feat: detect extensionless remote JSON responses

Remote HTTP sources without file extensions were always treated as markdown, even when the server identified the payload as JSON. That made API endpoints render as prose instead of a highlighted code block.

Record the HTTP Content-Type for remote sources and use JSON media types, including +json variants, as the code block language only when the URL has no extension.

Fixes #910

github.go

@@ -49,7 +49,7 @@ func findGitHubREADME(u *url.URL) (*source, error) {
 		}
 
 		if resp.StatusCode == http.StatusOK {
-			return &source{resp.Body, result.DownloadURL}, nil
+			return &source{reader: resp.Body, URL: result.DownloadURL}, nil
 		}
 	}
 

gitlab.go

@@ -53,7 +53,7 @@ func findGitLabREADME(u *url.URL) (*source, error) {
 		}
 
 		if resp.StatusCode == http.StatusOK {
-			return &source{resp.Body, readmeRawURL}, nil
+			return &source{reader: resp.Body, URL: readmeRawURL}, nil
 		}
 	}
 

glow_test.go

@@ -1,6 +1,9 @@
 package main
 
 import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
 	"testing"
 )
 
@@ -39,3 +42,53 @@ func TestGlowFlags(t *testing.T) {
 		}
 	}
 }
+
+func TestSourceDetectsExtensionlessJSONContentType(t *testing.T) {
+	src := source{
+		URL:         "https://api.example.com/endpoint",
+		contentType: "application/vnd.api+json; charset=utf-8",
+	}
+
+	if !src.isCode() {
+		t.Fatal("expected extensionless JSON response to render as code")
+	}
+	if got := src.codeBlockLanguage(); got != ".json" {
+		t.Fatalf("expected .json code block language, got %q", got)
+	}
+}
+
+func TestSourceMarkdownExtensionTakesPrecedenceOverContentType(t *testing.T) {
+	src := source{
+		URL:         "https://example.com/README.md",
+		contentType: "application/json",
+	}
+
+	if src.isCode() {
+		t.Fatal("expected markdown extension to render as markdown")
+	}
+}
+
+func TestSourceFromArgRecordsHTTPContentType(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/json; charset=utf-8")
+		fmt.Fprint(w, `{"ok":true}`)
+	}))
+	t.Cleanup(server.Close)
+
+	src, err := sourceFromArg(server.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		if err := src.reader.Close(); err != nil {
+			t.Fatal(err)
+		}
+	})
+
+	if got := src.contentType; got != "application/json; charset=utf-8" {
+		t.Fatalf("expected content type to be recorded, got %q", got)
+	}
+	if !src.isCode() {
+		t.Fatal("expected extensionless JSON HTTP source to render as code")
+	}
+}

main.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"io/fs"
+	"mime"
 	"net/http"
 	"net/url"
 	"os"
@@ -67,8 +68,9 @@ var (
 
 // source provides a readable markdown source.
 type source struct {
-	reader io.ReadCloser
-	URL    string
+	reader      io.ReadCloser
+	URL         string
+	contentType string
 }
 
 // sourceFromArg parses an argument and creates a readable source for it.
@@ -99,7 +101,11 @@ func sourceFromArg(arg string) (*source, error) {
 			if resp.StatusCode != http.StatusOK {
 				return nil, fmt.Errorf("HTTP status %d", resp.StatusCode)
 			}
-			return &source{resp.Body, u.String()}, nil
+			return &source{
+				reader:      resp.Body,
+				URL:         u.String(),
+				contentType: resp.Header.Get("Content-Type"),
+			}, nil
 		}
 	}
 
@@ -123,7 +129,7 @@ func sourceFromArg(arg string) (*source, error) {
 					}
 
 					u, _ := filepath.Abs(path)
-					src = &source{r, u}
+					src = &source{reader: r, URL: u}
 
 					// abort filepath.Walk
 					return errors.New("source found")
@@ -147,7 +153,34 @@ func sourceFromArg(arg string) (*source, error) {
 	if err != nil {
 		return nil, fmt.Errorf("unable to get absolute path: %w", err)
 	}
-	return &source{r, u}, nil
+	return &source{reader: r, URL: u}, nil
+}
+
+func (s source) isCode() bool {
+	if !utils.IsMarkdownFile(s.URL) {
+		return true
+	}
+	return filepath.Ext(s.URL) == "" && s.codeBlockLanguage() != ""
+}
+
+func (s source) codeBlockLanguage() string {
+	if ext := filepath.Ext(s.URL); ext != "" {
+		return ext
+	}
+	return codeBlockLanguageFromContentType(s.contentType)
+}
+
+func codeBlockLanguageFromContentType(contentType string) string {
+	mediaType, _, err := mime.ParseMediaType(contentType)
+	if err != nil {
+		mediaType = strings.TrimSpace(strings.Split(contentType, ";")[0])
+	}
+	mediaType = strings.ToLower(mediaType)
+
+	if mediaType == "application/json" || strings.HasSuffix(mediaType, "+json") {
+		return ".json"
+	}
+	return ""
 }
 
 // validateStyle checks if the style is a default style, if not, checks that
@@ -288,7 +321,7 @@ func executeCLI(cmd *cobra.Command, src *source, w io.Writer) error {
 		baseURL = u.String() + "/"
 	}
 
-	isCode := !utils.IsMarkdownFile(src.URL)
+	isCode := src.isCode()
 
 	// initialize glamour
 	r, err := glamour.NewTermRenderer(
@@ -303,7 +336,7 @@ func executeCLI(cmd *cobra.Command, src *source, w io.Writer) error {
 	}
 
 	content := string(b)
-	ext := filepath.Ext(src.URL)
+	ext := src.codeBlockLanguage()
 	if isCode {
 		content = utils.WrapCodeBlock(string(b), ext)
 	}