net/http: implement path value methods on Request

Add Request.PathValue and Request.SetPathValue,
and the fields on Request required to support them.

Populate those fields in ServeMux.ServeHTTP.

Updates #61410.

Change-Id: Ic88cb865b0d865a30d3b35ece8e0382c58ef67d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/528355
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 47 insertions, 0 deletions

diff --git a/src/net/http/request.go b/src/net/http/request.go
index 12039c9ae2..b66e6853f6 100644
--- a/src/net/http/request.go
+++ b/src/net/http/request.go
@@ -329,6 +329,11 @@ type Request struct {
 	// It is unexported to prevent people from using Context wrong
 	// and mutating the contexts held by callers of the same request.
 	ctx context.Context
+
+	// The following fields are for requests matched by ServeMux.
+	pat         *pattern          // the pattern that matched
+	matches     []string          // values for the matching wildcards in pat
+	otherValues map[string]string // for calls to SetPathValue that don't match a wildcard
 }
 
 // Context returns the request's context. To change the context, use
@@ -1415,6 +1420,48 @@ func (r *Request) FormFile(key string) (multipart.File, *multipart.FileHeader, e
 	return nil, nil, ErrMissingFile
 }
 
+// PathValue returns the value for the named path wildcard in the ServeMux pattern
+// that matched the request.
+// It returns the empty string if the request was not matched against a pattern
+// or there is no such wildcard in the pattern.
+func (r *Request) PathValue(name string) string {
+	if i := r.patIndex(name); i >= 0 {
+		return r.matches[i]
+	}
+	return r.otherValues[name]
+}
+
+func (r *Request) SetPathValue(name, value string) {
+	if i := r.patIndex(name); i >= 0 {
+		r.matches[i] = value
+	} else {
+		if r.otherValues == nil {
+			r.otherValues = map[string]string{}
+		}
+		r.otherValues[name] = value
+	}
+}
+
+// patIndex returns the index of name in the list of named wildcards of the
+// request's pattern, or -1 if there is no such name.
+func (r *Request) patIndex(name string) int {
+	// The linear search seems expensive compared to a map, but just creating the map
+	// takes a lot of time, and most patterns will just have a couple of wildcards.
+	if r.pat == nil {
+		return -1
+	}
+	i := 0
+	for _, seg := range r.pat.segments {
+		if seg.wild && seg.s != "" {
+			if name == seg.s {
+				return i
+			}
+			i++
+		}
+	}
+	return -1
+}
+
 func (r *Request) expectsContinue() bool {
 	return hasToken(r.Header.get("Expect"), "100-continue")
 }