1 files changed, 142 insertions, 0 deletions
diff --git a/microprom/handler_test.go b/microprom/handler_test.go
new file mode 100644
index 0000000..7278945
--- /dev/null
+++ b/microprom/handler_test.go
@@ -0,0 +1,142 @@
+// SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+package microprom_test
+
+import (
+	"context"
+	"errors"
+	"io"
+	"maps"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"go.xyrillian.de/gg/assert"
+	"go.xyrillian.de/gg/microprom"
+)
+
+func TestHandlerBasic(t *testing.T) {
+	// NOTE: Most happy path coverage is in `./testing/microprom`.
+	//       This only covers the SortOutput = false case.
+
+	h := microprom.Handler{
+		Families: map[microprom.MetricFamilyName]microprom.MetricFamilyInfo{
+			"process": {
+				Type: microprom.MetricTypeInfo,
+				Help: "Information about this process.",
+			},
+			"foo": {
+				Type: microprom.MetricTypeGauge,
+				Help: "This metric family will not have any collected metrics and thus go unreported.",
+			},
+		},
+		Collect: func(ctx context.Context, ms *microprom.MetricSet) error {
+			names := microprom.NewLabelNames("version")
+			labels := ms.FormatLabels(names, "1.2.3")
+			ms.Add("process", labels, 1.0)
+			return nil
+		},
+	}
+
+	// test normal behavior
+	status, body, headers := getMetrics(t, h, nil)
+	assert.Equal(t, status, http.StatusOK)
+	assert.Equal(t, headers, http.Header{
+		"Content-Type": {"text/plain; version=0.0.4; charset=utf-8; escaping=underscores"},
+	})
+	assert.Equal(t, body, strings.TrimSpace(`
+# HELP process_info Information about this process.
+# TYPE process_info info
+process_info{version="1.2.3"} 1
+	`)+"\n")
+}
+
+func TestHandlerErrors(t *testing.T) {
+	h := microprom.Handler{
+		Families: map[microprom.MetricFamilyName]microprom.MetricFamilyInfo{
+			"process": {
+				Type: microprom.MetricTypeInfo,
+				Help: "Information about this process.",
+			},
+		},
+		Collect: func(ctx context.Context, ms *microprom.MetricSet) error {
+			return errors.New("kaboom")
+		},
+	}
+
+	// test unacceptable content negotiation
+	status, body, headers := getMetrics(t, h, http.Header{"Accept": {"application/json"}})
+	assert.Equal(t, status, http.StatusNotAcceptable)
+	assert.Equal(t, headers.Get("Content-Type"), "text/plain; charset=utf-8")
+	assert.Equal(t, body, "supported formats are text/plain and application/openmetrics-text\n")
+
+	// test error during h.Collect()
+	status, body, headers = getMetrics(t, h, nil)
+	assert.Equal(t, status, http.StatusInternalServerError)
+	assert.Equal(t, headers.Get("Content-Type"), "text/plain; charset=utf-8")
+	assert.Equal(t, body, "kaboom\n")
+
+	// test panic from invalid metric family name
+	h.Families["what is this?"] = microprom.MetricFamilyInfo{
+		Type: microprom.MetricTypeGauge,
+		Help: "invalid metric family name",
+	}
+	msg := assert.PanicsWith[string](t, func() { getMetrics(t, h, nil) })
+	assert.Equal(t, msg, `in family "what is this?": invalid family name (does not match /^[a-zA-Z_:][a-zA-Z0-9_:]*$/)`)
+	delete(h.Families, "what is this?")
+
+	// test panic from invalid metric type
+	h.Families["invalid"] = microprom.MetricFamilyInfo{
+		Type: 100,
+		Help: "invalid metric type",
+	}
+	msg = assert.PanicsWith[string](t, func() { getMetrics(t, h, nil) })
+	assert.Equal(t, msg, `in family "invalid": invalid value for microprom.MetricType: 100`)
+	delete(h.Families, "invalid")
+
+	// test panic from invalid label name
+	h.Collect = func(ctx context.Context, ms *microprom.MetricSet) error {
+		names := microprom.NewLabelNames("app:version")
+		labels := ms.FormatLabels(names, "1.2.3")
+		ms.Add("process", labels, 1.0)
+		return nil
+	}
+	msg = assert.PanicsWith[string](t, func() { getMetrics(t, h, nil) })
+	assert.Equal(t, msg, `invalid label name: "app:version"`)
+
+	// test panic from wrong number of label values
+	h.Collect = func(ctx context.Context, ms *microprom.MetricSet) error {
+		names := microprom.NewLabelNames("version", "build_date")
+		labels := ms.FormatLabels(names, "1.2.3") // forgot build_date
+		ms.Add("process", labels, 1.0)
+		return nil
+	}
+	msg = assert.PanicsWith[string](t, func() { getMetrics(t, h, nil) })
+	assert.Equal(t, msg, `expected 2 label values, but got 1`)
+
+	// test panic from using an undeclared metric family
+	h.Collect = func(ctx context.Context, ms *microprom.MetricSet) error {
+		ms.Add("invalid", "", 1.0)
+		return nil
+	}
+	msg = assert.PanicsWith[string](t, func() { getMetrics(t, h, nil) })
+	assert.Equal(t, msg, `no such family: invalid`)
+}
+
+func getMetrics(t *testing.T, h http.Handler, requestHeaders http.Header) (status int, responseBody string, responseHeaders http.Header) {
+	t.Helper()
+	r := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/metrics", nil)
+	maps.Copy(r.Header, requestHeaders)
+
+	w := httptest.NewRecorder()
+	h.ServeHTTP(w, r)
+	resp := w.Result()
+
+	buf, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatal(err.Error())
+	}
+	return resp.StatusCode, string(buf), resp.Header
+}