Provide option to timeout functions earlier than configured

A header of X-Timeout can be passed to reduce the value of
exec_timeout. The consumer invoking the function is responsible
for tuning this value and making sure it has enough headroom.

The change was requested by @kevin-lindsay-1 for functions
which process batched data of varying size and could run for
24 hours or 2 minutes, but need to timeout in a timely
fashion, according to the input.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

README.md

@@ -128,6 +128,14 @@ In HTTP mode, the watchdog will append the X-Call-Id to its own HTTP log message
 2024/04/25 17:29:58 GET / - 301 Moved Permanently - ContentLength: 39B (0.0037s) [079d9ff9-d7b7-4e37-b195-5ad520e6f797]
 ```
 
+#### 1.5 Reducing timeouts
+
+If a function has a timeout set via `exec_timeout` of a large value like `1h`, but you need an individual request to timeout earlier, i.e. `1m`, then you can pass in a HTTP header of `X-Timeout` with a Go duration to override the behaviour.
+
+The value for `X-Timeout` must be equal to or shorter than the `exec_timeout` environment variable.
+
+`X-Timeout` cannot be set when the `exec_timeout` is set to `0` or hasn't been specified.
+
 ### 2. Serializing fork (mode=serializing)
 
 #### 2.1 Status

executor/http_runner.go

@@ -118,11 +118,13 @@ func (f *HTTPFunctionRunner) Run(req FunctionRequest, contentLength int64, r *ht
 	request.Host = r.Host
 	copyHeaders(request.Header, &r.Header)
 
+	execTimeout := getTimeout(r, f.ExecTimeout)
+
 	var reqCtx context.Context
 	var cancel context.CancelFunc
 
-	if f.ExecTimeout.Nanoseconds() > 0 {
-		reqCtx, cancel = context.WithTimeout(r.Context(), f.ExecTimeout)
+	if execTimeout.Nanoseconds() > 0 {
+		reqCtx, cancel = context.WithTimeout(r.Context(), execTimeout)
 	} else {
 		reqCtx = r.Context()
 		cancel = func() {
@@ -203,6 +205,20 @@ func (f *HTTPFunctionRunner) Run(req FunctionRequest, contentLength int64, r *ht
 	return nil
 }
 
+func getTimeout(r *http.Request, defaultTimeout time.Duration) time.Duration {
+	execTimeout := defaultTimeout
+	if v := r.Header.Get("X-Timeout"); len(v) > 0 {
+		dur, err := time.ParseDuration(v)
+		if err == nil {
+			if dur <= defaultTimeout {
+				execTimeout = dur
+			}
+		}
+	}
+
+	return execTimeout
+}
+
 func copyHeaders(destination http.Header, source *http.Header) {
 	for k, v := range *source {
 		vClone := make([]string, len(v))

executor/http_runner_test.go

@@ -0,0 +1,49 @@
+package executor
+
+import (
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+func TestGetTimeout_PicksLowerOverride(t *testing.T) {
+	override := time.Second * 10
+	r := httptest.NewRequest("GET", "http://localhost:8080", nil)
+	r.Header.Add("X-Timeout", override.Round(time.Second).String())
+
+	defaultTimeout := time.Second * 20
+	got := getTimeout(r, defaultTimeout)
+	want := time.Second * 10
+
+	if got != want {
+		t.Errorf("getTimeout() got: %v, want %v", got, want)
+	}
+}
+
+func TestGetTimeout_CapsOverrideToDefaultValue(t *testing.T) {
+	override := time.Second * 21
+	r := httptest.NewRequest("GET", "http://localhost:8080", nil)
+	r.Header.Add("X-Timeout", override.Round(time.Second).String())
+
+	defaultTimeout := time.Second * 20
+	got := getTimeout(r, defaultTimeout)
+	want := time.Second * 20
+
+	if got != want {
+		t.Errorf("getTimeout() got: %v, want %v", got, want)
+	}
+}
+
+func TestGetTimeout_NoDefaultMeansNoOverride(t *testing.T) {
+	override := time.Second * 10
+	r := httptest.NewRequest("GET", "http://localhost:8080", nil)
+	r.Header.Add("X-Timeout", override.Round(time.Second).String())
+
+	defaultTimeout := time.Nanosecond * 0
+	got := getTimeout(r, defaultTimeout)
+	want := time.Nanosecond * 0
+
+	if got != want {
+		t.Errorf("getTimeout() got: %v, want %v", got, want)
+	}
+}