Validate max_retries on both target forms (closes #221)

internal/handlers/target_retries.go

@@ -0,0 +1,119 @@
+package handlers
+
+import (
+	"errors"
+	"net/http"
+	"strconv"
+	"strings"
+)
+
+// maxTargetRetries bounds a target's max_retries.
+//
+// Both target forms already declare max="20" on the input, so this
+// enforces server-side what the UI has always advertised rather than
+// introducing a new limit.
+//
+// The number is not cosmetic. Every attempt writes a delivery_results
+// row that the event log then loads and renders, and the engine backs
+// off by 2^(n-1) seconds, so attempt 20 is already about six days
+// after the first. A value beyond this buys no additional durability
+// and only costs rows.
+const maxTargetRetries = 20
+
+// Errors returned when a max_retries form value cannot be turned into
+// a retry count.
+var (
+	// errRetriesInvalid signals a max_retries form value that is not
+	// a non-negative whole number.
+	errRetriesInvalid = errors.New(
+		"retries must be a whole number of attempts",
+	)
+
+	// errRetriesTooLarge signals a max_retries form value that is a
+	// whole number but above maxTargetRetries. It is distinguished
+	// from errRetriesInvalid so the message can name the ceiling
+	// instead of implying the input was not a number.
+	errRetriesTooLarge = errors.New("retries out of range")
+)
+
+// parseMaxRetries interprets a max_retries form value.
+//
+// An ABSENT value — the field empty or not submitted — yields
+// fallback, which lets the create path apply its default and the edit
+// path leave the stored value alone. A value that is SET BUT INVALID
+// is an error: unparseable, negative, or above maxTargetRetries.
+//
+// The distinction is the whole point of this function. max_retries=0
+// means fire-and-forget, so returning 0 for input the operator typed
+// but that did not parse silently disables retries on a
+// store-and-forward proxy — and on the edit path it destroys a
+// working retry configuration over a typo. A default answers a
+// question that was not asked; it never answers one that was asked
+// badly.
+//
+// A target stored with a count above the ceiling before this
+// validation existed keeps rendering and keeps delivering — nothing
+// clamps the row. Re-saving it from the edit form does have to bring
+// it into range, because the form submits the pre-filled value back
+// and accepting it would be the ceiling not applying to the edit
+// path. The 400 names the ceiling, so the fix is one field.
+func parseMaxRetries(raw string, fallback int) (int, error) {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return fallback, nil
+	}
+
+	v, err := strconv.Atoi(raw)
+	if err != nil || v < 0 {
+		return 0, errRetriesInvalid
+	}
+
+	if v > maxTargetRetries {
+		return 0, errRetriesTooLarge
+	}
+
+	return v, nil
+}
+
+// retriesErrorMessage returns the message the create and edit forms
+// show for a rejected max_retries value. Any error other than
+// errRetriesTooLarge falls back to the generic wording, so an
+// unrecognised parse failure still produces a sensible 400.
+func retriesErrorMessage(err error) string {
+	if errors.Is(err, errRetriesTooLarge) {
+		return errRetriesTooLarge.Error() +
+			": at most " + strconv.Itoa(maxTargetRetries) +
+			" retries"
+	}
+
+	return errRetriesInvalid.Error() +
+		", or 0 for fire-and-forget"
+}
+
+// targetMaxRetries reads and validates max_retries from a target form
+// submission, answering the request with a 400 and reporting false
+// when the value is set but invalid.
+//
+// Both the create and the edit path go through here, so the two
+// cannot come to disagree about what a valid retry count is. The
+// wording matches the timeout control on the same submission.
+func targetMaxRetries(
+	w http.ResponseWriter,
+	r *http.Request,
+	fallback int,
+) (int, bool) {
+	retries, err := parseMaxRetries(
+		r.PostFormValue("max_retries"), fallback,
+	)
+	if err != nil {
+		http.Error(
+			w,
+			"Invalid max retries: "+retriesErrorMessage(err),
+			http.StatusBadRequest,
+		)
+
+		return 0, false
+	}
+
+	return retries, true
+}