feat: deterministically set the colour of log scopes (#2814)

<img width="623" alt="image"
src="https://github.com/user-attachments/assets/bc875407-0586-4f15-9eed-dc716e0c65ee">

We should colour the module names in the status line too, so it's easier
to visually associate them with their logs.

internal/log/plain.go

@@ -1,21 +1,68 @@
 package log
 
 import (
+	"context"
 	"fmt"
+	"hash/fnv"
 	"io"
 	"os"
+	"strconv"
 	"time"
 
+	"github.com/alecthomas/types/once"
 	"github.com/mattn/go-isatty"
 )
 
-var colours = map[Level]string{
-	Trace: "\x1b[90m", // Dark gray
-	Debug: "\x1b[34m", // Blue
-	Info:  "\x1b[37m", // White
-	Warn:  "\x1b[33m", // Yellow
-	Error: "\x1b[31m", // Red
-}
+var (
+	// https://en.wikipedia.org/wiki/ANSI_escape_code#8-bit
+	// Colours for modules.
+	rgb256Colours = []string{
+		"000000", "008000", "808000", "000080", "800080", "008080", "c0c0c0", "808080",
+		"00ff00", "ffff00", "0000ff", "ff00ff", "00ffff", "ffffff", "000000", "00005f", "000087", "0000af",
+		"0000d7", "0000ff", "005f00", "005f5f", "005f87", "005faf", "005fd7", "005fff", "008700", "00875f",
+		"008787", "0087af", "0087d7", "0087ff", "00af00", "00af5f", "00af87", "00afaf", "00afd7", "00afff",
+		"00d700", "00d75f", "00d787", "00d7af", "00d7d7", "00d7ff", "00ff00", "00ff5f", "00ff87", "00ffaf",
+		"00ffd7", "00ffff", "5f0000", "5f005f", "5f0087", "5f00af", "5f00d7", "5f00ff", "5f5f00", "5f5f5f",
+		"5f5f87", "5f5faf", "5f5fd7", "5f5fff", "5f8700", "5f875f", "5f8787", "5f87af", "5f87d7", "5f87ff",
+		"5faf00", "5faf5f", "5faf87", "5fafaf", "5fafd7", "5fafff", "5fd700", "5fd75f", "5fd787", "5fd7af",
+		"5fd7d7", "5fd7ff", "5fff00", "5fff5f", "5fff87", "5fffaf", "5fffd7", "5fffff", "870000", "87005f",
+		"870087", "8700af", "8700d7", "8700ff", "875f00", "875f5f", "875f87", "875faf", "875fd7", "875fff",
+		"878700", "87875f", "878787", "8787af", "8787d7", "8787ff", "87af00", "87af5f", "87af87", "87afaf",
+		"87afd7", "87afff", "87d700", "87d75f", "87d787", "87d7af", "87d7d7", "87d7ff", "87ff00", "87ff5f",
+		"87ff87", "87ffaf", "87ffd7", "87ffff", "af0000", "af005f", "af0087", "af00af", "af00d7", "af00ff",
+		"af5f00", "af5f5f", "af5f87", "af5faf", "af5fd7", "af5fff", "af8700", "af875f", "af8787", "af87af",
+		"af87d7", "af87ff", "afaf00", "afaf5f", "afaf87", "afafaf", "afafd7", "afafff", "afd700", "afd75f",
+		"afd787", "afd7af", "afd7d7", "afd7ff", "afff00", "afff5f", "afff87", "afffaf", "afffd7", "afffff",
+		"d70000", "d7005f", "d70087", "d700af", "d700d7", "d700ff", "d75f00", "d75f5f", "d75f87", "d75faf",
+		"d75fd7", "d75fff", "d78700", "d7875f", "d78787", "d787af", "d787d7", "d787ff", "d7af00", "d7af5f",
+		"d7af87", "d7afaf", "d7afd7", "d7afff", "d7d700", "d7d75f", "d7d787", "d7d7af", "d7d7d7", "d7d7ff",
+		"d7ff00", "d7ff5f", "d7ff87", "d7ffaf", "d7ffd7", "d7ffff", "ff0000", "ff005f", "ff0087", "ff00af",
+		"ff00d7", "ff00ff", "ff5f00", "ff5f5f", "ff5f87", "ff5faf", "ff5fd7", "ff5fff", "ff8700", "ff875f",
+		"ff8787", "ff87af", "ff87d7", "ff87ff", "ffaf00", "ffaf5f", "ffaf87", "ffafaf", "ffafd7", "ffafff",
+		"ffd700", "ffd75f", "ffd787", "ffd7af", "ffd7d7", "ffd7ff", "ffff00", "ffff5f", "ffff87", "ffffaf",
+		"ffffd7", "ffffff", "080808", "121212", "1c1c1c", "262626", "303030", "3a3a3a", "444444", "4e4e4e",
+		"585858", "606060", "666666", "767676", "808080", "8a8a8a", "949494", "9e9e9e", "a8a8a8", "b2b2b2",
+		"bcbcbc", "c6c6c6", "d0d0d0", "dadada", "e4e4e4", "eeeeee",
+	}
+	scopeColours = once.Once(func(ctx context.Context) ([]string, error) {
+		out := make([]string, 0, len(rgb256Colours))
+		for i, c := range rgb256Colours {
+			r, g, b := parseRGB(c)
+			if (r > g*2 && r > b*2) || (r+g+b < 200) || (r == g && g == b) {
+				continue
+			}
+			out = append(out, fmt.Sprintf("\033[38;5;%dm", i))
+		}
+		return out, nil
+	})
+	levelColours = map[Level]string{
+		Trace: "\x1b[90m", // Dark gray
+		Debug: "\x1b[34m", // Blue
+		Info:  "\x1b[37m", // White
+		Warn:  "\x1b[33m", // Yellow
+		Error: "\x1b[31m", // Red
+	}
+)
 
