Host
+		cfg.BasePath = makeBasePath(parsedURL.Path)
 		// The Grafana client will always prefer HTTPS even if the URL is HTTP,
 		// so we need to limit the schemes to HTTP if the URL is HTTP.
 		if parsedURL.Scheme == "http" {
@@ -144,6 +149,7 @@ var ExtractGrafanaClientFromHeaders server.SSEContextFunc = func(ctx context.Con
 	if u != "" {
 		if url, err := url.Parse(u); err == nil {
 			cfg.Host = url.Host
+			cfg.BasePath = makeBasePath(url.Path)
 			if url.Scheme == "http" {
 				cfg.Schemes = []string{"http"}
 			}

mcpgrafana_test.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"testing"
 
+	"github.com/go-openapi/runtime/client"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -73,3 +74,35 @@ func TestExtractGrafanaInfoFromHeaders(t *testing.T) {
 		assert.Equal(t, "my-test-api-key", apiKey)
 	})
 }
+
+func TestExtractGrafanaClientPath(t *testing.T) {
+	t.Run("no custom path", func(t *testing.T) {
+		t.Setenv("GRAFANA_URL", "http://my-test-url.grafana.com/")
+		ctx := ExtractGrafanaClientFromEnv(context.Background())
+
+		c := GrafanaClientFromContext(ctx)
+		require.NotNil(t, c)
+		rt := c.Transport.(*client.Runtime)
+		assert.Equal(t, "/api", rt.BasePath)
+	})
+
+	t.Run("custom path", func(t *testing.T) {
+		t.Setenv("GRAFANA_URL", "http://my-test-url.grafana.com/grafana")
+		ctx := ExtractGrafanaClientFromEnv(context.Background())
+
+		c := GrafanaClientFromContext(ctx)
+		require.NotNil(t, c)
+		rt := c.Transport.(*client.Runtime)
+		assert.Equal(t, "/grafana/api", rt.BasePath)
+	})
+
+	t.Run("custom path, trailing slash", func(t *testing.T) {
+		t.Setenv("GRAFANA_URL", "http://my-test-url.grafana.com/grafana/")
+		ctx := ExtractGrafanaClientFromEnv(context.Background())
+
+		c := GrafanaClientFromContext(ctx)
+		require.NotNil(t, c)
+		rt := c.Transport.(*client.Runtime)
+		assert.Equal(t, "/grafana/api", rt.BasePath)
+	})
+}