feat: Show HTTP response body on 500 (#84)

#### PR Dependency Tree


* **PR #84** 👈

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[Charcoal](https://github.com/danerwilliams/charcoal)

Author: Peeja

transport/http/channel.go

@@ -3,6 +3,7 @@ package http
 import (
 	"context"
 	"fmt"
+	"io"
 	"net/http"
 	"net/url"
 
@@ -77,7 +78,22 @@ func (c *Channel) Request(ctx context.Context, req transport.HTTPRequest) (trans
 		return nil, fmt.Errorf("doing HTTP request: %w", err)
 	}
 	if !slices.Contains(c.statuses, res.StatusCode) {
-		return nil, NewHTTPError(fmt.Sprintf("HTTP Request failed. %s %s → %d", hr.Method, c.url.String(), res.StatusCode), res.StatusCode, res.Header)
+		const maxBodyLen = 4096
+		b, readErr := io.ReadAll(io.LimitReader(res.Body, maxBodyLen+1))
+		res.Body.Close()
+
+		msg := fmt.Sprintf("HTTP Request failed. %s %s → %d", hr.Method, c.url.String(), res.StatusCode)
+		if readErr != nil {
+			msg += fmt.Sprintf(" (failed to read response body: %s)", readErr)
+		} else if len(b) > 0 {
+			body := string(b)
+			if len(body) > maxBodyLen {
+				body = body[:maxBodyLen] + "... (truncated)"
+			}
+			msg += ": " + body
+		}
+
+		return nil, NewHTTPError(msg, res.StatusCode, res.Header)
 	}
 
 	ctx = extractTraceContext(ctx, res.Header)

transport/http/channel_test.go

@@ -2,9 +2,11 @@ package http
 
 import (
 	"context"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"strings"
 	"testing"
 
 	"go.opentelemetry.io/otel"
@@ -97,6 +99,83 @@ func mustSpanIDFromHex(t *testing.T, hex string) trace.SpanID {
 	return spanID
 }
 
+func TestChannelErrorIncludesResponseBody(t *testing.T) {
+	const errorBody = `{"error":"InternalServerError","message":"something went wrong"}`
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+		w.Write([]byte(errorBody))
+	}))
+	t.Cleanup(server.Close)
+
+	endpoint, err := url.Parse(server.URL)
+	if err != nil {
+		t.Fatalf("parsing server URL: %v", err)
+	}
+
+	channel := NewChannel(endpoint, WithClient(server.Client()))
+
+	_, err = channel.Request(context.Background(), NewRequest(http.NoBody, nil))
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+
+	if !strings.Contains(err.Error(), errorBody) {
+		t.Fatalf("expected error to contain response body %q, got: %s", errorBody, err.Error())
+	}
+}
+
+func TestChannelErrorHandlesEmptyBody(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	t.Cleanup(server.Close)
+
+	endpoint, err := url.Parse(server.URL)
+	if err != nil {
+		t.Fatalf("parsing server URL: %v", err)
+	}
+
+	channel := NewChannel(endpoint, WithClient(server.Client()))
+
+	_, err = channel.Request(context.Background(), NewRequest(http.NoBody, nil))
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+
+	expected := fmt.Sprintf("HTTP Request failed. POST %s → 500", server.URL)
+	if err.Error() != expected {
+		t.Fatalf("expected %q, got %q", expected, err.Error())
+	}
+}
+
+func TestChannelErrorTruncatesLongBody(t *testing.T) {
+	longBody := strings.Repeat("x", 5000)
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+		w.Write([]byte(longBody))
+	}))
+	t.Cleanup(server.Close)
+
+	endpoint, err := url.Parse(server.URL)
+	if err != nil {
+		t.Fatalf("parsing server URL: %v", err)
+	}
+
+	channel := NewChannel(endpoint, WithClient(server.Client()))
+
+	_, err = channel.Request(context.Background(), NewRequest(http.NoBody, nil))
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+
+	if !strings.Contains(err.Error(), "... (truncated)") {
+		t.Fatalf("expected error to contain truncation marker, got: %s", err.Error())
+	}
+	if strings.Contains(err.Error(), longBody) {
+		t.Fatal("expected error to NOT contain the full body")
+	}
+}
+
 func setTraceContextPropagator() func() {
 	prev := otel.GetTextMapPropagator()
 	otel.SetTextMapPropagator(propagation.TraceContext{})