Implement Go version of SQL Commenter

This implements the SQL Commenter specification for affixing comments to a
SQL query. Loosely inspired by url.Values [1] in the Go standard library.

The specification doesn't fully describe the following situation, so I'll
list the intended behavior of the Go implementation here:

- Empty keys: omitted and don't appear in the comment string.

- Keys with single quotes: this Go library uses the valid URL encoding
  of a single quote `%27` instead of escaping with a backslash. The
  spec has the confusing example: `name''` serialized to `name=\'\'`.
  Where did the equal sign come from? Also, the website shows fancy
  quotes, not plain ascii quotes.

- Already URL encoded values: this Go library double-encodes the value,
  meaning the key-pair `a=%3D` is encoded as `a='%253D'`. The spec exhibit
  doesn't double-encode, instead preserving the original percent encoding.
  That seems inadvisable since the parsing a serialized sql comment won't
  result in the same original values.

Fixes google#95.

[1]: https://pkg.go.dev/net/url#Values

go/sqlcommenter/go.mod

@@ -0,0 +1 @@
+module github.com/google/sqlcommenter/go/sqlcommenter

go/sqlcommenter/sqlcommenter.go

@@ -0,0 +1,60 @@
+package sqlcommenter
+
+import (
+	"net/url"
+	"sort"
+	"strings"
+)
+
+// Values maps a string key to a value for that key to attach to a SQL query
+// in a comment. Implements the SQL Commenter spec:
+// https://google.github.io/sqlcommenter
+type Values map[string]string
+
+// String returns the string representing all values according to the SQL
+// Commenter spec.
+func (vs Values) String() string {
+	if len(vs) == 0 {
+		return ""
+	}
+
+	pairs := make([]string, 0, len(vs))
+	for k, v := range vs {
+		if k == "" {
+			continue
+		}
+		pairs = append(pairs, serializeKey(k)+"="+serializeValue(v))
+	}
+
+	if len(pairs) == 0 {
+		return "" // we might have dropped only empty keys
+	}
+
+	// Spec requires sorted key-value pairs after running the serialization
+	// algorithm.
+	sort.Strings(pairs)
+
+	return "/*" + strings.Join(pairs, ",") + "*/"
+}
+
+// https://google.github.io/sqlcommenter/spec/#key-serialization-algorithm
+func serializeKey(s string) string {
+	esc := urlEncode(s)
+	return escapeMeta(esc)
+}
+
+// https://google.github.io/sqlcommenter/spec/#value-serialization-algorithm
+func serializeValue(s string) string {
+	esc := urlEncode(s)
+	return `'` + escapeMeta(esc) + `'`
+}
+
+func urlEncode(s string) string {
+	esc := url.QueryEscape(s)
+	// Go encodes spaces as "+"; use more standard %20.
+	return strings.Replace(esc, "+", "%20", -1)
+}
+
+func escapeMeta(s string) string {
+	return strings.Replace(s, `'`, `\'`, -1)
+}

go/sqlcommenter/sqlcommenter_test.go

@@ -0,0 +1,72 @@
+package sqlcommenter
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestValues_String(t *testing.T) {
+	tests := []struct {
+		name string
+		vs   Values
+		want string
+	}{
+		{name: "nil", vs: nil, want: ""},
+		{name: "empty", vs: Values{}, want: ""},
+		{name: "empty cast", vs: Values(map[string]string{}), want: ""},
+		{
+			name: "drop empty key",
+			vs:   Values(map[string]string{"": "val"}),
+			want: "",
+		},
+		{
+			name: "one",
+			vs:   Values(map[string]string{"key": "val"}),
+			want: "/*key='val'*/",
+		},
+		{
+			name: "two",
+			vs:   Values(map[string]string{"a": "1", "b": "2"}),
+			want: "/*a='1',b='2'*/",
+		},
+		{
+			name: "two reversed",
+			vs:   Values(map[string]string{"b": "2", "a": "1"}), // technically, Go map iteration is random
+			want: "/*a='1',b='2'*/",
+		},
+		{
+			name: "name=DROP TABLE FOO",
+			vs:   Values(map[string]string{"name": "DROP TABLE FOO"}),
+			want: "/*name='DROP%20TABLE%20FOO'*/",
+		},
+		{
+			name: `name''=DROP TABLE USERS'`,
+			vs:   Values(map[string]string{"name''": `DROP TABLE USERS'`}),
+			want: `/*name%27%27='DROP%20TABLE%20USERS%27'*/`,
+		},
+		{
+			name: `exhibit`, // https://google.github.io/sqlcommenter/spec/#sql-commenter-exhibit
+			vs: Values(map[string]string{
+				"action":      `%2Fparam*d`,
+				"controller":  `index`,
+				"framework":   `spring`,
+				"traceparent": `00-5bd66ef5095369c7b0d1f8f4bd33716a-c532cb4098ac3dd2-01`,
+				"tracestate":  `congo%3Dt61rcWkgMzE%2Crojo%3D00f067aa0ba902b7`,
+			}),
+			want: "/*" + strings.Join([]string{
+				"action='%252Fparam%2Ad'",
+				"controller='index'",
+				"framework='spring'",
+				"traceparent='00-5bd66ef5095369c7b0d1f8f4bd33716a-c532cb4098ac3dd2-01'",
+				"tracestate='congo%253Dt61rcWkgMzE%252Crojo%253D00f067aa0ba902b7'",
+			}, ",") + "*/",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.vs.String(); got != tt.want {
+				t.Errorf("\nwant: %v\ngot:  %v", tt.want, got)
+			}
+		})
+	}
+}