regexp: avoid copying mutex in (*Regexp).Copy.

There's nothing guaranteeing that the *Regexp isn't in active use,
and so copying the sync.Mutex value is invalid.

Updates #14839.

Change-Id: Iddf52bf69df1b563377922399f64a571f76b95dd
Reviewed-on: https://go-review.googlesource.com/20841Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/regexp/regexp.go

@@ -81,6 +81,14 @@ var debug = false
 // A Regexp is safe for concurrent use by multiple goroutines.
 type Regexp struct {
 	// read-only after Compile
+	regexpRO
+
+	// cache of machines for running regexp
+	mu      sync.Mutex
+	machine []*machine
+}
+
+type regexpRO struct {
 	expr           string         // as passed to Compile
 	prog           *syntax.Prog   // compiled program
 	onepass        *onePassProg   // onepass program or nil
@@ -93,10 +101,6 @@ type Regexp struct {
 	numSubexp      int
 	subexpNames    []string
 	longest        bool
-
-	// cache of machines for running regexp
-	mu      sync.Mutex
-	machine []*machine
 }
 
 // String returns the source text used to compile the regular expression.
@@ -109,10 +113,11 @@ func (re *Regexp) String() string {
 // When using a Regexp in multiple goroutines, giving each goroutine
 // its own copy helps to avoid lock contention.
 func (re *Regexp) Copy() *Regexp {
-	r := *re
-	r.mu = sync.Mutex{}
-	r.machine = nil
-	return &r
+	// It is not safe to copy Regexp by value
+	// since it contains a sync.Mutex.
+	return &Regexp{
+		regexpRO: re.regexpRO,
+	}
 }
 
 // Compile parses a regular expression and returns, if successful,
@@ -174,6 +179,7 @@ func compile(expr string, mode syntax.Flags, longest bool) (*Regexp, error) {
 		return nil, err
 	}
 	regexp := &Regexp{
+		regexpRO: regexpRO{
 			expr:        expr,
 			prog:        prog,
 			onepass:     compileOnePass(prog),
@@ -181,6 +187,7 @@ func compile(expr string, mode syntax.Flags, longest bool) (*Regexp, error) {
 			subexpNames: capNames,
 			cond:        prog.StartCond(),
 			longest:     longest,
+		},
 	}
 	if regexp.onepass == notOnePass {
 		regexp.prefix, regexp.prefixComplete = prog.Prefix()