 var _ Sink = (*plainSink)(nil)
 
@@ -52,7 +99,7 @@ func (t *plainSink) Log(entry Entry) error {
 	scope, exists := entry.Attributes[scopeKey]
 	if exists {
 		if t.isaTTY {
-			prefix += entry.Level.String() + ":\u001B[35m" + scope + "\u001B[0m: "
+			prefix += entry.Level.String() + ":" + scopeColour(scope) + scope + "\x1b[0m: "
 		} else {
 			prefix += entry.Level.String() + ":" + scope + ": "
 		}
@@ -63,7 +110,7 @@ func (t *plainSink) Log(entry Entry) error {
 	// Print
 	var err error
 	if t.isaTTY {
-		_, err = fmt.Fprintf(t.w, "%s%s%s%s\x1b[0m\n", colours[entry.Level], prefix, colours[entry.Level], entry.Message)
+		_, err = fmt.Fprintf(t.w, "%s%s%s%s\x1b[0m\n", levelColours[entry.Level], prefix, levelColours[entry.Level], entry.Message)
 	} else {
 		_, err = fmt.Fprintf(t.w, "%s%s\n", prefix, entry.Message)
 	}
@@ -72,3 +119,18 @@ func (t *plainSink) Log(entry Entry) error {
 	}
 	return nil
 }
+
+//nolint:errcheck
+func parseRGB(s string) (int, int, int) {
+	r, _ := strconv.ParseInt(s[:2], 16, 32)
+	g, _ := strconv.ParseInt(s[2:4], 16, 32)
+	b, _ := strconv.ParseInt(s[4:], 16, 32)
+	return int(r), int(g), int(b)
+}
+
+func scopeColour(scope string) string {
+	hash := fnv.New32a()
+	hash.Write([]byte(scope))
+	colours, _ := scopeColours.Get(context.Background()) //nolint:errcheck
+	return colours[hash.Sum32()%uint32(len(colours))]
+}