strings: Map does not handle invalid UTF-8 correctly

[rsc]

Map's loop does:

for i, c := range s {
	r := mapping(c)
	if r == c {

This is wrong if r is the error rune because s has invalid UTF-8. The loop should check this and flip to rewriting the string in that case.

[rsc]

bytes.Map is OK because it is defined to always make a copy.

/cc @robpike

[martisch]

Updated my comment: "This is wrong if r is the error rune because s has invalid UTF-8" the problematic case mentioned here is if c is the error rune due to s having invalid UTF-8 and the mapping of c also produces the error rune as r not leading the string to be rewritten.

It is ok if r is the error rune and c is the error rune because s contained an error rune that maps to an error rune.

[gopherbot]

Change https://golang.org/cl/131495 mentions this issue: strings: correctly handle invalid utf8 sequences in Map