fix: keep wrapping usage text after a word wider than the column

wrapN returned the whole remaining string whenever the first word did not
fit the description column, which ended the wrapping loop in wrap and left
every later word on that one line.

Break after the over-wide word instead, so only that word overflows and
wrapping resumes on the next one. Searching for the break point from i
covers both cases the old guard caught: no whitespace before i at all, and
whitespace only at index 0 where splitting would emit an empty line.

Fixes #501

flag.go

@@ -704,6 +704,13 @@ func wrapN(i, slop int, s string) (string, string) {
 
 	w := strings.LastIndexAny(s[:i], " \t\n")
 	if w <= 0 {
+		// There is nowhere to break within the first `i` runes, so the
+		// leading word is wider than the line. Let that one word overflow
+		// and break after it, rather than returning all of `s` and leaving
+		// everything that follows unwrapped.
+		if x := strings.IndexAny(s[i:], " \t\n"); x >= 0 {
+			return s[:i+x], s[i+x+1:]
+		}
 		return s, ""
 	}
 	nlPos := strings.LastIndex(s[:i], "\n")

printusage_test.go

@@ -3,6 +3,7 @@ package pflag
 import (
 	"bytes"
 	"io"
+	"strings"
 	"testing"
 )
 
@@ -72,3 +73,60 @@ func TestPrintUsage_2(t *testing.T) {
 		t.Errorf("Expected \n%s \nActual \n%s", expectedOutput2, res)
 	}
 }
+
+const expectedOutput3 = `      --mount string   a mount specification e.g.
+                       'type=bind,source=/opt,destination=/hostopt'.
+                       The rest of this description is
+                       wrapped.
+`
+
+// A word wider than the description column has to overflow, but only that
+// word: wrapping resumes on the next one.
+func TestPrintUsage_WrapsAfterUnbreakableWord(t *testing.T) {
+	f := NewFlagSet("test", ExitOnError)
+	f.String("mount", "", "a mount specification e.g. 'type=bind,source=/opt,destination=/hostopt'. The rest of this description is wrapped.")
+
+	res := f.FlagUsagesWrapped(60)
+	if res != expectedOutput3 {
+		t.Errorf("Expected \n%s \nActual \n%s", expectedOutput3, res)
+	}
+}
+
+// No line may be longer than the requested width unless it is a single word
+// that cannot be broken at all.
+func TestPrintUsage_OnlyUnbreakableWordsOverflow(t *testing.T) {
+	usages := []string{
+		"a mount specification e.g. 'type=bind,source=/opt,destination=/hostopt'. The rest of this description is wrapped.",
+		"https://example.com/a/very/long/url/that/will/not/fit/in/the/column and some words after it to wrap",
+		"averyveryverylongwordrightatthestart followed by several shorter words",
+		"short words then averyveryverylongwordinthemiddle and more words after that one",
+		"trailing text ending in anextremelylongunbreakabletokenattheveryend",
+	}
+
+	for _, width := range []int{40, 60, 80} {
+		for _, usage := range usages {
+			f := NewFlagSet("test", ExitOnError)
+			f.String("mount", "", usage)
+
+			lines := strings.Split(strings.TrimRight(f.FlagUsagesWrapped(width), "\n"), "\n")
+			if len(lines) < 2 {
+				t.Errorf("width %d, usage %q: expected the description to wrap", width, usage)
+				continue
+			}
+
+			// Continuation lines are padded to the description column, so
+			// their indent says where the description starts on every line.
+			indent := len(lines[1]) - len(strings.TrimLeft(lines[1], " "))
+
+			for _, line := range lines {
+				if len(line) <= width || indent >= len(line) {
+					continue
+				}
+				if strings.Contains(strings.TrimSpace(line[indent:]), " ") {
+					t.Errorf("width %d, usage %q: line is %d wide and still has a break point:\n%s",
+						width, usage, len(line), line)
+				}
+			}
+		}
+	}
+}