reflect: support multiple keys in struct tags

Fixes #40281

Change-Id: Ie624bce3a78a06d7ed71bba1f501e66802dffd13
Reviewed-on: https://go-review.googlesource.com/c/go/+/248341
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>

2 files changed, 194 insertions, 3 deletions

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 0684eab973..a12712d254 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -7165,6 +7165,176 @@ func TestMapIterDelete1(t *testing.T) {
 	}
 }
 
+func TestStructTagLookup(t *testing.T) {
+	var tests = []struct {
+		tag           StructTag
+		key           string
+		expectedValue string
+		expectedOK    bool
+	}{
+		{
+			tag:           `json:"json_value_1"`,
+			key:           "json",
+			expectedValue: "json_value_1",
+			expectedOK:    true,
+		},
+		{
+			tag:           `json:"json_value_2" xml:"xml_value_2"`,
+			key:           "json",
+			expectedValue: "json_value_2",
+			expectedOK:    true,
+		},
+		{
+			tag:           `json:"json_value_3" xml:"xml_value_3"`,
+			key:           "xml",
+			expectedValue: "xml_value_3",
+			expectedOK:    true,
+		},
+		{
+			tag:           `bson json:"shared_value_4"`,
+			key:           "json",
+			expectedValue: "shared_value_4",
+			expectedOK:    true,
+		},
+		{
+			tag:           `bson json:"shared_value_5"`,
+			key:           "bson",
+			expectedValue: "shared_value_5",
+			expectedOK:    true,
+		},
+		{
+			tag:           `json bson xml form:"field_1,omitempty" other:"value_1"`,
+			key:           "xml",
+			expectedValue: "field_1,omitempty",
+			expectedOK:    true,
+		},
+		{
+			tag:           `json bson xml form:"field_2,omitempty" other:"value_2"`,
+			key:           "form",
+			expectedValue: "field_2,omitempty",
+			expectedOK:    true,
+		},
+		{
+			tag:           `json bson xml form:"field_3,omitempty" other:"value_3"`,
+			key:           "other",
+			expectedValue: "value_3",
+			expectedOK:    true,
+		},
+		{
+			tag:           `json    bson    xml    form:"field_4" other:"value_4"`,
+			key:           "json",
+			expectedValue: "field_4",
+			expectedOK:    true,
+		},
+		{
+			tag:           `json    bson    xml    form:"field_5" other:"value_5"`,
+			key:           "non_existing",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           `json "json_6"`,
+			key:           "json",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           `json:"json_7" bson "bson_7"`,
+			key:           "json",
+			expectedValue: "json_7",
+			expectedOK:    true,
+		},
+		{
+			tag:           `json:"json_8" xml "xml_8"`,
+			key:           "xml",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           `json    bson    xml    form "form_9" other:"value_9"`,
+			key:           "bson",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           `json bson xml form "form_10" other:"value_10"`,
+			key:           "other",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           `json bson xml form:"form_11" other "value_11"`,
+			key:           "json",
+			expectedValue: "form_11",
+			expectedOK:    true,
+		},
+		{
+			tag:           `tag1`,
+			key:           "tag1",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           `tag2 :"hello_2"`,
+			key:           "tag2",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           `tag3: "hello_3"`,
+			key:           "tag3",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           "json\x7fbson: \"hello_4\"",
+			key:           "json",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           "json\x7fbson: \"hello_5\"",
+			key:           "bson",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           "json bson:\x7f\"hello_6\"",
+			key:           "json",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           "json bson:\x7f\"hello_7\"",
+			key:           "bson",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           "json\x09bson:\"hello_8\"",
+			key:           "json",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+		{
+			tag:           "a\x7fb json:\"val\"",
+			key:           "json",
+			expectedValue: "",
+			expectedOK:    false,
+		},
+	}
+
+	for _, test := range tests {
+		v, ok := test.tag.Lookup(test.key)
+		if v != test.expectedValue {
+			t.Errorf("struct tag lookup failed, got %s, want %s", v, test.expectedValue)
+		}
+		if ok != test.expectedOK {
+			t.Errorf("struct tag lookup failed, got %t, want %t", ok, test.expectedOK)
+		}
+	}
+}
+
 // iterateToString returns the set of elements
 // returned by an iterator in readable form.
 func iterateToString(it *MapIter) string {
diff --git a/src/reflect/type.go b/src/reflect/type.go
index 44c96fea82..a3a616701b 100644
--- a/src/reflect/type.go
+++ b/src/reflect/type.go
@@ -1130,6 +1130,9 @@ func (tag StructTag) Lookup(key string) (value string, ok bool) {
 	// When modifying this code, also update the validateStructTag code
 	// in cmd/vet/structtag.go.
 
+	// keyFound indicates that such key on the left side has already been found.
+	var keyFound bool
+
 	for tag != "" {
 		// Skip leading space.
 		i := 0
@@ -1149,11 +1152,29 @@ func (tag StructTag) Lookup(key string) (value string, ok bool) {
 		for i < len(tag) && tag[i] > ' ' && tag[i] != ':' && tag[i] != '"' && tag[i] != 0x7f {
 			i++
 		}
-		if i == 0 || i+1 >= len(tag) || tag[i] != ':' || tag[i+1] != '"' {
+		if i == 0 || i+1 >= len(tag) || tag[i] < ' ' || tag[i] == 0x7f {
 			break
 		}
 		name := string(tag[:i])
-		tag = tag[i+1:]
+		tag = tag[i:]
+
+		// If we found a space char here - assume that we have a tag with
+		// multiple keys.
+		if tag[0] == ' ' {
+			if name == key {
+				keyFound = true
+			}
+			continue
+		}
+
+		// Spaces were filtered above so we assume that here we have
+		// only valid tag value started with `:"`.
+		if tag[0] != ':' || tag[1] != '"' {
+			break
+		}
+
+		// Remove the colon leaving tag at the start of the quoted string.
+		tag = tag[1:]
 
 		// Scan quoted string to find value.
 		i = 1
@@ -1169,7 +1190,7 @@ func (tag StructTag) Lookup(key string) (value string, ok bool) {
 		qvalue := string(tag[:i+1])
 		tag = tag[i+1:]
 
-		if key == name {
+		if key == name || keyFound {
 			value, err := strconv.Unquote(qvalue)
 			if err != nil {
 				break