Quote a string setting that YAML would read as a number (closes #229)

config set wrote every value as an unquoted YAML scalar, and config.Load
reads the file through untyped YAML, so an access key 00112233 loaded as
38043 and a hostname 007 as 7.

config set now looks the key up in config.Config by the fields' yaml
tags. A string setting is tagged !!str, which the encoder quotes
wherever YAML would read a number or a boolean. Other settings stay
unquoted, so compression_level 9 is still a number. A value that is not
valid UTF-8 stays untagged: the encoder refuses it as !!str and writes
it as !!binary, which loads back unchanged.

Judgement call: the type comes from reflection over config.Config.
Quoting every value YAML would change avoids that, but would also quote
odd numbers such as compression_level 03 and make them fail to load.

Model: opus-5-5

internal/cli/config.go

@@ -10,6 +10,7 @@ import (
 	"reflect"
 	"strconv"
 	"strings"
+	"unicode/utf8"
 
 	"github.com/spf13/cobra"
 	"gopkg.in/yaml.v3"
@@ -592,7 +593,10 @@ func yamlPathSet(root *yaml.Node, keys []string, value string) error {
 	// unquoted 00112233 into the number 38043 and 1e5 into 100000. Tagging
 	// a string setting as a string makes the encoder quote such a value.
 	// Other settings stay unquoted, so compression_level 9 is a number.
-	if configKeyIsString(keys) {
+	// The encoder refuses to write a value that is not valid UTF-8 as a
+	// string. Left untagged, such a value is written as base64 !!binary and
+	// loads back unchanged.
+	if configKeyIsString(keys) && utf8.ValidString(value) {
 		node.Tag = yamlStringTag
 	}
 

internal/cli/config_test.go

@@ -116,6 +116,8 @@ func TestConfigSetStringLooksLikeNumber(t *testing.T) {
 			func(cfg *config.Config) string { return cfg.S3.Prefix }},
 		{"s3.bucket", "true",
 			func(cfg *config.Config) string { return cfg.S3.Bucket }},
+		{"s3.region", "FALSE",
+			func(cfg *config.Config) string { return cfg.S3.Region }},
 		{"snapshots.home.exclude.0", "1.10",
 			func(cfg *config.Config) string { return cfg.Snapshots["home"].Exclude[0] }},
 	}
@@ -134,6 +136,22 @@ func TestConfigSetStringLooksLikeNumber(t *testing.T) {
 	}
 }
 
+// TestConfigSetNonUTF8Path checks that config set still accepts a value that
+// is not valid UTF-8, such as a path with a Latin-1 file name, and that
+// config.Load returns it unchanged.
+func TestConfigSetNonUTF8Path(t *testing.T) {
+	t.Parallel()
+
+	const dir = "/srv/caf\xe9"
+
+	cfg := loadAfterConfigSet(t, "snapshots.home.paths.0", dir)
+
+	got := cfg.Snapshots["home"].Paths[0]
+	if got != dir {
+		t.Errorf("snapshots.home.paths.0 = %q, want %q", got, dir)
+	}
+}
+
 // TestConfigSetNumberStaysNumber checks that a number set for an integer
 // setting is still read as a number, not as a quoted string.
 func TestConfigSetNumberStaysNumber(t *testing.T) {