net/url: do not percent-encode valid host characters

The code in question was added as part of allowing zone identifiers
in IPv6 literals like http://[ipv6%zone]:port/foo, in golang.org/cl/2431.

The old condition makes no sense. It refers to §3.2.1, which is the wrong section
of the RFC, it excludes all the sub-delims, which §3.2.2 (the right section)
makes clear are valid, and it allows ':', which is not actually valid,
without an explanation as to why (because we keep :port in the Host field
of the URL struct).

The new condition allows all the sub-delims, as specified in RFC 3986,
plus the additional characters [ ] : seen in IP address literals and :port suffixes,
which we also keep in the Host field.

This allows mysql://a,b,c/path to continue to parse, as it did in Go 1.4 and earlier.

This CL does not break any existing tests, suggesting the over-conservative
behavior was not intended and perhaps not realized.

It is especially important not to over-escape the host field, because
Go does not unescape the host field during parsing: it rejects any
host field containing % characters.

Fixes #12036.

Change-Id: Iccbe4985957b3dc58b6dfb5dcb5b63a51a6feefb
Reviewed-on: https://go-review.googlesource.com/13254Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/net/url/url.go

@@ -75,6 +75,18 @@ func shouldEscape(c byte, mode encoding) bool {
 		return false
 	}
 
+	if mode == encodeHost {
+		// §3.2.2 Host allows
+		//	sub-delims = "!" / "$" / "&" / "'" / "(" / ")" / "*" / "+" / "," / ";" / "="
+		// as part of reg-name.
+		// We add : because we include :port as part of host.
+		// We add [ ] because we include [ipv6]:port as part of host
+		switch c {
+		case '!', '$', '&', '\'', '(', ')', '*', '+', ',', ';', '=', ':', '[', ']':
+			return false
+		}
+	}
+
 	switch c {
 	case '-', '_', '.', '~': // §2.3 Unreserved characters (mark)
 		return false
@@ -97,10 +109,6 @@ func shouldEscape(c byte, mode encoding) bool {
 			// that too.
 			return c == '@' || c == '/' || c == '?' || c == ':'
 
-		case encodeHost: // §3.2.1
-			// The RFC allows ':'.
-			return c != ':'
-
 		case encodeQueryComponent: // §3.4
 			// The RFC reserves (so we must escape) everything.
 			return true
@@ -110,13 +118,6 @@ func shouldEscape(c byte, mode encoding) bool {
 			// everything, so escape nothing.
 			return false
 		}
-
-	case '[', ']': // §2.2 Reserved characters (reserved)
-		switch mode {
-		case encodeHost: // §3.2.1
-			// The RFC allows '[', ']'.
-			return false
-		}
 	}
 
 	// Everything else must be escaped.

src/net/url/url_test.go

@@ -382,6 +382,26 @@ var urltests = []URLTest{
 		},
 		"",
 	},
+	// issue 12036
+	{
+		"mysql://a,b,c/bar",
+		&URL{
+			Scheme: "mysql",
+			Host:   "a,b,c",
+			Path:   "/bar",
+		},
+		"",
+	},
+	// worst case host
+	{
+		"scheme://!$&'()*+,;=hello!:port/path",
+		&URL{
+			Scheme: "scheme",
+			Host:   "!$&'()*+,;=hello!:port",
+			Path:   "/path",
+		},
+		"",
+	},
 }
 
 // more useful string for debugging than fmt's struct printer
@@ -1130,6 +1150,7 @@ var shouldEscapeTests = []shouldEscapeTest{
 	{'a', encodeUserPassword, false},
 	{'a', encodeQueryComponent, false},
 	{'a', encodeFragment, false},
+	{'a', encodeHost, false},
 	{'z', encodePath, false},
 	{'A', encodePath, false},
 	{'Z', encodePath, false},
@@ -1154,6 +1175,29 @@ var shouldEscapeTests = []shouldEscapeTest{
 	{',', encodeUserPassword, false},
 	{';', encodeUserPassword, false},
 	{'=', encodeUserPassword, false},
+
+	// Host (IP address, IPv6 address, registered name, port suffix; §3.2.2)
+	{'!', encodeHost, false},
+	{'$', encodeHost, false},
+	{'&', encodeHost, false},
+	{'\'', encodeHost, false},
+	{'(', encodeHost, false},
+	{')', encodeHost, false},
+	{'*', encodeHost, false},
+	{'+', encodeHost, false},
+	{',', encodeHost, false},
+	{';', encodeHost, false},
+	{'=', encodeHost, false},
+	{':', encodeHost, false},
+	{'[', encodeHost, false},
+	{']', encodeHost, false},
+	{'0', encodeHost, false},
+	{'9', encodeHost, false},
+	{'A', encodeHost, false},
+	{'z', encodeHost, false},
+	{'_', encodeHost, false},
+	{'-', encodeHost, false},
+	{'.', encodeHost, false},
 }
 
 func TestShouldEscape(t *testing.T) {